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Constraining coral sensitivity to climate and environmentalchange: an integrated experimental approachGeorgiou, L. (2017). Constraining coral sensitivity to climate and environmental change: an integratedexperimental approach
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Download date: 11. Jun. 2018
Constraining coral sensitivity to climate and environmental change:
an integrated experimental approach
Lucy Georgiou
This thesis is presented for the degree of Doctor of Philosophy of The University of
Western Australia
School of Earth and Environment
2017
I
II
Acknowledgements
The work contained within this thesis would not have been possible without the
knowledge, expertise and support of many other individuals. Professionally I would
like to extend a special thanks to my immediate supervisory team at the University of
Western Australia (Professor Malcolm McCulloch, Dr Julie Trotter, Dr Juan D'Olivo
Cordero and Dr Jim Falter) for providing me with guidance, focus and freely giving me
their time and expert knowledge. I would also like to extend warm gratitude to my
other co-authors for taking the time to review my work and share their insightful
wealth of knowledge.
I would like to thank the postdoctoral fellows within the ARC Centre for Excellence for
Coral Reef Studies (Dr Verena Schoepf, Dr Juan D'Olivo Cordero and Dr Michael
Holcomb) for their insightful prospective on early career research as well as their
subject knowledge but above all for their personal support and friendship. For
technical support and guidance, I would like to say how grateful I am to the staff at
the UWA AGFIOR facility, Dr Kai Rankenburg and Dr Hans Oskierski (now at Murdoch
University). Groups of scientists working at Heron Island and KAUST (Saudi Arabia)
obtained the coral samples used for analysis in this thesis. I would like to acknowledge
their work (fieldwork is never easy) and dedication.
To my fellow Ph.D. candidates in the School of Earth and Environment I am especially
grateful for the laughs, the distractions, and the trips to the coffee shop. I would like
to take this opportunity to thank Mark Wilson and Selena Black for their unending
support. To Mark Wilson especially for those conversations where we resolved all the
world’s problems while making our millions, long may they continue!
Finally and most importantly, I would like to thank my family (Gilly, Costas, Bethany,
Alex, Steve, Andy, Nick and the rest of the Georgiou family) for giving me both
emotional and financial support when needed.
III
Publications arising from this thesis
Georgiou et al. (2015) pH homeostasis during coral calcification in a Free Ocean CO2
Enrichment (FOCE) experiment, Heron Island reef flat, Great Barrier Reef. PNAS, 112
(43), 13219–13224 (Chapter 2)
Georgiou et al. (2017) Long-Term Impacts of the 1998 El Nino-Southern Oscillation
(ENSO) Event on the Growth and Skeletal Geochemistry of Porites lutea In The Red
Sea. PLOSOne, IN REVIEW. (Chapter 3)
Georgiou et al. (2017) Response of Coral Calcification Chemistry to Short and Long-
Term Changes in Climate in the Red Sea. To be submitted to Biogeosciences. (Chapter
4)
IV
Thesis Declaration
I, Lucy Georgiou, certify that:
This thesis has been substantially accomplished during enrolment in the degree.
This thesis does not contain material which has been accepted for the award of any
other degree or diploma in my name, in any university or other tertiary institution.
No part of this work will, in the future, be used in a submission in my name, for any
other degree or diploma in any university or other tertiary institution without the
prior approval of The University of Western Australia and where applicable, any
partner institution responsible for the joint-award of this degree.
This thesis does not contain any material previously published or written by another
person, except where due reference has been made in the text.
The work(s) are not in any way a violation or infringement of any copyright,
trademark, patent, or other rights whatsoever of any person.
This thesis contains published work and/or work prepared for publication, some of
which has been co-authored.
Signature
Date: 8.03.2017
V
Table of Contents
Acknowledgements ................................................................................................................................... ii
Publications arising from this thesis ......................................................................................................... iii
THESIS DECLARATION ............................................................................................................................... IV
TABLE OF CONTENTS ................................................................................................................................. V
List of Figures ............................................................................................................................... viii
ABSTRACT ............................................................................................................................................ 1
CHAPTER 1 ........................................................................................................................................ 4
1.0 INTRODUCTION ......................................................................................................................... 4
1.1 Coral Sensitivity and Adaptability ............................................................................................. 6
1.2 Seawater Carbonate Chemistry and the Calcification of Marine Organisms .......................... 10
1.2.1 Coral Calcification ........................................................................................................................... 12
1.3 Incorporation of trace elements in the coral skeleton ............................................................ 16
1.4 Boron Isotope Systematics in Marine Carbonates .................................................................. 20
1.5 Thesis outline .......................................................................................................................... 24
CHAPTER 2 ...................................................................................................................................... 25
2.0 CORAL PROTO – FREE OCEAN CARBON ENRICHMENT SYSTEM (CP-FOCE) EXPERIMENTAL
DESIGN .......................................................................................................................................... 25
PH HOMEOSTASIS DURING CORAL CALCIFICATION IN A FREE OCEAN CO2 ENRICHMENT (FOCE) EXPERIMENT, HERON
ISLAND REEF FLAT, GREAT BARRIER REEF ..................................................................................................... 28
ABSTRACT ............................................................................................................................................. 29
SIGNIFICANCE STATEMENT ....................................................................................................................... 29
2.1 INTRODUCTION ................................................................................................................................ 30
2.1.1 Heron Island reef flat ........................................................................................................... 32
2.1.2 The Free Ocean CO2 Enrichment (FOCE) experiment ........................................................... 34
2.2 MATERIALS AND METHODS ................................................................................................................ 35
2.2.1 CP-FOCE system ................................................................................................................... 35
2.2.2 Sampling protocol and geochemical methods ..................................................................... 35
VI
2.2.3 Constraining growth chronology with Sr/Ca analysis .......................................................... 36
2.2.4 Extension and calcification rates ......................................................................................... 36
2.2.5 Boron isotope-pH proxy ....................................................................................................... 36
2.2.6 Modelling Ωcf and coral calcification ................................................................................... 37
2.2.7 Statistics ............................................................................................................................... 38
2.3 RESULTS AND DISCUSSION .................................................................................................................. 38
ACKNOWLEDGEMENTS ............................................................................................................................ 45
CHAPTER 3 ...................................................................................................................................... 46
3.0.1 Characterisation of the Red Sea with focus on the Saudi Arabian Red Sea ......................... 46
3.0 LONG-TERM IMPACTS OF THE 1997-1998 BLEACHING EVENT ON THE GROWTH AND SKELETAL GEOCHEMISTRY OF
PORITES LUTEA FROM THE CENTRAL RED SEA ............................................................................................... 49
Abstract ........................................................................................................................................ 50
3.1 INTRODUCTION ................................................................................................................................ 51
3.2 MATERIALS AND METHODS ................................................................................................................ 54
3.2.1 Site description and climatology .......................................................................................... 54
3.2.2 Coral core sampling ............................................................................................................. 58
3.2.3 Geochemical analysis ........................................................................................................... 59
3.2.4 Analytical procedure ............................................................................................................ 59
3.2.5 Coral chronology and skeletal growth parameters.............................................................. 60
3.2.6 Statistical analysis ................................................................................................................ 65
3.3 RESULTS ......................................................................................................................................... 66
3.3.1 Coral growth rates ............................................................................................................... 66
3.3.2 TRACE ELEMENT –SST PROXIES ........................................................................................................ 68
3.4 DISCUSSION ..................................................................................................................................... 74
3.5 Conclusions ............................................................................................................................. 81
Acknowledgments ........................................................................................................................ 82
CHAPTER 4 ...................................................................................................................................... 83
VII
RESPONSE OF CORAL CALCIFICATION AND CALCIFYING FLUID COMPOSITION TO CLIMATE
CHANGE AND THERMAL STRESS IN THE RED SEA ......................................................................... 83
ABSTRACT ................................................................................................................................................ 84
4.1 Introduction ............................................................................................................................ 85
4.1.1. Site description .............................................................................................................................. 87
4.1.2 Satellite and Reconstructed SST data ............................................................................................. 88
4.1.3 Records of seawater carbonate chemistry...................................................................................... 89
4.2 Materials and Methods .......................................................................................................... 90
4.2.1 Coral core sampling ........................................................................................................................ 90
4.2.2 Geochemical analysis ...................................................................................................................... 93
4.2.3 Calculation of pH of the calcifying fluid .......................................................................................... 94
4.2.4 Calculation of carbonate ion and DIC of the calcifying fluid ........................................................... 94
4.2.5 Modelled pHsw/pHcf and growth rates ................................................................................. 95
4.3 Results ..................................................................................................................................... 96
4.3.1 Seasonal Geochemical Signal and ENSO disruption ........................................................................ 96
4.3.2 Annual changes in coral skeletal chemistry from 1938 - 2013 ........................................................ 99
4.3.3 Growth over the past 75 years (1940-2012) ................................................................................. 101
4.4 Discussion .............................................................................................................................104
4.4.1 Regulation of internal carbonate chemistry and environmental stress ........................................ 104
4.4.2 Annual changes in coral carbonate chemistry and growth over the past 75 years (1938-2013) .. 109
4.5 Conclusions ...........................................................................................................................114
Acknowledgements .....................................................................................................................115
CHAPTER 5 .................................................................................................................................... 116
5.0 DISCUSSION AND CONCLUSIONS ..........................................................................................116
APPENDICES ........................................................................................................................................122
Chapter 2: Supporting Information .............................................................................................122
Chapter 3: Supporting Information .............................................................................................136
Chapter 4: Supporting Information .............................................................................................139
PUBLICATIONS .....................................................................................................................................141
VIII
CONSOLIDATED REFERENCES...................................................................................................................142
List of Figures
Figure 1-1. Bjerrum plot of inorganic carbon species in seawater ......................................................... 11
Figure 1-2. Simplified diagram of the coral organism ............................................................................ 15
Figure 1-3. Boron speciation in seawater ............................................................................................... 21
Figure 2-1. Environmental parameters at Heron Island ......................................................................... 33
Figure 2-2. Boron isotope composition and derived pH of the calcifying fluid ....................................... 40
Figure 2-3. Boxplot of pH of the calcifying fluid of nubbins. ................................................................... 41
Figure 2-4. Extension rates and calcification rates of the coral nubbins ................................................ 43
Figure 3-1. Regional map of the Red Sea and Saudi Arabian Coastline. ................................................ 56
Figure 3-2. Climatological data for the central Red Sea region and El Nino index ................................. 57
Figure 3-3. Coral core x-ray images from cores from the coast of Saudi Arabia .................................... 64
Figure 3-4. Relative x-ray optical density (intensity) and Li/Mg records ................................................ 65
Figure 3-5. Growth parameters of inner-shelf and outer-shelf cores. .................................................... 67
Figure 3-6. Annual records of local summer (JAS) temperature ............................................................. 67
Figure 3-7. Near-monthly records of trace lement ratios of corals from the central Red Sea ................ 70
Figure 3-8. Comparison between trace element ratios for central Red Sea cores; ................................ 74
Figure 4-1. Location map of reef site Shi'b Nazar and adjacent coast. .................................................. 88
Figure 4-2. Seasonal SST records (1994-2012) ....................................................................................... 89
Figure 4-3. X-ray images of core (Shi'b Nazar) showing sampling tracks............................................... 92
Figure 4-4. Seasonal geochemical and SST records ................................................................................ 98
Figure 4-5. Annual records of boron isotopes and sea surface temperature .......................................100
Figure 4-6. Growth characteristics of the massive Porites coral at Shib Nazar ....................................103
Figure 4-7. Seasonal time series of corals internal carbonate chemistry .............................................106
Figure 4-8. Relationship of seasonally resolved sea surface temperature with carbonate chemistry
profile ...................................................................................................................................................108
Figure 4-9. Annual changes in the carbonate chemistry of the calcifying fluid from 1938-2013 .........113
IX
1
ABSTRACT
The regulation of the corals carbonate chemistry at the site of calcification is central
to the maintenance of the reef system. The process of calcification itself relies on the
elevation of the aragonite saturation state (Ωcf) in order to drive mineral precipitation
kinetics, which is driven by raising dissolved inorganic carbon (DICcf) and pH of the
calcifying fluid (pHcf). Evaluating the response of these drivers to changes in the
external environment is imperative as we try to assess how corals will survive as sea
surface temperatures rise and the oceans carbonate chemistry is modified in response
to rising levels of atmospheric CO2. Studying the internal chemistry of the coral can be
challenging, but by employing geochemical tracers as proxies for variables such as
DICcf and pHcf we can investigate these responses in corals growing in natural reef
systems.
The ratio of stable boron isotopes (11B) in coral can be used as a reliable measure of
pHcf due to its unique speciation in seawater and incorporation into marine
carbonates. We employ 11B analysis in corals (Porites cylinidrica) from an in situ Free
Ocean Carbon Enrichment (FOCE) experiment at Heron Island (Great Barrier Reef),
which were exposed to low pH seawater levels while maintaining the seasonal regime
of the natural environment. The pHcf was reconstructed over the duration of the
experiment (6-months) to determine how pHcf changes in response to reductions in
external seawater pH. Our findings indicate pHcf in these corals from dynamic
environments can maintain almost a constant pHcf regardless of how much the pH of
the external seawater (pHsw) is changing (range ~ 7.7 – 8.3). This implies that some
2
corals have a strong capacity to control their internal chemistry to drive normal rates
of calcification even under highly acidified (i.e., low pH) scenarios.
Long-term records of trace elements (e.g. Li/Mg, Sr/Ca and Mg/Ca) from the cored
skeletons of massive corals (e.g. Porites spp.) have become important tools for
assessing past environmental parameters such as sea surface temperature (SST) in the
absence of existing instrumental records. Discrepancies in the calibration and
correlation of these records can occur when thermal stress disrupts the normal
incorporation of elements into the aragonite lattice of the coral skeleton. This was
evaluated in cores taken from the central Red Sea off Saudi Arabia where corals
experience temperatures that exceed those thought to accommodate coral
calcification (often over 32 oC) and the seasonal temperature variability is ~10 oC. A
distinct and prolonged disruption (2-4 years), in measured trace elements (e.g., Sr/Ca,
B/Ca and Li/Mg) occurred following a bleaching event triggered by the 1998 El Niño
event, the most intense El Niño on record and part of a particular strong El Niño-
Southern Oscillation (ENSO). Following this period of disruption there is significant
reduction in the correlation between the trace element ratio and SST records in both
cores; the exception being Li/Mg, which is a relatively new SST proxy and until now
has thought to be largely reserved for temperate and cold water species. These
finding indicate the robust nature of the Li/Mg proxy that could filter out the biological
effects that otherwise compromise traditional trace element signals.
Assessment of 11B-B/Ca in the same coral core (from the outer-shelf of the central
Red Sea) provided a means to resolve the complete carbonate chemistry profile of the
coral over two time scales; seasonally over 20 years (1994-2013) and annually over 75
3
years (1938-2013). This long-term record of Ωcf, DICcf and pHcf revealed co-varying
seasonal signal between pHcf and Ωcf, DICcf. However, the DICcf seasonality was
impacted during two periods were bleaching and heat stress had been reported in the
region (ENSO: 1998-2002 and 2007-2010). Annually resolved data suggest pHcf, DICcf
and Ωcf are declining; however, the regulatory control on pHcf (pH up-regulation)
appears strong in this coral, which is facilitating increased rates of calcification.
Therefore, we conclude that temperature is driving a compensatory mechanism
whereby pHcf continues to be enhanced to facilitate calcification based on mineral
precipitation kinetics.
4
Chapter 1 1.0 INTRODUCTION
Coral reefs are one of the most biologically diverse ecosystems on earth. They provide
coastal protection and are of high economic importance in terms of the endemic
fisheries and tourism they support. However, they are facing increasing uncertainty
in the light of global climate change, which is widely accepted being brought about by
increasing concentrations of atmospheric gasses such as CO2. Pre-industrial levels of
atmospheric pCO2 were around ~280 ppm and are currently more than 40% higher at
~400 ppm (as of 2015). Even the most conservative predictions estimate that CO2
levels could reach ~550 ppm by 2100 (2). This is believed to be an unprecedented rate
of change in recent geological history initiating scientists to investigate the potential
effects these scenarios will have on marine organisms, communities and ecosystems.
Changes to the ocean's carbonate chemistry system are expected to occur as a result
of increased pCO2; most notably through reductions in seawater pH and the
saturation state (Ω) of calcium carbonate (CaCO3) minerals; a process termed ‘ocean
acidification’ (detailed in Section 1.1) (3). Ocean acidification has thus been cited as a
significant threat to coral as it may impede the production of the coral's calcium
carbonate skeleton 1 (4, 5). The abrupt temperature anomalies associated with
climate change may also negatively impact corals by initiating bleaching (loss of the
corals symbiotic zooxanthellae), slowing growth, and ultimately causing mortality (6).
In addition to these changes in regional and global climate, corals are also
experiencing numerous local stresses including over-fishing as well as unmitigated
coastal development, run-off (sediments and fresh water), and pollution. As these
5
pressures are taking place simultaneously, it is imperative to understand how
changing climates will influence coral organisms and the wider reef community in
order to direct management and mitigation efforts.
The link between increased sea surface temperature (SST) and coral bleaching has
long been established (6) and more recently laboratory evidence points to ocean
acidification negatively influencing the production of the calcium carbonate skeleton
of corals and other marine calcifiers (7). However, the relationship between increasing
temperatures, ocean acidification, and local stressors may not be straightforward.
Coral reef systems are physically and biologically complex with constantly changing
wave, tidal, light, nutrient and heat regimes according to regional climatology (8). In
addition, coral reefs are home to a diverse array of organisms whose metabolic
activities (e.g., photosynthesis, respiration and calcification) can alter chemical
seawater composition. Though corals are often described as simple organisms,
physiologically they are quite complex, which is reflected in our lack of understanding
of the mechanisms behind calcification and the influence of interactions between the
coral and its associated symbiont.
This thesis aims to bring together our current knowledge of the resilience of coral to
environmental threats and further this understanding through the use of geochemical
tracers. It also aims to test the application of geochemical tracers as environmental
proxies under stressful conditions. The first section of this introduction gives an
overview of what we have learnt from studies of natural reef systems and laboratory
experiments. The second section provides a synopsis of trace element and boron
6
isotopes in coral and there use as environmental proxies and tools for understanding
biological mechanisms.
1.1 Coral Sensitivity and Adaptability
The majority of coral reefs occupy a narrow range of environmental conditions where
temperatures range from ~ 20-30 °C, light attenuation is low, and waters are clear.
There ecological range is also constrained by aragonite saturation levels (Ωarg) and
bathymetry (9) Temperature sensitivity is often regarded as particularly pertinent to
coral health and survival. Many corals live within ~1 °C of their upper thermal limit and
exceeding this by ~1 °C above the maximum monthly mean (MMM) can induce coral
bleaching (10). Reductions in coral calcification and increased episodes of bleaching,
due to thermal stress and associated increased light intensity (11) has been recorded
with increasing frequency over the past decade (1). The most notable worldwide
bleaching event occurred in response to the strong 1997-98 El Niño phase of the El
Niño-Southern Oscillation cycle (ENSO) (9, 12, 13). Among the notable effects of the
1997-1998 El Niño rates of coral calcification appeared to decrease, such as those
recorded in the central Red Sea, which decreased by 30% following the 1998 event
(14). Impaired calcification rates have also been highlighted in areas such as the Great
Barrier Reef (GBR) (15); however, these changes seem to be evident in nearshore sites
only where other factors such as pollution are also impacting coral health (16, 17).
The role thermal stress has on calcification is clear but can be complicated by the fact
that thermal thresholds vary according to region, local climatology, coral taxa, and
7
symbiont clade (18, 19). Within a very narrow geographic location the variability in
bleaching response within taxa can be profound. One explanation for this is the
unique hydrographic systems that are intrinsically part of the reef environment and
often incorporate complex topography, currents and upwelling systems (20, 21). One
of these complex reef systems is found in the central Red Sea (22). A bleaching event
in this area of the Red Sea in 2009-2010 resulted in a higher prevalence of bleaching
in the shallow inshore regions of the reef compared to deeper offshore areas (23, 24).
Seawater carbonate chemistry (i.e. dissolved inorganic carbon, saturation state and
pH) is also thought to be an important regulatory factor in coral reef health, providing
a basis for calcification to occur. Several laboratory studies have demonstrated this
apparent need by manipulating seawater carbonate chemistry parameters. Dove, et
al. (7) conducted mesocosm experiments that simulated natural patch-reef
environments reducing pH and increasing temperature in line with predicted changes
outlined by the IPCC (2). Species such as Acropora, Monitpora and Stylophora within
the elevated pCO2 and temperature tank systems (both reduced emission and
business-as-usual scenarios) bleached and died, while species such as Goniasterea,
Lobophylia, Porites and Fungia bleached to some extent but survived. Daylight
calcification rates were also reduced in coral species exposed to these two scenarios
compared to pre-industrial and control mesocosms. These finding support other
studies (4, 25) that suggest resilience to changing ocean environments is variable
among taxa yet the reasons for these differences has yet to be determined.
However, natural reef environments often provide evidence, which contradict
laboratory experiments such as reefs that survive around natural CO2 seeps where pH
8
and Ωarg are considered too low for coral to survive. The reefs around Palau’s Rock
Island and Papua New Guinea in the Western Pacific are naturally exposed to low-pH
waters. Their coral communities still thrive in areas where pCO2 levels are around 720
ppm and (Ωarg) average 2.7 and (Ωarg) can be as low as 1.9 in some localities (26, 27).
This is in comparison to laboratory experiments where calcification showed a strong
inverse linear relationship to Ωarg (e.g. (28-30)). It has been generally considered that
when Ωarg approaches ~3.3 (pCO2 of around 480 ppm) coral calcification rates falls too
low to counter the effects of erosion (31). However, bioerosion occurred frequently
in corals at low pH sites at Palau, while coral community composition appeared to be
shaped by other environmental variables such as light, nutrients, water flow and
temperature rather than just pH alone (26). At CO2 seeps located in the Milne Bay
Province of Papua New Guinea, environmental variables (light, nutrients etc.) are
relatively constant across the natural pH gradient of the site. Species richness around
the seeps was negatively affected by pH reduced waters with massive boulder corals
such as Porites spp. dominating these sites. The more structurally complex corals (i.e.
branching corals) were reduced or absent from these low-pH areas (25, 32) indicating
coral resilience and adaptability to these sites are species specific. On the other side
of the world, freshwater springs of the Yucatan Peninsula, Mexico create naturally low
pH sites where corals are still able to thrive. Massive corals of (Porites astreoides)
showed a slowdown in calcification rates along the pH gradient, which was expressed
mainly through reduced skeletal densities but not linear extension rates. The authors
therefore concluded these corals were not adapting to their low-pH environments
(33).
9
Although the sites and habitats described above are atypical environments for corals
to inhabit (34), even natural reef environments can exhibit highly dynamic pH,
chemistry and temperature regimes (e.g., Kline, et al. (8)). Many reef sites experience
variable and dynamic regimes in carbonate chemistry (and temperature), which
expose corals to short-term carbonate conditions that may occur under high pCO2
emission scenarios in the coming decades (35-37). These dynamic regimes are often
due to a combination of the hydrodynamic processes driving mixing and circulation as
well as the underlying biologically mitigated benthic carbon fluxes resulting from
photosynthesis, respiration and calcification (34). In these areas, where organisms
may experience aragonite saturation levels (Ωarg) < 1, an adaptive response may be
occurring that is mitigating the adverse effects of ocean acidification. Short-term
acclimatisation and longer duration adaptation (with responsive gene expression) to
high temperatures has been documented in some corals (38). Acropora hyacinthus
were taken from environments with highly variable temperature regimes. These
corals were particularly resistant to thermal stress conditions, while corals from more
stable environments showed less resistance to the same conditions (38). Palumbi,
Barshis, Traylor-Knowles and Bay (38) study showed gene expression provided the
acclimatisation mechanism for thermal tolerance rather than any modification in
symbiont clade. Whether these acclimatisation/adaptive responses we see in
temperature are available against changes in ocean chemistry is still to be tested
thoroughly.
There is still no consensus on how corals reefs will respond to the combined effects
of changing carbonate chemistry and temperature. However, it is only through
10
understanding the physiological mechanisms behind calcification that will enable us
to constrain the response of calcifying organisms to these climate shifts. The next
section discusses our current understanding of coral calcification and the relationship
of this process to seawater carbonate chemistry.
1.2 Seawater Carbonate Chemistry and the Calcification of
Marine Organisms
Carbon dioxide [CO2] is exchanged between the air-sea interface at a rate determined
by the difference in concentrations of the gas, and the exchange coefficients related
to the reaction (equilibrium between [CO2]air and [CO2]sw) necessary for the gas
exchange to take place (39). As sea temperatures continue to rise the ability of surface
seawater to absorb [CO2] decreases. Surface water movement, deep-water upwelling
and global circulation systems all affect the capacity of the air-sea gas exchange,
which means oceanic uptake of [CO2] is not uniform spatially or temporally over the
globe. However, as atmospheric [CO2] steadily increases diffusion of [CO2] into surface
oceans increases, affectively shifting the balance of ions and increasing H+ (therefore
lowering pH).
Within seawater, the carbonate system largely involves reactions between five main
species of ions; aqueous carbon dioxide [CO2], bicarbonate [HCO3-], carbonate ions
[CO32-], hydroxide ions [OH-] and hydrogen ions [H+] (Figure 1-1). Note H2CO3 (carbonic
acid) is combined with [CO2] in this text. The first three in this list are inorganic species
of carbon that equate to the total dissolved inorganic carbon in seawater [DICt] (eq.1).
Apart from the incursion of atmospheric [CO2] photosynthesis/respiration and
11
calcium carbonate, dissolution will regulate [DICt] (along with weathering on longer
time scales).
2
2 3 3tDIC CO HCO CO ------- (Eq.1)
Figure 1-1. Bjerrum plot of inorganic carbon species in seawater carbon dioxide [CO2],
bicarbonate [HCO3-], carbonate ions [CO3
2-], as a function of pH. The concentrations of the
two main species of boron in seawater, borate ion [B(OH)4-] and boric acid [B(OH)3] are shown
as a function of pH (Adapted from Zeebe and Wolf-Gladrow (39)).
In seawater, the concentrations of each carbonate species (eq. 2) are in equilibrium,
depending on seawater temperature (T), salinity (S), and pressure (P) variables
(stoichiometric equilibrium constants [k]). Concentrations of each carbonate species
is therefore relative to one another and drive pH levels within the systems. Changes
in the production and dissolution of calcium carbonate also affect total alkalinity (TA)
12
(eq.3), which is defined as the charge balance in a seawater ‘packet’, which is also a
crucial measure when considering the carbonate system.
2 2
2 2 2 3 3 32CO H O H CO H HCO H CO --------- (Eq.2)
23 4 + 2 TA HCO CO B OH OH H ----- (Eq.3)
The saturation state (Ω) of calcium carbonate (CaCO3) expressed in equation 4 where
K*sp is the solubility product of CaCO3 (one of the two main forms aragonite or calcite)
under the variables of T, S and P. The effect of pressure is significant and produces the
effect of a reduced saturation state at depth (saturation horizon; Ω=1), this depth
differs for Ωcal and Ωarg (with aragonite being the less stable form in seawater) and
between regions. Below the saturation horizon, CaCO3 starts to become unstable (and
dissolve). The calcite or aragonite compensation depths (CCD and ACD respectively)
are defined as the depth where the rate of dissolution is equal to the rate of
precipitation and these compensation depths may decrease as atmospheric CO2
increases (40).
2 3
*
[ ] [ ]
sp
Ca CO
K
------- (Eq.4)
1.2.1 Coral Calcification
The pathways of transport and the inorganic species of carbon that facilitate
calcification (eq. 4) are not thoroughly understood, although it is thought the
carbonate ion [CO32-] is the species used to precipitate calcium carbonate at the site
13
of calcification. However, several studies have suggested that bicarbonate ions are
the main building blocks of the corals skeleton (eq. 5), which are transported into the
calcifying fluid and then converted into carbonate ions for the formation of CaCO3 (41,
42). This is generally accepted as; 1) there are no known active transport mechanisms
for carbonate ions but there are for bicarbonate ions, and 2) bicarbonate is the most
dominant carbon species under the pH range found within seawater and the calcifying
fluid (~7.94-8.53) (43).
2 2
3 3Ca CO CaCO ------- (Eq.5)
2
3 3Ca HCO CaCO H ------ (Eq.6)
The role of respiratory CO2 on the calcification reaction (eq. 6) is also currently unclear
although some studies suggest this is the major source of carbon for some coral
species (44). An active transport mechanism may be present to facilitate the
movement of CO2 to the site of calcification, which again is dependent of the enzyme
carbonic anhydrase (42-44).
------- (Eq. 7)
During the process of calcification H+ ions are pumped away from the site of the
calcification by Ca2+ATPase anhydrase proton pump increasing pH of the calcifying
fluid (pHcf) and modulating the chemistry of the corals internal environment (Figure
1-2C). We are now understanding how high the degree of regulation occurring at the
site of calcification is (45-47), which extends to the regulation of dissolved inorganic
carbon (DICcf) and aragonite saturation state (Ωcf) (48) constituting the whole of the
22 2 3 2Ca CO H O CaCO H
14
coral internal carbonate chemistry profile. However, the strength of this regulation
appears to be dependent on the health of the coral-symbiont relationship. This aspect
of the coral’s physiology is further explored in this thesis.
15
Figure 1-2. Simplified diagram of the coral organism including (a) the anatomy of the polyp
and calcium carbonate skeletal structure, (b) the cellular layers of the coral polyp and adjacent
skeleton, and (c) the biochemical processes occurring at the site of calcification to facilitate
the calcification process. CA represents those processes occurring with carbonic anhydrase.
Diagrams adapted from (a) Sabourault, Ganot, Deleury, Allemand and Furla (49) (b) Allemand,
Tambutté, Zoccola and Tambutté (50) (c) (50-52).
16
1.3 Incorporation of trace elements in the coral skeleton
Measuring the abundance of trace elements (e.g., Sr, Li, Mg, B, U, etc.) and isotopes
(e.g., 11B, 13C, 18O) incorporated into the coral skeleton as it forms provide a means
to determine past environmental variances in the absence of instrumental records
(53, 54). Trace element and isotopic measurements taken from the length of the
growth axis of a massive coral from genera such as Porites in the Indo-Pacific and
Diploria in the Caribbean can help resolve past climate history over many decades by
providing proxy data on key local natural environmental parameters such as sea
surface temperature, salinity, nutrients, and flood events. Global scale climate
forcing, such as El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation
(NAO) and can also be resolved though analysis of measured geochemical tracers as
well as anthropogenic enforced modifications to the environment such as pollution
(see Druffel (54) for review). Massive corals are particularly useful tools for
paleoclimate studies as they are generally slow growing (around 6-20 mm per year)
(55) and can provide up to several hundred years of seasonal and annual data (56,
57). Another important characteristic of cores from these types of corals are their
high and low density bands which alternate with changing seasons providing a reliable
means of dating and constructing an age model for the selected section of the coral
(usually a core drilled vertically from the top of the colony to obtain a single, vertical
major growth axis). In most situations one high and one low density band constitutes
a year of growth (58). X-radiograph images of slices of the core clearly reveal the
density bands providing the initial assessment for constraining the growth rates
(linear extension) and age model of the coral (17, 59, 60).
17
Assessment of a diverse range of trace elements in marine carbonates have revealed
a variation in their reliability and accuracy as paleoclimate proxies. Sr/Ca ratios are
one of the most studied and considered a robust method for reconstructing sea
surface temperature (SST) variability. Sr2+ is believed to substitute readily for Ca2+ in
the CaCO3 lattice with ratios showing a strong (inverse) dependence on temperature
(61, 62). Additionally, strontium in seawater is assumed to generally be stable over
long time periods (~2Myr) within a given reef system (63) and independent of
seawater salinity variations (64). The inverse relationship between SST and the Sr/Ca
ratios in long-term coral records has shown strong calibration coefficients in corals
from several regions including the Great Barrier Reef (17, 65-67), the Pacific (68-70),
the Gulf of Mexico (71), the South China Sea (72) and the northern Red Sea (73).
Though it appears SST is the overriding control on Sr/Ca incorporation into the CaCO3
lattice there is still some debate on how calcification rates (74, 75) and species specific
biological effects (76) also impact the Sr/Ca (and other trace element) ratios
measured. However, close agreement in measured ratios from two coral colonies
from the same area, as shown in some studies (65, 68) strengthens our confidence of
using Sr/Ca as a paleothermometer. It has been documented however that extreme
temperatures (high and low) can affect the strontium signal, which may stem from
biological artefacts in growth and a breakdown in the symbiodinium association (77).
To what degree this disruption to can affect the trace element/Ca-SST (TE/Ca-SST)
signal is explored in chapters 2 and 3.
Some potential trace element ratios are less reliable as temperature proxies. For
example, the ratio of magnesium to calcium (Mg/Ca) has proven to be a valid
18
paleothermometer in biogenic calcites under certain circumstances (78-80). However,
biogenic aragonite calibrations between Mg/Ca and SST can be highly variable
between specimens (81-83). This is mainly due to differences in the crystallographic
structure of the two carbonate minerals, where calcite is rhombohedral and aragonite
is orthorhombic. Both theoretical and experimental X-ray adsorption near edge
structure (XANES) spectroscopy has shown that differences in the crystal lattice
structures favours some trace elements, like Mg, to substitute readily for calcium in
calcite but not aragonite (84). Similarly, uranium (U/Ca) was once thought a
potentially robust paleothermometer (85, 86); however, biological effects and
environmental variables such as salinity heavily influence U/Ca ratios within the coral
lattice (87).
In order to overcome these biological effects to the TE/Ca signature and improve our
confidence in these proxies it is important to understand the biomineralization
process occurring in the calcifying fluid (CF) and the partitioning of elements into the
calcium carbonate skeleton. The calcifying fluid is thought to be the medium where
aragonite precipitation occurs in isolation from the surrounding seawater; as Ωarg
reaches supersaturation, precipitation occurs automatically (51). This is thought to
occur extracellularly within the calcifying fluid encased by epithelial membrane that
prevents passive diffusion (88, 89). Any level of permeability of this membrane is
expected to be negligible, as some regulation has been established, such as an
elevated Ωarg/pH compared to the surrounding seawater. The calcifying fluid appears
to be replenished as Ca2+ ions as they are depleted (45, 46, 90, 91). There is still
however, some uncertainty about the way the calcification system operates in
19
relation to seawater movement in and away from the site of calcification (92).
Numerical models to explain Rayleigh fractionation (93-95) support the process of
aragonite precipitation and trace element incorporation occurring in a semi-enclosed
space. Rayleigh-Based Multi-Element (RBME) proxy reconstruction models remove
uncertainties in the trace element temperature proxy record by removing the level of
biological sensitivity. RBME uses known temperature dependent distribution co-
efficient (DMe or Kd
Me value) of each trace elements measured (calculated through
inorganic precipitation experiments) (96, 97). RBME equations also incorporate the
change in the precipitation rate of Ca2+ from the calcifying fluid from a ‘batch’ of fluid
at a point in time, which is also temperature dependent. The fractionation process is
as such in corals that as precipitation of Ca2+ increases (during summer periods) trace
elements such as Sr2+ and Li+ decrease, while conversely Li and Mg2+ increases. This
pattern is reversed during lower calcification stages such as winter. The atomic radii
of elements such as Mg2+ and Ba2+ are different from Ca2+ and therefore may not be
transported readily by ion channels or enzymes whose purpose is to carry Ca2+ to the
calcification site. Surface entrapment is another potential mechanism by which trace
elements are confined in the aragonite lattice of the coral (96). This is the most likely
scenario for elements such as Mg2+, Li+ and U+ (98, 99). Mg2+ appears to be
concentrated in a disordered inorganic phase in areas of defects in the skeleton and
in centres for calcification (COCs) (100, 101).
Clearly, there are still several areas of ambiguity relating to trace element
incorporation and the mechanisms behind the process of calcification. One aspect of
this thesis will focus on assessing the nature of trace element incorporation into the
20
aragonite lattice with consideration of how thermal stress events may affect the
mechanism of incorporation and our interpretation of paleoclimate records.
1.4 Boron Isotope Systematics in Marine Carbonates
The ratio of stable isotopes can provide another source of vital information about
environmental conditions during the time of the formation of the coral skeleton.
Oxygen and carbon isotopes have provided a means to reconstruct past climate
variables such as El Niño-Southern Oscillations and temperature and salinity
variability e.g. (102, 103); however, in the present thesis I will concentrate on the
boron isotope (11B) and its development as a pH proxy in marine carbonates.
The borate ion [B(OH)4-] and boric acid [B(OH)3] are the two main species of boron
found in modern seawater. These species have a distinct fractionation (~ 27.2‰) with
boric acid being isotopically heavier compared to the borate ion. The equilibrium
between these two species is highly pH dependent (39) (figure 1-3; eq.7) making them
an ideal tool to examine the relationship between the pH of the calcifying fluid (pHcf)
of modern coral species and seawater pH (pHsw) (45, 91).
------- (Eq. 7)
During the calcification process, B(OH)4- is predominately incorporated into the
aragonite lattice in concentrations proportional to that of the external seawater (39,
104). Thus, measurements in 11B in modern marine carbonates (11Bcarb) have been
examined as pH proxy tool following changing B(OH)4- in past and present oceans
(105-107). pHcf values are derived from measured 11Bcarb values using the following
eq. 8 (39):
23 4 B OH H O B OH H
21
-------- (Eq.8)
Where pKB is the dissociation constant (8.957), 11Bsw = 39.61 after Foster, Pogge von
Strandmann and Rae (108) and αB3-B4 is the fractionation factor of the boron isotopes
(B(OH)4- and B(OH)3) in seawater where αb3-b4 equals 1.0272 (109)
Figure 1-3. Boron speciation in seawater (a) Relative amounts of the borate ion [B(OH)4-] (red)
and boric acid [B(OH)3] (blue) against a given seawater pH. (b) Relative 11B composition for
each boron species seawater (borate ion [B(OH)4-] red and boric acid [B(OH)3] blue at a given
pH). pH total scale (pHt) environemtal parameters at 20 oC and 35 ppt salinity.
The modifications to the calcifying fluid discussed earlier (Section 2.1) impart a
biological affect specific to the 11Bcarb of the precipitated skeleton. This can be
3 4 3 4
11 11
11 11 1000 1B B B B
carbsw
carb sw
cf B
B B
B BpH pK log
22
identified through the offset (pHcf – pHsw) seen from the theoretical boron curve (109).
Trotter, et al. (91) quantified the pHcf– pHsw offset for several temperate and tropical
coral species through calibrating the difference in pH at the site of calcification (pHcf)
to that of the ambient seawater (pHsw) expressed as ΔpH= pHcf - pHsw. Calibration
equations showed a linear systematic species specific increase (between 0.3-0.6 pH
units) and supported the theory of a biologically enhanced pHcf (pH up-regulation).
Up-regulating pH infers biological controlled DIC and Ω at the site of calcification to
maintain calcification rates (46, 110). The Internal pH regulation of the calcifying fluid
with abiotic calcification model (IpHRAC model) developed by McCulloch, Falter,
Trotter and Montagna (45), further quantified the sensitivity of coral species (and
other calcifying organisms) to changes in seawater pH by calculating the effect of pHcf
up-regulation on carbonate precipitation (G) via known abiotic rate kinetics (eq. 9)
(111):
(Eq.9)
Where k is the rate law constant, Ωcf is aragonite saturation state at the site of
calcification and n is the order of magnitude (temperature dependent). The initial
model produced two ‘classes’ of coral organisms, those with the ability to raise
internal pH above ambient seawater (described as ‘up-regulators’) and corals that had
little or no ability to do this (described as ‘non up-regulators’). Further to quantifying
calcification against Ωcf the authors showed that the free energy required to maintain
the elevated pH gradient between the calcifying fluid and the ambient seawater was
trivial relative to the amount of energy being generated by photosynthesis, given
known ratios of calcification and net production in reef-building coral. Incorporating
( 1)ncfG k
23
future predictions on pCO2 and associated changes in Ωarg to the IpHRAC model also
allows a species specific assessment of calcification rates under these scenarios.
Chapter 2 of this thesis builds on the work of McCulloch, et al., (45) and the IpHRAC
model in order to assess the potential for a third class of coral organism, which exhibit
higher rates of modification to its internal environment and therefore more resilience
to ocean acidification.
Additionally to providing a proxy of pHcf skeletal, the boron isotope in conjunction
with the B/Ca ratio potentially provides us with an insight into the carbonate
parameters of the calcifying fluid, such as the concentrations of dissolved inorganic
carbon (CO32- relative to HCO3
-) at the site of calcification. One theory is that B(OH)4-
competes with CO32- for incorporation in the coral lattice and therefore
measurements of B/Ca are potentially a proxy for CO32- ion concentration. This
method has been approached with calcific foraminifera with some success (112-114)
and is also being explored using coral (aragonite) species (115).
Our use of the boron isotope pH proxy has evolved to provide us with a means to
evaluate the biological processes occurring at the site of calcification in the coral
organism. Currently this is being used to further our knowledge of how corals will
respond to the effects of ocean acidification. Chapter 2 and 4 in this thesis aims to
contribute to this knowledge through the application of B isotope and B element
(B/Ca and B/Mg) analysis.
24
1.5 Thesis outline
The following three chapters contained in this body of work comprise of three
manuscripts and additional material where deemed appropriate. The first of these
manuscripts (Chapter 2) was published in the Proceedings of the National Academy
of Sciences (PNAS) (47) and describes a study using the boron isotope as a proxy for
pHcf in corals from a highly dynamic reef environment and sheds light on the response
of corals from such environments to low pH conditions. The second manuscript
(Chapter 3) has been submitted to the journal PloSOne and pertains to disruptions to
long-term trace element records in two cores from the Saudi Arabian central Red Sea,
due to strong prolonged ENSO phases. The third manuscript (Chapter 4) will be
submitted to Biogeosciences after review from the co-autors. Chapter 4 investigates
the long-term change in the saturation state (Ωcf), pH (pHcf) and dissolved inorganic
carbon (DICcf) of the corals calcifying fluid (using the 11B-B/Ca proxy) on massive
Porites spp. collected from the central Red Sea region. A summary and discussion on
the wider implications of the finding in these chapters are discussed in chapter 5.
25
Chapter 2 2.0 CORAL PROTO – FREE OCEAN CARBON ENRICHMENT
SYSTEM (CP-FOCE) EXPERIMENTAL DESIGN
Section 2.1 is a manuscript published in the Proceedings of the National Academy of
Sciences (PNAS) (47). Supplementary Information can be found in Appendices 1 of this
thesis. The coral samples used for geochemical analysis in this manuscript were
supplied from a Coral Proto-Free Carbon Ocean Enrichment System (CP-FOCE)
experiment conducted at Heron Island Reef Flat in 2010 overseen by Dr David Kline of
the Scripps Institute. The following is a synopsis of the system, which gives more detail
then the manuscript.
The Coral Proto-Free Carbon Ocean Enrichment System (CP-FOCE) is a unique
engineering development that aimed to address the limitations of ocean acidification
laboratory experiments. Designed and modified from the terrestrial FACE (Free Air
Carbon Dioxide Enrichment) see Hendrey, Lewin and Nagy (116) for full review. This
in situ mesocosm system allows manipulation of pH (with CO2 injections) while tracing
the natural cycle of pH changes occurring on the reef flat (ambient pH) without
compromising all other environmental parameters (i.e. light, temperature, nutrient
and tidal regimes). pH within the open-ended flume systems are manipulated by pCO2
injection at an offset to the ambient pH while still following the environments natural
cycle in real time (8, 117).
The CP-FOCE system injects carbon dioxide into a volume of water, which is partially
cut off from the surrounding seawater controlling the pH of the water within the
26
system while flow conditioners control water flow and provide passive mixing of the
carbonate enriched seawater and the ambient seawater. Low pH seawater is mixed
into the environmental seawater via the flow conditioners. CO2 flow rate is
maintained by keeping a positive pressure of pure CO2 in the tank that is slightly higher
than the atmospheric pressure. The seawater is left to equilibrate to the new pH level
and is partially closed for most of the time. To measure oxygen level and carbon
chemistry values the system is completely enclosed for short periods. (From changes
in oxygen, the net production and respiration rates can be calculated while the
alkalinity measurements can be used to calculate net rates of calcification i.e. the net
production/respiration and the calcification/dissolution rates can be determined for
each chamber within the system.
The pH control systems mirrors the diurnal changes in pH seen in the surrounding
seawater. Therefore, there is an offset between the ambient seawater and the
experimental pH; this is used to determine the chamber pH set point and the
measured pH in the chamber. A ‘proportional integral control algorithm is produced
– controlling the power of the dosing pumps. See Marker, et al. (117) and Kirkwood,
et al. (118) for a detailed review of the development of the CP-FOCE system.
27
Figure 2.0.1. Design of Foce system at Heron Island, GBR schematic diagram of FOCE chambers
and CO2 enriched seawater injection sites (reverse and forward injection) (b) photograph of
FOCE setup at Heron Island Reef flat courtesy of David Klien.
28
pH homeostasis during coral calcification in a Free Ocean CO2
Enrichment (FOCE) experiment, Heron Island reef flat, Great
Barrier Reef
Lucy Georgioua,b*, James Faltera,b , Julie Trottera, David I. Klined,f, Michael Holcomba,b,
Sophie G. Doved,e, Ove Hoegh-Guldbergc,e, Malcolm McCullocha,b
aSchool of Earth and Environment, The University of Western Australia, Crawley WA
6009
bARC Centre of Excellence for Coral Reef Studies, The University of Western Australia,
Crawley WA 6009
cGlobal Change Institute, dSchool of Biological Sciences, eARC Centre of Excellence in
Coral Reef Studies, University of Queensland St. Lucia, QLD 4072, Australia
f(current address) Scripps Institution of Oceanography, Integrative Oceanography
Division, University of California, San Diego, La Jolla, CA 92093, USA
*Correspondence to: – Lucy Georgiou, The University of Western Australia and ARC
Centre of Excellence for Coral Reef Studies, Crawley WA 6009. Email:
Keywords: ocean acidification, Heron Island, coral resilience, FOCE, pH regulation
29
Abstract
Geochemical analyses (11B and Sr/Ca) are reported for the coral Porites cylindrica grown
within a Free Ocean Carbon Enrichment (FOCE) experiment, conducted on the Heron Island
reef flat (Great Barrier Reef) for a six month period from June to early-December of 2010. The
FOCE experiment was designed to simulate the effects of CO2-driven acidification predicted to
occur by the end of this century (scenario RCP4.5), while simultaneously maintaining the
exposure of corals to natural variations in their environment under in-situ conditions. Analyses
of skeletal growth (measured from extension rates and skeletal density) showed no systematic
differences between low-pH FOCE treatments (ΔpH = ~-0.05 to -0.25 units below ambient) and
present-day controls (ΔpH = 0) for calcification rates, or the pH of the calcifying fluid (pHcf); the
latter derived from boron isotopic compositions (11B) of the coral skeleton. Furthermore,
individual nubbins exhibited near constant 11B compositions along their primary apical growth
axes (±0.02 pHcf units) regardless of the season or treatment. Thus, under the highly dynamic
conditions of the Heron Island reef flat, P. cylindrica up-regulated the pH of its calcifying fluid
(pHcf ~8.4 to 8.6), with each nubbin having near-constant pHcf values independent of the large
natural seasonal fluctuations of the reef flat waters (pH ~7.7 to ~8.3) or the superimposed
FOCE treatments. This newly discovered phenomenon of pH-homeostasis during calcification
indicates that coral living in highly dynamic environments exert strong physiological controls on
the carbonate chemistry of their calcifying fluid, implying a high degree of resilience to ocean
acidification within the investigated ranges.
Significance Statement
In-situ FOCE experiments and geochemical analyses (11B, Sr/Ca) conducted on corals
(Porites cylindrica) from the highly dynamic Heron Island reef flat of the Great Barrier
Reef show that this species exerts strong physiological controls on the pH of their
calcifying fluid (pHcf). Over a ~6 month period, from mid-winter to early summer, we
show that these corals maintained their pHcf at near constant elevated levels
independent of the highly variable temperatures and FOCE-controlled carbonate
chemistries to which they were exposed, implying they have a high degree of tolerance
to ocean acidification.
30
2.1 Introduction
Atmospheric CO2 has risen by over 30% during the past century causing a reduction in
seawater pH of ~0.1 units relative to pre-industrial times, with a further reduction of 0.1 to 0.4
units predicted to occur by the end of this century (119). This process, commonly known as
‘ocean acidification’, is expected to have severe impacts on calcifying marine organisms due
to its effect on the thermodynamics of biomineralization (120). Our current understanding of
the sensitivity of coral calcification to declining seawater pH has mainly been inferred from
short-term laboratory-based studies that do not fully simulate real-world reef conditions,
particularly the daily to seasonal variations in temperature, light, and pH (121-123). To address
these short-comings we have applied Free Ocean Carbon Enrichment (FOCE) technologies
(8, 124-126) to manipulate water chemistry in situ and thereby provide more realistic
experimental conditions to investigate how future levels of acidification could affect marine
organisms according to different Representative Concentration Pathways (RCPs) (127). The
FOCE system uses a flow-through flume design that allows organisms to experience near
natural conditions, in particular the daily and seasonal regimes of fluctuating temperature, light,
and nutrients, whilst maintaining offsets in flume-water pH below that of ambient environmental
conditions. This is accomplished by the controlled introduction of small volumes of low-pH
modified seawater into the open-ended flumes at levels necessary to simulate future declines
in ambient seawater pH. The FOCE system therefore enables realistic simulations of the
effects of ocean acidification within natural reef environments at levels of atmospheric pCO2
that are predicted to occur by the end of this century (127).
The influence that external seawater chemistry (i.e. pH and saturation state) has on
biomineralization and ion transport processes during skeleton formation is central to
understanding how ocean acidification will effect coral calcification, and therefore their general
ability to maintain the balance between reef growth and erosion (120). Although a clear
understanding of the physico-chemical mechanisms controlling coral calcification is still only
emerging, an important means of biological control is up-regulation of pH (91, 128) at the site
of calcification (pHcf). This is thought to occur predominantly by the biologically mediated action
of Ca-ATPase ion transporters that exchange 2H+ for Ca2+ (51, 129), but how such biological
31
controls are affected by changes in the ambient marine environment is still poorly understood.
It is also becoming increasingly apparent that the ‘natural’ level of environmental variability to
which coral-reef systems are subjected to also influences their potential to adapt and/or
acclimatise to environmental change (38, 130, 131). Understanding how corals living in
dynamic environments physiologically respond to large diel and seasonal changes in seawater
temperature and pH can thus provide important insights into the resilience or vulnerability of
corals to ocean warming and acidification. The Heron Island FOCE experiment was thus
designed to explore these questions through the application of targeted increases in pCO2 over
an extended time-scale to corals living in the highly variable environment of the Heron Island
reef flat (8).
The boron isotopic composition (11B) of carbonate skeletons essentially records the
biologically mediated pH of the calcifying fluid as calcification occurs (91, 128, 132). The use
of 11B as a pH proxy is based on the selective incorporation of the pH-sensitive and isotopically
distinct borate-ion, B(OH)4-, into the corals’ calcium carbonate skeleton (105, 133). Prior studies
have shown that the 11B of coral skeletons exhibit a significant positive offset from the
theoretical borate curve, equivalent to an elevation of ~0.5 to 0.8 units in the pHcf relative to
that of the external seawater pH (91) due to the ability of corals to manipulate their pHcf using
energy dependant ion transporters (129). These observations have been independently
corroborated by similar measurements of pHcf using electrodes and pH sensitive dyes (132,
134, 135). Biological controls on calcification impart significant species-specific but highly
systematic increases in pHcf relative to ambient seawater (91). The thermodynamic cost of pH
up-regulation within the calcifying fluid, however, is still relatively small compared to the amount
of metabolic energy available given normal rates of photosynthesis, respiration and calcification
in reef-building corals (128). Elevation of pHcf above ambient seawater results in higher
aragonite saturation states in the calcifying fluid (Ωcf) that in turn drives higher rates of mineral
precipitation (128, 134, 136, 137). Understanding the response of pHcf to ocean acidification is
therefore critical to predict the effects that increasing levels of atmospheric CO2 will have on
calcification and net reef growth.
32
Here, we report the sensitivity of pHcf within and between colonies of P. cylindrica grown in situ
within a FOCE experiment comprising of flumes subject to both natural (and often extreme)
diel and seasonal changes in seawater temperature and pH, as well as enhanced shifts
(decreases) in seawater pH to simulate future conditions (127). These experiments were
conducted within the highly dynamic reef flat of Heron Island in the Great Barrier Reef (GBR)
and covered a range of pCO2 scenarios (8). We show that P. cylindrica corals living in this
highly dynamic environment exhibit a previously unrealised strong pHcf homeostasis, despite
the highly variable conditions on the reef flat as well as the superimposed pH offsets (~-0.05
to -0.25 units) in FOCE treatments simulating future seawater chemistry in a high-pCO2
atmosphere. We then explore what this apparent lack of sensitivity in pHcf to changes in
ambient seawater pH implies for the growth of P. cylindrica living in such dynamic environments
as well as how these corals may respond to increasing acidification in a high-pCO2 world.
2.1.1 Heron Island reef flat
Heron Island (23.442°S, 151.914°E) is a sub-tropical coral cay situated in the southern area of
Great Barrier Reef (GBR) located ~80 km off the coast of Queensland (Figure S2-1). The
dominant substrate of the inner and mid-reef flat are carbonate sands that are sporadically
populated by coral patches several meters across which typically comprise species of Acropora
and Porites. Tides at Heron Island are semi-diurnal and reef flat waters are separated from the
open-ocean for a few hours each day at low tide. The shallow depth of the reef flat and periodic
isolation at low tide combined with the active metabolism of its benthic communities result in
strong diel and seasonal variations in the chemistry of the reef waters (8) (Figure 2-1).
Water temperatures offshore of Heron Island range from around 22°C in winter to around 27°C
during summer. This seasonal temperature pattern is mirrored by the Heron Island reef flat
(20°C to 28°C respectively), albeit with slightly larger seasonal amplitudes (6.5°C vs 5.5°C)
and stronger diel variations (3-4°C, Figure 2-1a). This is due to the shallow reef flat waters
being more susceptible to local atmospheric heating and cooling, especially during low tides
(8, 138). Similarly seasonal changes in net benthic metabolism (139) result in inter-seasonal
33
variation in the pH (8) of reef flat waters (~8.24 in June to 8.04 in December) that are much
greater than in offshore waters (Figure 2-1b, Figure 2-1c), the latter having relatively constant
pH throughout the year (pH ~8.0 - 8.1) (140). Reef flat waters on Heron Island are also subject
to strong diel variations that can change by up to 0.75 pH units within a 24 hour period (Figure
S2-2) (8).
Figure 2-1. Environmental parameters at Heron Island (a) Daily average water temperatures
both offshore (green) and on the Heron Island reef flat (black) from January 2010 – December
2010 (b) Daily (3hr interval) pHT measurements from the end of May 2010 to early December
2010 period from offshore (black) and on the reef flat (green) as well as in both control (blue)
and treatment flumes (red) same colour reference for (c) (but no black offshore data), grey
vertical bars indicate time intervals of milled δ11B samples. Missing data due to the passing of
a tropical storm. Offshore temperature data obtained from a NOAA PMEL CO2 buoy near
‘Harry’s bommie’ (22.46°S, 151.93°E) managed by the Marine and Atmospheric Research
Division of the Commonwealth Scientific and Industrial Research Organization (140)
http://cdiac.ornl.gov/oceans/Moorings/Heron_Island.html. Reef flat temperature data taken
from IMOS relay pole #2 (http://data.aims.gov.au/metadataviewer). Offshore pH data were
calculated from records of fCO2 data obtained from Heron Island NOAA buoy assuming an
34
offshore total alkalinity of 2275 ueq/kg (125). See supplementary information for location of
loggers and method details.
The FOCE system was constructed on the Heron Island reef flat with four submerged flumes
(two controls and two treatments) oriented parallel to the shore, each flume open to the
environment at both ends and on the bottom. Five living parent colonies of the branching coral
P. cylindrica were harvested from the Heron Island reef flat and transplanted into both treatment
and control flumes, such that each flume contained a representative selection of each colony.
The FOCE flumes were deployed for over ~6 months, from the end of May to mid-December
2010.
2.1.2 The Free Ocean CO2 Enrichment (FOCE) experiment
Colonies of P. cylindrica collected from Heron Island reef flat were first acclimatised for 4 weeks
before the experiment which commenced in the winter of 2010 (June). In the 1st phase of the
experiment (June to July), all flumes were initially set to follow the same ambient pH conditions
of the reef flat waters (mean winter pH ~8.24) to allow colonies to acclimatize further and
recover from transplantation. In the 2nd phase of the experiment (July to September), the pH of
the FOCE treatment flumes were progressively offset relative to the ambient reef flat water by
-0.05 in July, then by -0.15 in August, and finally by -0.25 pH units in September (Figure 2-1c).
The treatment flumes were maintained at this reduced pH offset (-0.25) until early October
when the experiment was interrupted by a strong tropical storm that led to a temporary
cessation of the pH offset in the FOCE treatments. The experiment then resumed in late
November when treatment flumes were again subjected to pH offsets of around -0.25 relative
to the ambient seawater pH which continued until the end of the experiment in mid-December.
As noted above, both the controls and treatments were subject to strong natural diel and
seasonal forcing independent of the FOCE experiment. This strong combined diel and
seasonal variation in reef flat pH allowed us to simultaneously examine the response of each
coral’s internal chemistry to both high levels of natural variability in ambient pH together with
systemic shifts in pH expected to occur over this century.
35
2.2 Materials and Methods
2.2.1 CP-FOCE system
In May 2010, five colonies of Porites cylindrica were collected from the Heron Island reef flat and divided
into sub-samples that were allowed to recover on the reef flat for four weeks prior to commencing the
experiment. In June 2010, the sub-samples from each colony were transplanted to each flume in order to
maximize the diversity of the fragments within the flumes, and allowed to acclimatise in situ within all four
FOCE chambers for a further four weeks prior to lowering pH within the flume systems. The pH levels
within treatment flumes were then incrementally lowered relative to ambient reef flat water in July (-0.05
pH units), August (-0.15 pH units) and September (-0.25 pH units). The pH within the treatment flumes
were then maintained at a constant offset of -0.25 units relative to the controls until early October, when
a passing tropical storm interrupted the CO2 injections and the acquisition of water quality data. The
treatments were re-initiated in late November and ran until the end of the experiment (December 2010).
Discrete water samples were collected daily over the course of the experiment for the analysis of
Dissolved Inorganic Carbon (DIC), pH, Total Alkalinity (AT) and dissolved oxygen. Continuous
measurements of pH were made with MBARI digital pH sensors (Nido Instruments, CA, USA),
conductivity, temperature and depth were measured with a Seabird SBE-16plusV2 SEACAT (Sea-Bird
Electronics, WA, USA). For a full description of the FOCE systems, environmental data monitoring and
instrument models/specifications see (8, 124, 125).
2.2.2 Sampling protocol and geochemical methods
Coral nubbins were first bleached and then sliced in half to expose the central growth (extension) axis of
the coral. One half was used for the analysis of trace elements (to constrain growth chronology) while
the other half was used for boron isotope analysis. Samples for trace elements were extracted by milling
along the major growth axis at 0.5mm intervals to a depth of 1mm (see Figure S2-3 for diagram) using a
video microscope mounted micromill (New Wave Research MicroMill Sampling System, WA Dept. of
Fisheries). Powdered samples (0.5-1mg) were processed in the ultra-clean laboratory of the Advanced
Geochemical Facility for Indian Ocean Research (AGFIOR, University of Western Australia) for
dissolution and dilution to 10ppm Ca solutions. Trace element ratios were determined using the Thermo
X-series-2 Quadropole-Inductively Coupled Plasma Mass Spectrometer (Q-ICPMS) housed in the UWA
36
AGFIOR facility. Trace element ratios were corrected against an in-house (Davis Reef, NEP) and inter-
laboratory (JCp-1) standards.
2.2.3 Constraining growth chronology with Sr/Ca analysis
Strontium/Calcium (Sr/Ca) ratios from each coral nubbins (samples of 0.5-1 mg) were measured to
provide high resolution (2-4 weeks) sampling of ambient seawater temperatures over a roughly one year
period that began well before commencement of the experiment (Figure. S2-3, Dataset S2-1). Like many
other coral, seasonal changes in the Sr/Ca ratios of P. cylindrical nubbins were inversely proportional to
average monthly reef flat temperatures (141) (Figure S2-4). Low Sr/Ca ratios at the apex below the tissue
zone of the nubbin (0-1mm) coincided with higher spring temperatures compared to high Sr/Ca ratios
measured between 5-7 mm from the apex that coincided with lower winter temperatures. Porites
cylindrica is reportedly a relatively slow growing species which is consistent with our trace element data
that indicates extension rates of ~1 to 1.5 mm/month. Separate Sr/Ca analyses on 5 mg samples
conducted during preliminary work also revealed seasonal timing in reef flat temperature minima and
maxima that were consistent with the higher resolution Sr/Ca sampling (Figure S2-4). This confirmed the
seasonal timing of our boron isotope samples (see following) and our ability to follow the primary growth
axis back in time.
2.2.4 Extension and calcification rates
Linear extension rates were estimated from the physical distance between two points along the primary
growth axis identified as occurring in mid-winter and early summer according to the Sr/Ca-SST proxies
(see above). Density was calculated using the buoyant weight technique (142), where corals were
weighed dry in air before being vacuum sealed in plastic to minimize the effect of air and then submerged
and weighed in distilled water. Calcification rates along the primary growth axis were estimated for
individual nubbins from multiplying linear extension rates by skeletal densities.
2.2.5 Boron isotope-pH proxy
Carbonate samples for boron isotope analysis were extracted along the primary growth axis (Figure S2-
3) using a small hand held dental drill (Saeshin Strong 206/103L), fitted with diamond burs, at the UWA
AGFIOR facility. Sample sizes of 2.5 mg were drilled at the last growth period, representing November
(just under the tissue zone), September, August and the beginning of the experiment (June) as
37
determined from the higher resolution Sr/Ca data (0.5-1mg samples, see previous), from which the time-
series of seasonal temperatures were derived (Figure S2-4). Samples for 11B determinations were
dissolved in 0.58N HNO3 and the boron quantitatively separated on ion-exchange columns according to
McCulloch et al. 2014 (143). Boron Isotope analyses was measured using the Thermo Neptune Plus Multi
Collector-ICPMS and NU Plasma II MC-ICP-MS (AGFIOR, University of Western Australia). Isotope data
are reported in the permil notation relative to the NIST SRM 951 boron isotopic standard where:
11 11 10 11 10 / / / 1 1 000 ‰sample stdB B B B B
------------- Eq. 1
The pH of the calcifying fluid were derived from measured skeletal 11Bcarb values according to the
following equation (39):
11 11
3 411 11
3 4
1000 1
sw carb
B B B
B B carb sw
B BpHcf pK log
B B
------------ Eq. 2
Where pKB is the dissociation constant dependent on temperature and salinity, 11Bsw = 39.61 Foster,
Pogge von Strandmann and Rae (108) and αB3-B4 is the boron fractionation factor for the pH-dependent
equilibrium of B(OH)4- and B(OH)3 in seawater equal to 1.0272 (109).
Off-axis sampling (Figure S2-5a) yielded significant differences in 11B compositions of up to 2.65‰
(Figure S2-5b), or up to 0.17 pH units. These significant fine scale spatial controls on 11B compositions
(132) are also indicative of strong physiological controls on pHcf at the corallite level. To ensure the
repeatability of our results and the absence of such sampling artefacts, the initial 5 mg coral nubbin
samples (Figure S6) were replicated by resampling at a higher spatial resolution using 2.5 mg samples
(Figure 2-2). 11B derived from both the 2.5 and 5 mg samples are in close agreement (Figures 2-2 and
S2-6).
2.2.6 Modelling Ωcf and coral calcification
Following the IpHRAC model of McCulloch et al. (2012), the pHcf of the calcifying fluid was derived from
the 11B of the nubbin material (Eq. 2) with the Dissolved Inorganic Carbon of the calcifying fluid (DICcf)
assumed to be twice that of the treatment and control seawater DIC as calculated from the treatment and
control pH as well as the ambient seawater TA (8). Ωcf for each individual colony were then calculated
from pHcf and DICcf as well as ambient temperature and salinity (8) using CO2SYS (144) and averaged
38
over the entire growth period considered (July-December). Hypothetical rates of abiotic calcification (G)
were calculated over a domain of Ωcf encompassing the average Ωcf of all colonies (Ωcf = 10-18, Figure.
2-4b) and range of temperatures encompassing the experimental period (T = 20-27°C) according to G =
k(Ωcf -1)n where k is the rate law constant, Ωcf is the saturation state within the calcifying fluid and n is the
order of the reaction (k and n being temperature dependent) (128). Since the absolute amount of
biomineral surface area during calcification is not known at present (145), relative calcification rates were
calculated by dividing all modelled calcification rates by the calcification rate achieved when the internal
saturation state Ωcf was equal to 18.
2.2.7 Statistics
We used a linear mixed effects model to examine the general dependency of skeletal δ11B and internal
pHcf in P. cylindrica on changes in ambient seawater pH (see Figure 2-2b) where each colony was treated
as an individual group. All statistical results were generated using the function nlmefit.m in Matlab v8.3.0
(Table S2-3).
2.3 Results and Discussion
The 11B compositions (Dataset S2-2) were measured from a series of sub-samples collected
along the major growth axis of the skeletons of the P. cylindrica nubbins from each of the four
FOCE flumes (Figure S2-3). High-resolution profiles of strontium to calcium ratios (Sr/Ca, see
methods) were used to determine seasonally resolved chronologies and extension rates.
Calcification rates along the primary growth axis were calculated from the product of extension
rates and density, with the latter determined using the buoyant weight method. Based on the
Sr/Ca chronology, the11B compositions were determined for four distinct periods of growth:
June, August, late-September, and late-November 2010 (Figure 2-1c). The August and
especially the late-September periods were representative of particularly low pH regimes within
the treatment flumes (Figure 2-1). Samples analysed for 11B represent ~3 weeks of growth
and hence incorporate the shorter-term variations in calcification driven by, for example, diel
cycles in the supply of metabolic carbon from photosynthesis and respiration (132, 146). During
the initial phase of the experiment in mid-winter, when the reef flat waters were characterised
39
by relatively high pH (~8.3 units, Figure 2-2), the coral nubbins exhibited a wide range of 11B
values (~22‰ to ~26‰). During the early spring-phase (i.e. late-September) of the experiment,
coral nubbins again exhibited the same range in 11B (~22‰ to ~26‰) despite the significant
reduction in pH within the FOCE treatments and in the ambient reef flat pH. Importantly,
individual nubbins exhibited near constant 11B compositions along their major growth axis over
each of the four growth periods measured, regardless of whether they were grown under
treatment or control conditions (Figure 2-2a and S2-7a). These near constant 11B
compositions equate to near constant internal pHcf (Figure 2-2b and S2-7b) irrespective of
treatment and season and declined by less than 0.1 units per unit decrease in external pHsw (
cf swpH pH = 0.067, p = 0.078, df = 36, Table S2-3, Figure 2-2b). This reflects the capacity
of these coral to homeostatically maintain a pHcf of ~8.4 to 8.6 at the site of calcification (Figure
3) and, thus, near constant up-regulation of pHcf during the calcification process. As such, these
findings are in marked contrast to earlier laboratory studies in which corals grown under stable
and constant pH conditions exhibited a stronger sensitivity to ambient seawater pH whereby
pHcf decreased by up to 0.5 units for each unit decrease in ambient seawater pH. However,
under the naturally and highly dynamic pH conditions within the Heron Island reef flat, corals
seemingly exert a much stronger physiological control of pHcf, which overrides the seasonal
ambient depression in seawater pH as well as the superimposed FOCE induced decrease in
seawater pH. Re-interpretation (91) of previous laboratory work using P.cylindrica colonies
under depressed pCO2 conditions (147) indicates that pH-up regulation was taking place at the
site of calcification in this species; these previous experiments, however kept CO2 constant
throughout the experiment and therefore did not capture the dynamic nature of many natural
reef environments. We note that the finding from our study of strong pH homeostasis as
exhibited along the major growth axis of the coral skeletons occurs despite the large range in
11B for inter-colonial (Figure 2-2) and off-axis intra-colony specimens (Figure S2-5). The
observation of higher 11B values for off-axis compared to the primary growth-axis of the
nubbins (Figure S2-5) is unexpected since growth is higher along the primary growth axis, so
we can only speculate that factor(s) other than direct pHcf controls on CO32−, such as the
internal transport of carbon and energy to sites of calcification (148), are driving faster axial
growth rates.
40
Figure 2-2. Boron isotope composition and derived pH of the calcifying fluid (a) Measured
symbols) flumes versus the average measured pH seawater conditions at June, August,
September and November 2010 within control and treatment flumes during the FOCE
experiment. The boron isotope composition of all samples are elevated relative to the abiotic
curve provided by Klochko et al. (109). (b) pH of the calcifying fluid (pHcf) derived from the
modest dependency of pHcf on pHsw ( cf swpH pH = 0.067, p = 0.078, df = 36, Table S2-3,
Figure 2-2b).
41
Figure 2-3. Boxplot of pH of the calcifying fluid of nubbins. Boxplots showing range (coloured
vertical lines) and the mean (diamond) and (±1) standard error from the mean (black bars) of
pHcf of P. cylindrica nubbins taken at four intervals of growth (June, August, September and
November) grown under treatment (red) and control (blue) conditions. Also shown are the
mean (circle) and standard error (coloured horizontal bars) of the pH of the seawater within
both treatment (red) and control (blue) FOCE flumes.
Extension rates of P. cylindrica nubbins used in both the treatment and control flumes were
similar to P. cylindrica from elsewhere in the Great Barrier Reef (149), however, this is not
entirely surprising given that coral can maintain similar rates of extension under low-pH
conditions at the cost of reduced skeletal density and hence, rates of calcification (150).
Nonetheless, we find that the skeletal densities in the low pH FOCE corals differed by less than
3% from the densities of control corals (Table S2-2). Furthermore, the calcification rates that
we measured were comparable to or higher than rates reported in prior studies (151, 152),
which further demonstrates the capacity of P. cylindrica from Heron Island reef flat to calcify
and grow at normal rates despite the highly variable and disparate thermal and chemical
conditions in the controls and in the FOCE treatments.
To better understand how extension and calcification rates varied within and between colonies,
we used a ‘bio-inorganic model’ of calcification based on the model of Internal pH Regulation
Abiotic Calcification (IpHRAC) (128). In this model, the rates of calcification depend primarily
on the saturation state of the calcifying fluid (Ωcf) according to known abiotic rate kinetics (137),
42
which are in turn dependent on elevated levels of pH and DICcf in the calcifying fluid relative to
ambient seawater conditions (see methods). We found no significant relationship between
rates of extension and estimated saturation state inside the calcifying fluid (Figure 2-4a); thus
providing further evidence that physical rates of coral extension are not strictly dependent on
the chemical conditions driving rates of biomineralization within the calcifying fluid.
Nonetheless, rates of coral calcification, were positively correlated with Ωcf (r2 = 0.41, p = 0.05,
n = 10, Figure. 2-4b), albeit modestly, due to both uncertainties in derived calcification rates
(~20%) and the very narrow dynamic range of internal saturation states over which this
dependency could be determined (Ωcf = 13.5 to 16.4, or a relative variation of just 19%). The
limited range in Ωcf among experimental colonies was, of course, the direct result of the
maintenance of near-constant internal pHcf across treatments and seasons (Figure 2-3). Thus,
while coral calcification is a ‘biologically mediated’ process influenced by both external (e.g.
light, temperature, food abundance (153)) and internal (e.g. production of organic matrices as
templates (154) for skeletal formation) processes, our simple bio-inorganic model nonetheless
explains the first order functional dependence of calcification rates on internal Ωcf (Figure 2-4a
and 2-4b).
43
Figure 2-4. Extension rates and calcification rates of the coral nubbins (a) Extension rates of
P. cylindrica (y-axis) from both ambient (circles) and elevated pCO2 (triangles) treatments
versus aragonite saturation state of the calcifying fluid (Ωcf) calculated from boron isotope
measurements according to the IpHRAC model of McCulloch et al. (11, see Methods, r2 = 0.21,
p = 0.19, n = 10). (b) Calcification rates (left y-axis) of ambient (circle) and treatment (triangle)
P. cylindrica corals versus Ωcf (r2 = 0.41, p = 0.05, n = 10). Also shown are predicted rates of
aragonite precipitation (right y-axis) as a function of Ωcf relative to Ωcf = 18 at both 20°C and
27°C according to the reported abiotic rate kinetics of aragonite precipitation (137).
Our study now provides a process-based mechanism (up-regulation and/or homeostasis of
pHcf) that can account for the increased tolerance of corals growing under ranges of ambient
seawater pH in highly dynamic reef systems. How sensitive the growth of marine calcifiers is
to ocean acidification thus depends on the particular strategies (or lack thereof) for controlling
pH at the site of calcification. Based on our findings, we propose three types of strategies; (1)
the ‘passive’ strategy whereby pH up-regulation does not occur and rates of calcification follow
an abiotic mineral precipitation curve that is highly dependent on ambient pH and pCO2; growth
in these organisms will be highly sensitive (128) to ocean acidification (e.g., some species of
44
foraminifera). (2) the ‘partial regulation’ strategy, whereby some up-regulation occurs but the
calcifying fluid pHcf is still partially modulated by the external environment (91, 128) and rates
of calcification are only partially dependent on ambient pH and pCO2; growth in these
organisms will be only moderately sensitive to ocean acidification; and (3) the highly resilient
or ‘homeostatic’ strategy, as identified in this study, where a near constant internal
(extracellular) pHcf of the calcifying fluid is maintained at a fixed level to optimize calcification
rates independent of external conditions; growth in these organisms will be the least sensitive
to ocean acidification. This homeostasis strategy was also observed in the massive corals
Porites lutea and P. lobata during microcosm experiments (155) where the synergetic effects
of increased temperature and low pH showed no net effect on calcification rate. The most likely
or realistic scenario is that most reef-building corals fall somewhere within the second and third
categories; that is being weaker or stronger partial regulators of internal pHcf depending on
their taxonomy, habitat, and/or symbiont composition (91, 128, 135, 145). Indeed, Porites
appears to be a genus whose adult colonies are highly resilient to ocean acidification, as also
shown from their occurrence around CO2 seeps (25), yet their skeletal densities and
calcification rates can nevertheless decline when exposed to particularly caustic seawater
chemistries (Ωsw < 2, (156). The variation between and within coral species to maintain robust
rates of calcification in low pH (or low Ωarg) conditions may also be a function of their ability to
access additional sources of metabolic energy via particle feeding (157). Nonetheless, naturally
occurring ‘acidified’ reefs (Ωsw < 3) can still support reasonable levels of coral diversity and
growth (158) despite any taxa-dependent differences in pH sensitivity.
The capacity of corals to adapt to a changing environment is also expected to vary among
species depending on their ability to isolate their growth and metabolism from changes in
ambient conditions (128, 159, 160). The results presented herein shows that P. cylindrica
corals from the Heron Island reef flat maintained elevated extracellular pHcf at the levels
necessary to support rates of skeletal growth similar to those recorded in other Porites species.
Whilst this capacity for pHcf homeostasis appears to be strongest for P. cylindrica colonies
growing in more extreme and highly dynamic chemical environments, it is still uncertain how
well these coral can maintain this level of pHcf homeostasis with the additional stress of
45
increased temperature (123). To better understand how future reefs may change under high-
pCO2 environments, it is further necessary to assess this potential biological resistance in a
range of coral species from environments with different chemical and thermal regimes (123),
as well as assess the metabolic cost associated with persistently high pH up-regulation when
ambient pH is very low. Regardless, the ability of pH-homeostatic coral to survive and grow in
these extreme pH environments may provide them with a greater resilience to the increased
levels of ocean acidification expected to occur over the coming decades and centuries.
Acknowledgements
The authors would like acknowledge the work of the scientists and engineers that carried out the FOCE
experiment at Heron Island in 2010 and thank Dr Kai Rankenburg from the UWA AGFIOR facility for
technical assistance with the trace element and isotope analyses. We appreciate assistance from Jan
Richards from the Department of Fisheries (WA) for the use of their micromill. The CP-FOCE experiments
were funded by the Australian Research Council (ARC) Linkage Infrastructure Equipment and Facilities
grant #LE0989608 (OHG,DK, SD, MM), ARC Linkage grant #LP0775303 (OHG, ARC Centre of
Excellence grant #CE0561435 (OHG, SD), a Queensland Government Smart State Premier's Fellowship
to OHG, and a Pacific Blue Foundation grant (DK).This project was funded by the Australian Research
Council (ARC) Centre of Excellence for Coral Reef Studies (UWA and UQ). Funding support was also
provided to LG from an Australian Government IPRS/APA scholarship. MM was supported by a Western
Australian Premier’s Fellowship and an ARC Laureate Fellowship. MH acknowledges support from an
ARC Super Science Award. Permits from the Department of Environment and Resource Management
(#CSCE00874010) and the Great Barrier Reef Marine Park Authority (#G09/29996.1) were provided to
conduct this research.
46
Chapter 3 3.0.1 Characterisation of the Red Sea with focus on the Saudi
Arabian Red Sea
The length of the Red Sea (2000 km) lies over a rift valley with a maximum width of
355 km. Shallow areas below 50 m comprise around 40% of the Red Sea area while
the central axis has recorded to depths of over 3000 m. Saudi Arabia is the largest of
nine countries that flank the east and west coasts of the Red Sea (161). The Red Sea
has a long paleaogeographic history this has helped to shape the regions fauna and
flora, which contain a high percentage of endemic species (162). A narrow, shallow
opening links the Red Sea to the Suez Canal, which only opened in 1867, allows only
limited water exchange into the northern end of the Red Sea. The strait of Bab-el-
Mandeb at the southern opening of the Red Sea is only around 20 km across and
therefore water exchange is still limited (161). The hot arid climate of the countries
either side of the sea provide little to no freshwater input through either rainfall or
riverine discharge. Until recently, the main focus of Red Sea marine research was
centred on the northern reefs where access was proliferated by heavy tourist traffic.
Saudi Arabia is largest of the nine countries that surround the Red Sea and occupies
much of the coastline of the central eastern side of the Red Sea where the sea is
almost tideless (161). However annual water fluctuations are significant due to high
evaporation (~2 m year-1), this controls much of the natural salinity variation, which
ranges from 38.5% in the winter and 39.8% in summer.
47
Sea surface temperatures in the central Red Sea range from 25 oC in February to about
31 °C in September but there is strong diurnal variability in water temperatures
around reef platforms. Wave exposed areas of reefs can typically range between 0.5–
1.5 oC while the wave protected side of the same reef will experience changes of 2-5
oC over the same period (22). Spatial temperature variability on reefs has been
attributed to atmospheric conditions such as evaporative winds and currents that
move across and along the shelf of the reef. (22). Sea level rises during the winter
season (October – March) due to the inflow of water from the Bab-el-Mandab strait,
which exceeds the outflow of water and evaporation. During the summer (April –
September) sea levels decrease over the length of the Red Sea. Stratification of the
water column is governed by heat fluctuations, currents caused by wind and tidal
movement (163, 164).
The high temperatures experienced by Red Sea corals makes them ideal subjects in
assessing acclimatisation and tolerance to future high SSTs. Some of the more
pressing questions to answer include; are these corals at their upper thermal
tolerance? Therefore, are they more susceptible to increasing temperatures or does
their familiarity with high temperatures make them more thermally tolerant? A major
mass bleaching event occurred in the summer of 1998 in response to above average
temperature due to the 1998 El Niño, which was the worst El Niño ever recorded
(165). The following sections in this chapter look at how this event affected the
calcification and trace element/isotope incorporation in two massive corals (Porites
lutea) obtained in 2013 from reefs off the coast of Jeddah. These cores were collected
48
as a collaborative effort by teams of scientists from King Abdullah University of
Science and Technology (KAUST) and UWA.
Section 3.2 of this chapter contains a manuscript, which has been submitted
to the journal PlosOne and is currently under review. The supplementary material for
this manuscript has been placed in the appendices. Below is a synopsis and
characterisation of the unique Red Sea environment, with particular focus of the
central Saudi Arabian region.
49
3.0 Long-term impacts of the 1997-1998 bleaching event on the
growth and skeletal geochemistry of Porites lutea from the
central Red Sea
1, 2*Lucy Georgiou, 1, 2Juan Pablo D’Olivo Cordero, 1, 2Jim Falter, 1Kai Rankenburg,
3 #b #cXabier Irigoien, 3Christian R. Voolstra, 3, #c Cornelia Roder 1, 2Malcolm McCulloch
1School of Earth and Environment, The University of Western Australia, Crawley,
Western Australia
2ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia,
Crawley, Western Australia
3King Abdullah University of Science and Technology (KAUST), Red Sea Research
Center (RSRC), Biological and Environmental Sciences & Engineering Division (BESE),
Thuwal, Saudi Arabia
#aAlfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Shelf Sea
System Ecology, Helgoland, Germany
#bAZTI - Marine Research, Herrera Kaia, Portualdea Pasaia (Gipuzkoa), Spain
#cIKERBASQUE, Basque Foundation for Science, Bilbao, Spain
Corresponding author: Lucy Georgiou ([email protected])
# Current addresses
50
Abstract
This study investigates the long-term impact of high sea surface temperatures related
to one of the most intense El Niño Southern Oscillation (ENSO) cycles in recent history
(1997-2001) on the trace element geochemistry (Sr/Ca, Li/Mg, Mg/Ca and U/Ca) of
two massive Porites lutea coral cores from the central Saudi Arabian Red Sea. Prior to
1998, the Sr/Ca and Li/Mg records showed strong correlations with sea surface
temperature (SST). However, during the prolonged high temperatures phase
associated with the 1998 El Niño event the trace element ratio (TER) seasonal signals
were disrupted, which coincided with a reduction in both extension and calcification
rates. This disruption in highly correlated seasonal TER-SST relationships lasted for
approximately two years in the inner-shelf site and nearly four years in the outer-shelf
site. Although the seasonal signal quality of the TER-SST correlations did eventually
recover, their calibrations (e.g., regression coefficients) were significantly different
compared to pre-1998 levels (p < 0.0001); the only exception being the relatively
robust Li/Mg paleo-thermometer. However, extension and calcification rates
following the passing of the ENSO cycle were comparable to pre-1998 levels in both
cores. This suggests that prolonged thermal stress induced subtle but long-lasting
physiological changes that affected the elemental composition of the coral’s calcifying
fluid. Prolonged disruption of seasonal TER-SST relationships similar to the ones we
observed in the central Red Sea may provide a useful ‘fingerprint’ with which to
identify past thermal stress events.
51
3.1 Introduction
Discrete episodes of anomalously elevated temperature can severely affect the health
of the coral community even in areas already exposed to naturally high temperatures
(166). In the Red Sea, coral communities were severely affected by the 1997-98 El
Niño (167), which until the recent 2015/16 El Niño, was the most intense global coral
thermal stress event yet recorded. The 1997-98 El Niño event began in the winter of
1997 and continued until the summer of 1998 causing prolonged periods of elevated
sea surface temperature and mass coral bleaching throughout many areas of the
world. As a result approximately 16% of reefs worldwide were affected by this single
event (168) with areas of the Red Sea experiencing up to 14 degree heating weeks
(169). Although in many places El Niño and La Niña result in opposing temperature
trends (i.e. colder than average temperatures in response to La Niña and warmer than
average temperatures in response to El Niño), there are regions where both La Niña
and El Niño can result in thermal anomalies of the same sign (positive or negative). In
the summer of 2010, during the second strongest La Niña on record, corals of the
Thuwal coastal region in the central eastern Red Sea experienced 9-11 degree heating
weeks (DHW) of stress (23). This event resulted in mass coral bleaching followed by a
drastic shift in coral community composition (23). Consequently, it is imperative that
we develop a better understanding of how coral reefs will respond to periods of
severe thermal stress as these events become more frequent and intense.
The skeletal geochemistry of long-lived massive corals such as Porites spp. has
provided valuable information on both climate variability (65, 170-172) as well as the
52
response of coral calcification to these changes (173). The translation of coral skeletal
geochemistry into robust records of past climate has; however, been limited by an
incomplete understanding of the mechanisms governing biogenic calcification and its
influence on the incorporation of trace elements into the crystal lattice of the
precipitated minerals (83, 96, 174). Thus, although the incorporation of trace
elements like strontium (Sr) into the coral’s aragonite skeleton are strongly correlated
with sea surface temperature (SST), their partitioning can also be affected by a range
of physiological processes or ‘vital effects’ (76, 77, 175, 176); particularly during
periods of climate extremes and the associated physiological stress they cause.
Although Sr/Ca, the most widely used coral paleothermometer, can be measured in
the skeletons of massive corals with an analytical precision of better than 0.1% (64,
65, 67, 68, 170), its specific relationship with temperature for any given coral is still
subject to physiological controls which can become altered during periods of
environmental stress. For example, Sr/Ca positive anomaly values have been
observed during periods of anomalously high temperature (such as occurs during
coral bleaching events (173, 177, 178)). It is thought that thermal stress disrupts the
enzymes responsible for active Ca2+ transport (Ca2+ATPase), thereby causing an
increase in the abundance of trace elements relative to calcium within the carbonate
skeleton (77). Because Ca2+ATPase is also responsible for the removal of protons
during calcification (51), thermal stress will likely also influence the pH and aragonite
saturation state (Ωarg) in the corals’ calcifying fluid (28, 45, 90, 91).
It has been suggested that Mg2+ and Li+ are transported to the site of calcification and
incorporated into aragonite by similar mechanisms (82, 179); however, the nature of
53
these transport mechanisms are still uncertain (180). Inorganic experiments suggest
that Li+ is incorporated into aragonite through heterovalent substitution for Ca2+ (as
opposed to homovalent substitution by Mg2+) (181, 182). However, other evidence
suggests that both Li and Mg are incorporated through defects of the aragonite crystal
(179) and/or other entrapment processes (98, 99, 183). Regardless of the specific
mechanisms, differences in biogenic crystal growth mechanics appear to have little
effect on the Li/Mg ratio in the coral skeleton; thus making this a robust proxy for
ambient seawater temperature that is essentially independent of vital effects (179,
184). Despite its success as a paleo-temperature proxy for cold and temperate coral
(179, 180, 184), the reliability of the Li/Mg temperature proxy has not yet been fully
evaluated in corals living at higher (tropical) temperatures, nor those exposed to
prolonged periods of thermal stress, which given present climate trends, is of
increasing importance.
Here we use a suite of trace element ratios (Sr/Ca, Mg/Ca, U/Ca and Li/Mg) to
document the response of two massive Porites lutea coral colonies from the inner and
outer-shelf of the central eastern Red Sea to a strong ENSO cycle that occurred
between 1998 and 2001. We first examine the impact of these events on the
incorporation of trace elements into the aragonite lattice and then examine how
thermal stress affects the relationships of these trace element ratios with SST. This is
achieved by measuring each trace element at near monthly intervals for an ~18 year
period starting from the year 1995 in the inner-shelf and for a ~21 year period in the
outer-shelf starting in 1992. The relationships between trace element ratios and
regional SST are evaluated over three time periods: For 3 to 6 year periods before the
54
1998 El Niño (pre-ENSO), during the ‘1998’ bleaching event (ENSO+), and after the
ENSO cycle (post-ENSO+) to better understand the long-term influence of these
elevated regional SSTs on the trace element proxies. We then discuss the implications
of these findings on the reliability of this comprehensive suite of trace element ratios
as proxies for past SST. We also discuss how our results can help identify the
mechanisms of calcification and whether the trace element signatures can provide a
distinctive fingerprint of thermal stress events.
.
3.2 Materials and Methods
3.2.1 Site description and climatology
The Red Sea is a semi-enclosed water body roughly 2000 km long and 200-300 km
wide, where depths reach below 2500 m in the central axis. It is highly saline (>39)
due to low levels of rainfall in the surrounding region and limited exchange of water
with other rivers or seas (161). The Red Sea can be divided into three ecologically
distinct regions: 1) the northern region which is connected to the Mediterranean Sea
via the Suez Canal in the northwest and to the Gulf of Aqaba in the northeast, 2) the
central region (20 – 24 N) which is extremely arid with minimal rainfall and riverine
input, causing a nutrient poor environment, and 3) the southern region which is most
heavily influenced by the Indian Ocean via exchange through the narrow Bab-al-
Mandeb Strait (161). SST in the southern part of the Red Sea are influenced by the
incursion of Indian Ocean waters governed by the Asian Monsoon system (and ENSO)
(163, 164), while the northern part is influenced by the North Atlantic Oscillation
55
(NAO) and Arctic Oscillation (AO) (171, 185). Our work was conducted in the central
region of the Red Sea, close to the coast of Saudi Arabia (around 22 N) (Fig 3-1), which
is mainly influenced by ENSO. The coastal area is comprised of a narrow shelf ranging
from 30-90 m in depth, which supports a complex set of reef systems and island
features.
Daily local satellite sea surface temperature data (1980-2013) derived from the
Pathfinder Advanced Very High Resolution Radiometer (AVHRR) (186) were obtained
online from the NASA THREDDS (v5.2) data server
(http://thredds.jpl.nasa.gov/thredds/catalog_oceantemperature.html) at a spatial
resolution of 25 km. In situ SST data at inner-shelf and outer-shelf reef sites were
obtained by the King Abdullah University of Science and Technology (KAUST) at daily
resolution from September 2012 to September 2013 (187, 188). Sea surface
temperatures in the central Red Sea range from around 25 °C in February to about 31
°C in September, with strong diurnal variability around reef platforms (186). Wave
exposed areas of reefs can typically range between 0.5–1.5 C while the wave
protected side of the same reef will experience changes of 2-5 C over the same
diurnal period (22). Spatial temperature variability on reefs in the central Red Sea has
been attributed to a combination of regional climate forcing, such as wind-driven
cooling, wave-driven circulation and large-scale buoyancy driven circulation (22, 189).
56
Figure 3-1. Regional map of the Red Sea and Saudi Arabian Coastline. Red Box represents
locality of sampling area (insert), stars indicate the locations of the reef sites where the Porites
lutea cores were retrieved; Abu Shosha (inner-shelf) and Shi’b Nazar (outer-shelf).
Although the worldwide bleaching event was associated with the 1997/98 El Niño, the
three regions of the Red Sea were not affected equally. No bleaching was observed in
the northern region of the Red Sea due to the cooler conditions and notable upwelling
in the region (167). However, severe bleaching was documented in the southern
region of the Red Sea; particularly around the central Saudi Arabian coast (161, 167).
In the central Saudi Arabian coast ~14 DHW were experienced during the summer of
1998, with seawater temperatures that were ~1.3 °C higher than normal from the
summer of 1997 to the summer of 1998 (AVHRR v5.2) (Fig 3-2) (190). Given that
widespread bleaching and mortality is likely to occur at around eight DHW (191), this
created a thermally stressful environment for coral reefs, which periodically
experienced temperatures that were ~2 °C above the maximum monthly mean
57
(MMM) (Fig 3-2). Following the 1998 El Niño event summer SST and summer SST
anomalies remained high for the next four years (Fig 3-2) (190). In addition to the
episodic El Niño related events, the Red Sea’s average SST are increasing at a faster
rate than typical tropical regions (14) and indeed other regions (192), with models
forecasting a 2.5 - 3 °C temperature rise in the central Red Sea by the end of the 21st
century (193).
Figure 3-2. Climatological data for the central Red Sea region and El Nino index for the region
3.4. (A) Annual summer (July-September; JAS) mean sea surface temperatures from 1985-
2014 (blue solid line) and JAS SST anomalies from 1985-2014 (red solid line). Data extracted
from Pathfinder Advanced Very High Resolution Radiometer (AVHRR v5.2)
(http://thredds.jpl.nasa.gov/thredds/catalog_oceantemperature.html). Maximum monthly
mean SST (MMM-SST) calculated from average August SST per year for the time periods after
the onset of El Niño (1997-2013; black dashed line) and before the onset of El Niño (1985-
58
1996; blue dashed line) (194) (B) The Niño 3.4 index SST anomaly data (ERSSTv4 -
http://www.cpc.ncep.noaa.gov/data/indices/) showing positive El Niño (red) and negative La
Niña conditions (blue). Light grey bar highlights worldwide 1997/98 El Niño event and dark
grey bar highlights prolonged La Niña conditions 1999-2001 (strong ENSO cycles).
3.2.2 Coral core sampling
Coral cores were collected from two sites: Abu Shosha on the inner-shelf (~5 km from
shore, 6.6 m depth, 39 02 86.7 E, 22 18 13.1” N) and Shi’b Nazar on the outer-
shelf (~25 km from shore, 5 m depth: 38 56 41.14 E, 22° 19 1.94 N) (Fig 3-1). Core
samples (5 cm in diameter and up to 50 cm in length) were taken from living massive
colonies of Porites lutea during April 2013. Cores were first cleaned with fresh water,
and then dried and packed for transport to the University of Western Australia (UWA)
for processing and analysis. The cores were cut into slices 6-7 mm thick along the
length of the core and the resulting slices were bleached overnight in a 1:1 NaClO
solution before being washed thoroughly in deionized water, ultrasonicated 3 times
and dried at ~50 °C in a laboratory oven. A set of slices from the middle of each coral
core were taken for X-radiography and the resulting images processed using Clarity
Pacs (jCRco) software. Profiles of optical density along each core were digitized and
extracted using the software CoralXDS (Ver 4.6, NOVA Southeastern University). High-
resolution powder samples were taken at 1.25 mm intervals for both cores with a
computer controlled Zenbot drill. This sampling strategy resolved 10-11 samples per
year for the outer-shelf core and 12-13 samples per year for the inner-shelf core;
during years unaffected by temperature anomalies.
59
3.2.3 Geochemical analysis
Powdered core samples were processed in the (Advanced Geochemical Facility for
Indian Ocean Research) AGFIOR ultra-clean laboratory based at UWA. All sample and
standard solutions were prepared using sub-boiling distilled HNO3 (Savillex DST-1000)
and 18.2 MΩ water (Millipore). Coral powder samples were weighed (10 ± 0.2 mg)
and dissolved in 0.562M HNO3 before being centrifuged. An aliquot of 31 μl was
pipetted into 2.97 ml 2% HNO3 (≡ 100 ppm Ca for Li, Mg and B analysis), and further
diluted to 10 ppm Ca using 2% HNO3 spiked solution containing ~19 ppb Sc, 19 ppb Y,
0.19 ppb Pr, and 0.095 ppb Bi (to correct for variable instrumental mass bias) (195).
One aliquot of the JCp-1 coral standard was processed along with every 30 samples in
order to assess the quality and reproducibility of our full chemical treatment.
3.2.4 Analytical procedure
Trace element measurements were carried out on a Thermo Fisher Scientific (Bremen,
Germany) X Series II quadrupole inductively coupled plasma mass spectrometer (ICP-
MS) using the standard Xt interface with the plasma screen fitted. The sample
introduction system consisted of an autosampler (Elemental Scientific SC4 DX)
attached to a self-aspirating Teflon microflow nebulizer (Elemental Scientific PFA-ST)
using a 100 μL/min capillary fitted to a quartz spray chamber cooled to 2 °C (Elemental
Scientific PC3). All analyses were then carried out in peak-hopping pulse counting
mode; washout and uptake times were 20 and 100 seconds per sample, respectively.
Typical sensitivity of the instrument was ~300-500 kcps for 43Ca aspirating a solution
containing 10 ppm Ca. For the determination of Li/Mg and 7Li, 25Mg and 46Ca were
acquired in 100 sweeps and 6 repetitions for a total acquisition time of 112 seconds
60
per sample ([Ca] = 100 ppm). Elements 25Mg, 43Ca, 86Sr, and 238U were acquired for
the determination of metal to Ca ratios in 100 sweeps and 8 repetitions for a total
acquisition time of 224 seconds per sample ([Ca] = 10ppm). Long-term reproducibility
(representative of ~1 year of data collection) of Me/Ca derived from repeated
analyses of individual aliquots of the JCp-1 coral powder are as follows: Mg/Ca = 1.2%,
Sr/Ca = 0.3%, U/Ca = 1.2% and Li/Mg = 1.8% (2s RSD; n=60). These uncertainties are
assumed to be representative for individual analysis of sample powders. This has been
confirmed by occasional repeat analyses of sample powders.
3.2.5 Coral chronology and skeletal growth parameters
Initial chronology was ascertained from X-radiograph images using a visual inspection
of the annual high and low density band patterns (Fig 3-3). Establishing the chronology
from X-ray images was particularly challenging in these cores due to double-banding
and density band changes associated with the cessation or slow growth occurring in
late 1998 to early 1999 (Fig 3-3). Fluorescent bands formed by river runoff events,
which provide chronological markers in other systems (17, 196) are absent from the
corals in this region. Intensity measurements of high and low density bands were
undertaken using CoralXDS software taken along the length of the sampled core to
resolve some of the uncertainties in the chronologies (Fig 3-4). Slices were cut along
the axis of maximum growth to expose the most appropriate surface for geochemical
sampling as identified from the X-ray images (Fig 3-3). Breaks and fissures along the
core were avoided to provide the most homogenous sampling areas.
61
The final age model was assigned using AnalySeries (197) by fixing the timing of
maxima in Sr/Ca ratios to the known timing of minima SST records and interpolating
the resulting chronologically fixed trace element time series at near monthly
resolution. Additional tie points were used to maximize the correlation with the SST
time series, based on the in-built correlation function included in Analyseries (172).
The resulting chronology was crosschecked against maxima and minima Li/Mg ratios,
a robust SST proxy for a number of coral species (179, 198).
Annual linear extension rates were initially measured from profiles of skeletal optical
density taken along the primary growth axis and later re-calculated by measuring the
distance between summers in the Sr/Ca data as defined in the final chronology. Paired
high and low density were measured and taken to be one 12 month period of linear
growth (58). Calcification rates were calculated using the resolved linear extension
measurements and density calculations.
Rectangular sections ~50 × 20 × 7 mm encompassing 3-4 years of growth were cut
from the core along the primary growth axis using a precision circular saw and their
bulk skeletal densities measured using the buoyant weight method (17, 199). All
density samples were first weighed in air (Wair), then vacuum-sealed in plastic (with
any residual plastic removed), and finally re-weighed in distilled water (Wwater) using
a precision balance (±0.001 g). Coral density was calculated using the following
equation after correcting for the weight and density of the plastic wrapping:
air watercoral
air water
W
W W
62
where ρwater is the density of water at the temperature the measurements were
made.
U-series dating (200) was applied on coral core samples from both the inner and outer
reef locations in an attempt to independently constrain the start and end dates of the
anomalous trace element patterns observed in these cores during the late 1990’s (see
results and supplementary information Table S3-1). From each core a sample of ~200
mg was dated prior to the commencement of the anomalous period, with a second
reference sample dated from the younger upper portion of the core whose age was
clearly constrained from the density bands and trace element seasonality. The
purpose of the reference samples (Feb-2006 for the inner core and Feb-2003 for the
outer core) was to provide constraints on the initial 230Th/232Th ratio whose value is
critical to obtaining accurate age estimates at sub-annual resolution (200). The
uncertainty associated with these corrections is however still significant (see table S3-
1), being equivalent to ~2 years in this region. For the outer reef site (Shi’b Nazar), the
U-series ages thus indicate a hiatus of 6 ± 2 years consistent within errors to those
estimates based on X-ray TE growth rate constraints. However, the ~2 year anomalous
period for the inner reef site (Abu Shosha) is not resolvable by U-series given the ± 2
year uncertainty associated in estimating the initial 230Th/232Th.
We applied the following reasoning to constrain the duration and chronology for the
anomalous trace element data during and shortly after the strong ENSO cycle. First,
the seasonal signal of the trace elements and density banding provided unequivocal
evidence for the end year of the anomalous data period. Then, we seasonally phase-
locked the SST time series with the measured Sr/Ca and Li/Mg records prior to the
63
ENSO event by aligning seasonal maxima in Sr/Ca and Li/Mg with seasonal minima in
SST, but then allowed the SST time series to lag or lead the trace element ratio records
by discrete annual increments ranging from -3 to +3 years. Having done this we found
that the best correlations occurred when the anomalous period was set to begin in
mid-1998 for both coral cores, or when the strong ENSO cycle was known to have
commenced. While the anomalous Sr/Ca and Li/Mg data for the inner-shelf (Abu
Shosha) core appear to resemble only one seasonal cycle the reduced amplitude
(compared to normal years) of this chronology is inconsistent with the doubling in
sampling resolution expected from condensing the data into one year. Furthermore,
in the case of the inner-shelf core the anomalous trace element data could not
plausibly fit in a single year as this would require a growth rate of 21.5 mm per year
which is ~60% higher than the average growth in unaffected years (~13.7 mm/yr).
Finally, a seasonal signal (albeit weak) is apparent during the anomalous years in
records of Li/Mg and Sr/Ca from the outer reef and in records of Mg/Ca from the inner
core.
64
Figure 3-3. Coral core x-ray images from cores from the coast of Saudi Arabia (A) the inner-
shelf core (Abu Shosha) and (B) outer-shelf core (Shi’b Nazar). Red and blue lines indicate
sampling tracks for trace elements at near monthly resolution. Double bands apparent in
some years; particularly in the outer-shelf core.
65
Figure 3-4. Relative x-ray optical density (intensity) and Li/Mg records for the inner-shelf core
(top, red) and the outer-shelf core (bottom, blue). The intensity was measured on the X-ray
images of the core slices along the same paths used for the trace element to determine
chronology and measure linear extension. Track A (dashed line) and track B solid line
represent a change in the sampled path as outlined in Fig 3-3. Anomalous years of growth and
corresponding anomalous trace element measurements are highlighted for years 1998-2001
in the inner core and 1998-2002 in the outer core.
3.2.6 Statistical analysis
The four most recent (from the top of the core) samples measurements were omitted
from any statistical analysis as these include residue from the coral tissue layer. All
statistical analyses (regressions and ANOVA) were performed using SigmaPlot 13.0
and R-3.2.2.
66
3.3 Results
3.3.1 Coral growth rates
Records of skeletal growth (linear extension, density, and calcification rates) are
presented for the inner and outer core in increments of three to four years (~triennial)
for the period from 1980 to 2012 (Fig 3-5). Annual linear extension rates derived from
trace element chronology and from banding patterns observed in the X-radiographs
are presented in Fig 3-6. Given that seasonality was reduced or not evident in the TE
ratios, and the periodicities of the density bands were irregular (e.g multiple annual
bands) during the affected years, linear extension rates were calculated by dividing
the entire length of the anomalous period by its respective durations: 1998-2001 (~3
years) in the inner-shelf core and 1998-2002 (4 years) in the outer-shelf core. One-
Way Analysis of Variance (Kruskal-Wallis using ranks) indicates that mean rates of
annual linear extension (~5 mm/yr, Fig. 3-6) were significantly lower during the
anomalous TE period than before or after (p< 0.0001, Table S3-2). These low extension
rates are identical to those obtained for Porites corals from the GBR during similar
thermal stress events (17, 201), where the presence of distinctive luminescent
banding in the corals allow for a very precise chronology despite the disruption in
seasonal growth patterns and skeletal trace element chemistry.
67
Figure 3-5. Growth parameters of inner-shelf and outer-shelf cores. Records of (A) linear
extension (B) density and (C) calcification, for inner-shelf core (left) and outer-shelf core
(right). All records were mean centered by subtracting the mean of the entire record from all
of the values of the record. Linear extension and density variations are from three-four year
bulk measurements. Calcification rates are a product of linear extension measurements and
density calculations. Arrows represent the start of 1998 El Niño event.
Figure 3-6. Annual records of local summer (JAS) temperature (AVHRR v5.2) data and coral
skeletal growth. (A) Annual summer (July-September; JAS) mean sea surface temperatures
from 1992-2012 (left y-axis blue solid line) and JAS SST anomalies from 1992-2012 (right y-
68
axis red solid line). Maximum monthly mean SST (MMM-SST) calculated from average August
SST per year for each time period (1991-1996; blue dashed line and 1997-2013; black dashed
line) in accordance with NOAA methods (194) (B) Annual linear extension measurements from
1992 -2012 for inner-shelf core (red line) and outer-shelf core (blue line). Extension rates for
trace element anomalous years (1998-2001 at inner-shelf and 1998-2002 at outer-shelf) were
averaged over all anomalous years, represented by dark to light shaded areas for the inner-
shelf core and grey for the outer-shelf core respectively.
3.3.2 Trace element –SST proxies
All trace element ratios showed clear seasonal cycles in both cores; however, the
consistency of this seasonality was interrupted between 1998 and 2001 in the inner-
shelf core and between 1998 and 2002 in the outer-shelf core (Fig 3-7). Based on the
relationship of the trace elements with temperature we divided each time series into
three distinct periods: 1) ‘pre-ENSO’ representing the period prior to the onset of the
strong ENSO event 2) ‘ENSO+’ which represents both the period of strong ENSO
forcing as well as the period that followed when the trace element chemistry of the
coral still appeared to be disturbed (1998 to 2001 for the inner-shelf and 1998 to 2002
for the outer-shelf) and 3) ‘post-ENSO+’ which represent the period from when trace
element ratio-SST (TER-SST) relationships returned to more normal regression values
and correlations to the present day. Trace element ratios prior to the onset of the
ENSO cycle showed clear and strong seasonal signals (Table 3-1, Fig 3-7) whereas
during the ENSO+ years they were not significantly correlated with SST. A re-
emergence of the high correlations between seasonal variability in TER and SST was
observed during the post-ENSO+ period; however, all TER-SST correlations were lower
post-ENSO+ than pre-ENSO with the exception of Li/Mg (Table 3-1).
69
70
Figure 3-7. Near-monthly records of trace lement ratios of corals from the central Red Sea
(black) from massive Porites lutea cores from the inner-shelf (Abu Shosha, top), and outer-
shelf (Shi’b Nazar, bottom) of the central eastern Red Sea compared against monthly sea
surface temperature (blue) taken from satellite data (186). Grey area indicates anomalous
region of trace element values (see text).
71
Table 3-1. Regression (r2) and significance (p-values) of regression slopes and
intercepts (y-int) of trace element ratios (Li/Mg, Sr/Ca, Mg/Ca, U/Ca) versus SST for
the inner-shelf core (top) and outer-shelf core (bottom). Values were calculated for
the whole records and for the periods pre-ENSO, ENSO+, and post-ENSO+ and all years
combined.
Trace element ratio-SST relationships from pre-ENSO, ENSO+ and post-ENSO+ were
variable between cores and amongst trace element ratios measured (Table 3-1 and 3-
2, Fig 3-7). Sr/Ca ratios correlated strongly with SST in both cores during the pre-ENSO
phase (r2 =0.65; p < 0.001 and r2 = 0.73; p < 0.001, inner-shelf and outer reef
respectively). The breakdown in this relationship was evident for both cores during
the ENSO years (r2 = 0.23; p = 0.27 outer core and r2 = 0.25; p = 0.18 inner core).
Following the affected years there was a recovery of the Sr/Ca-SST relationship;
however, the strength of the correlations were slightly reduced in comparison to the
pre-ENSO years for the inner (r2 = 0.58; p < 0.001) and outer-shelf (r2 = 0.65; p < 0.001)
72
cores. Li/Mg from the inner-shelf showed similar time-dependent changes (r2 = 0.80
pre, r2 = 0.62 post; both: p < 0.001) to that of Sr/Ca; however, in the outer-shelf the
Li/Mg–SST post 2002 relationship fully recovered to pre-ENSO values (r2 = 0.72 pre, r2
= 0.71 post; both: p < 0.001).
Even though Li/Mg and Sr/Ca showed relatively strong correlations with SST, post-
ENSO+ SST calibrations showed significant shifts (p < 0.001) in slope compared to pre-
ENSO calibrations (Table 3-1). Pre-ENSO Sr/Ca calibrations in the inner-shelf core did
not return to the same fit once the seasonal signal returned (Table 3-1). This pattern
was repeated in the outer core with pre-ENSO and post-ENSO+ calibrations showing
a distinct offset (p < 0.001). Applying the pre-ENSO calibration to the post-ENSO+
Sr/Ca data results in an offset of up to ~5 oC (±0.14 Standard error of the mean; SEM)
in the inner and up to ~0.8 oC (±0.02 SEM) at the outer-shelf. The Li/Mg calibrations
in the inner-shelf also showed shifts from pre-ENSO years to post-ENSO+ years (Table
3-1). Applying the pre-ENSO Li/Mg calibration to the post-ENSO+ data equated up to
a ~2.3 oC (±0.18 SEM) in the inner-shelf and ~0.6oC (±0.02 SEM) offset in the calculated
SST in the outer-shelf.
The Mg/Ca and U/Ca ratios from the outer-shelf core provided relatively robust SST
proxies prior to the onset of the 1998 El Niño event (r2 = 0.62; p = < 0.001 and r2 =
0.59; p < 0.001, respectively). However, these relationships also broke down (r2 = 0.09;
p =0.64 and r2= 0.06; p =0.75, Mg/Ca and U/Ca respectively) over the four years ENSO+
period (Fig 3-7, Table 3-1). After this period, both Mg/Ca and U/Ca again started to
exhibit some degree of seasonal periodicity; albeit their statistical relationship with
temperature was reduced in the post-ENSO+ years (r2 = 0.32; p < 0.59, r2 = 0.24; p <
73
0.004, Mg/Ca and U/Ca respectively). In the inner-shelf we find that the Mg/Ca and
U/Ca were not statistically significant proxies for SST even before the onset of the
1998 El Niño (Table 3-1).
The strength of correlations between different trace element ratios (e.g., Sr/Ca and
Li/Mg) varied among different trace elements ratios combinations and time intervals
(Fig 3-8 and Table S3-3). Relationships between most TE ratios pre-ENSO and post-
ENSO+ period were strongly correlated and significant in both the inner and outer
core, even during the ENSO period when many TER-SST relationships broke down (r2
= 0.29 to 0.82, median = 0.59).
74
Figure 3-8. Comparison between trace element ratios for central Red Sea cores; for the inner-
shelf core, Abu Shosha (top) and outer-shelf core (Shi’b Nazar). Trace element ratios divided
into time intervals, pre ENSO (black circle), ENSO (red triangle) and ENSO+ (blue square).
Regression lines (colour dashed) applied to each time interval plot with 95% confidence
intervals (colour solid). Correlation coefficients and r2 and p values are presented in Table S3-
1.
3.4 Discussion
This study demonstrates that the use of massive corals to provide continuous records
of local SST’s can potentially be compromised by episodic thermal stress events. Both
coral records showed a reduction in linear extension and therefore calcification
accompanied by a loss of seasonality in the TE’s signal and therefore a reduced
correlation with temperature changes that started during the summer of 1998 and
75
lasted 3 to 4 years (Fig 3-6, Table 3-1). Disruption of trace element ratios in response
to thermal stress has previously been documented (77, 173, 177) and in one instance
have been shown to last up to five years in a coral from the Japanese islands of
Ogasawara in the western North Pacific Ocean (202). Similar to our corals it took 3-4
years for growth in Porites sp. from the inner Great Barrier Reef (GBR) to return to
pre-ENSO El Niño rates (17, 201). The effect of the 1997/98 El Niño instigated warming
event on trace element ratios has also been documented in coral core records from
other locations such as the Indian Ocean and the Lakshadweep Archipelago, Arabian
Sea, where trace element seasonality recovered over annual time scales (177, 178).
In this sense, the unusually warm temperatures during the summer of 1998 that
coincided with the intense El Niño (Fig 3-2) were likely responsible for the disruption
in trace element ratios and decrease in skeletal growth. The length of the disruption
(3-4 years) observed in the Red Sea corals; although not unprecedented in its duration
on a global scale, presents evidence of the previously unrealized severity of this event
on coral in this region. The corresponding decline in calcification rates during this
period returned to comparable pre-ENSO rates as the trace element ratios regained
their strong seasonal correlation with SST (Fig 3-6, Table 3-1).
The major decline in calcification and TER-SST relationship observed in the Red Sea
Porites corals coincided with and was clearly initiated by the intense ENSO cycle (Fig
3-2), the reason behind the length of the disturbance of 3-4 years is less certain. It
may have been a result of additional regional thermal stress and associated with
strong ensuing La Niña which, in this case, also caused an extended period of elevated
76
ocean warming and consequent mass coral bleaching (203). In this sense, despite
there being no records of bleaching in this region during the following La Niña phase
in 1998-2001, high summer sea surface temperatures, higher than the long-term
maximum monthly mean, are nonetheless evident from satellite data collected in this
region (Fig 3-2). Furthermore, during the summer of 2010 another intense La Niña
episode brought high SSTs and thermal stresses of 10-11 DHW to the central Red Sea
that caused severe coral bleaching (23). Yet the corals in this study show no evidence
of reduced growth or trace element disruption during that time. This lack of a
response in the geochemical record could suggest that some degree of acclimation
may have taken place between these two events thus increasing the resilience of
these corals to subsequent thermal stress events (204, 205). Alternatively, it could
have been a combination of the strength and duration of the 1997-2001 ENSO event
that caused a noticeable disruption in calcification and trace element incorporation
that was not replicated during the milder 2010 La Niña event. Therefore, while it was
the 1997-98 El Niño that instigated the disruption in the normal incorporation of trace
elements into the corals’ skeleton we can only surmise the additional periods of
elevated thermal stress brought on by La Niña conditions in the following four years
of varying intensity (Fig 3-2) likely helped sustain this disruption.
It is not surprising that the recovery in extension rates and density occurred in
conjunction with the recovery in trace elements in both cores is not surprising as the
biological mechanisms that facilitate calcification also influence trace element
incorporation (53, 81). However, it is surprising that the linear extension and density
77
regained their pre-ENSO levels following the ENSO impacted years even though
correlations between trace element ratios and SST did not; thus suggesting a
prolonged (maybe even permanent) shift in the mechanism of trace element
incorporation.
The severity of this episode on the reefs from the central Saudi Arabian coast may not
have been previously realised. Though described the reduction in calcification of
~30% in a central Red Sea coral (Diploastrea heliopora) since the 1998 warming event.
Although both corals (inner-shelf and outer-shelf) were affected by the 1998 event a
clear intra-colonial or site-specific response was observed. The longer recovery time
for the outer-shelf core (4 years) compared to the inner-shelf core (3 years) for the
trace element ratio-SST correlations may be a result of how the local thermal
environment affects the physiology and resilience of the coral. While wind- and
thermohaline-driven circulation (189) combined with regional net heating can cause
substantial temperature anomalies on the shelf scale (24), wave-driven circulation
combined with local net heating can cause substantial temperature anomalies on the
reef (local) scale (20, 22). The micro-climates resulting from this multi-scalar forcing
of the thermal regime can, in turn, influence the resilience of corals at a particular site
to a given regional warming event (24) as well as the rate of recovery following a mass
bleaching event. Offshore reefs are sometimes considered refugia for corals as they
are not subject to the same local variability in temperature and light that are typical
of more inshore environments (206). However, as inshore reefs may experience
higher more variable temperature regimes they can potentially host more resilient
coral species (207) or individuals with a wider tolerance of thermal/UV regimes (166).
78
In this sense, the coral from the inshore environments can be more resilient to
environmental stress, which may explain the spatial differences in recovery seen here.
In situ temperature data, collected from exposed areas from inner-shelf and outer-
shelf reef sites between September 2012 and October 2013 (187) suggests that
minima and maxima temperatures between the inner-shelf and outer-shelf sites are
within ~1 °C of each other (though critical summer temperature data for the inner reef
is missing) (Fig S3-1). This difference could be critical during a warming event and may
indicate that our inner-shelf coral, being acclimatized to higher temperatures,
recovered quicker as temperatures began to fall after the initial summer SST
anomalies.
Though the high correlations between most trace element ratios and SST returned
after three four years following the breakdown at both shelf sites, they were generally
more robust on the outer-shelf than on the inner-shelf core (Fig 3-7, Table 3-1). This
was especially true for the Li/Mg–SST correlation where the relationship returned to
the same level it was prior to the La Niña phase of the ENSO cycle in the outer-shelf
but not the inner-shelf (Table 3-1). The differences recorded between the inner and
outer reef sites is indicative of physiological changes that affect the mechanism by
which trace elements are incorporated into the coral skeleton, this may be governed
by individual colony adaptability to maintain calcification at a near constant level.
Although knowledge of the physicochemical mechanisms by which each trace
element is incorporated into the aragonite matrix is incomplete, the coherent
behavior of trace element ratios (TE/Ca) during the thermally impacted period, and
79
pre-ENSO and post-ENSO+ disturbance periods suggest that most ratios (apart from
U/Ca) are being affected in similar ways (Fig 3-8, Table S3-3).
With the possible exception of Sr, which is incorporated through direct substitution
of Ca, the behavior of Li/Mg and Mg/Ca is not unexpected since Mg and Li are likely
incorporated into the lattice through entrapment processes (99, 183). Furthermore,
Mg may also be concentrated in a disordered inorganic phase where it is readily
assembled in areas known as the centers of calcification or areas of defects (53, 100).
In addition, U speciation may be influenced by the changing composition of the
calcifying fluid (86, 208). Thus, despite differences in TE incorporation mechanisms
the dominant controller of TE/Ca is Ca2+, and hence the reduced efficiency of the
Ca2+ATPase enzyme during higher than average temperatures (88, 209) and its effect
on Ca2+. This would also reduce the pH and therefore saturation state at the site of
calcification, as removal of H+ ions (also influencing the composition of dissolved
inorganic carbon) would slow/cease. Some studies have recorded an increase in the
abundance of TE relative to Ca2+ thought to be a consequence of reduced activity of
the Ca2+ATPase (50, 88, 96). Our study however sees no such increase in TE relative to
Ca2+ but a muted effect on the seasonal signal (Fig 3-7) with opposite trends (to those
expected) observed in TE values (relative to Ca2+) during summer periods. This
suggests an additional facilitator of the calcification process, the zooxanthellae, is
imparting an important level of control on mechanisms behind the calcification
process (52). When the zooxanthellae–coral host relationship is compromised due to
thermal stress (6) disruptions in the TER-SST relationship are recorded.
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The subsequent reduction in the saturation state of the calcifying fluid may also affect
the structure of the aragonite crystals; i.e. bundles of elongated crystals or granular
type material (210). At high saturation states granular type structures readily grow
and form the centers of calcification (COC) while lower saturation levels facilitate the
growth of elongated bundles radiating from the COCs (210). Mg and Li are
preferentially incorporated, and therefore concentrated, into centers of calcification
for example (82) so our data of lower Mg and Li values during ENSO+ suggests lower
saturation levels are disrupting crystal formation, which may also add to the
disruption in trace element incorporation seen in our corals.
Our finding that Li/Mg-SST was the most robust proxy of seawater temperature
measured in corals from both the inner and outer-shelf further reinforces the role
that Ca2+ plays in controlling the veracity of TE/Ca thermometry. Therefore, although
Sr/Ca also eventually recovered, the strength of the post-ENSO+ correlation was
reduced and the calibration parameters were nonetheless considerably altered (Table
3-1), which could potentially bias the calibration in post-ENSO+ data by up to 5 °C.
This would have serious implications for the accuracy of longer-term temperature
reconstructions derived from modern TER-SST calibrations. Our data indicates that
heat stress can cause prolonged changes in the physiological mechanisms behind
calcification and therefore alter trace element incorporation even after coral growth
recovers. We therefore suggest some caution when using Sr/Ca ratios to construct
long-term climate records as past environmental perturbations could have altered the
SST-calibration being applied. However, these disruptions also provide important
markers for past heat stress events, which can flag-up where trace element climate
81
records need to be thoughtfully processed (i.e. excluded or corrected). This highlights
the importance of the application of additional independent paleo-thermometers
such as Li/Mg that are less sensitive to vital effects. Thus, although both Li and Mg
incorporation into coral aragonite is still subject to perturbations from intense
thermal stress, we saw a strong recovery of the Li/Mg ratio from the 1997-1998
initiated perturbation. This argues that both Li/Ca and Mg/Ca are subject to similar
growth-rate related disturbances that are largely ignored by the Li/Mg ratio.
3.5 Conclusions
Our data reveal how extreme thermal stress events can alter the incorporation of
multiple trace elements into the coral aragonite skeleton. Trace elements are already
known to be heavily influenced by the physiology of biomineralization under normal
conditions (i.e. U and Mg) but are even more sensitive to alterations of coral
physiology during periods of prolonged stress, such as when the zooxanthellae-coral
host relationship is compromised. The trace element ratios measured in this study,
especially those indexed to Ca2+, showed prolonged anomalous period where the
expected seasonal cycle of trace element incorporation became compromised. The
strength of the trace element ratio-SST correlations were significantly reduced and/or
their calibrations significantly altered relative to relationships prior to the El Niño
event even after the high SSTs of the 1997-1998 El Niño event had passed and even
for the most popular coral paleothermometer, Sr/Ca. This could partly undermine the
use of Sr/Ca or other trace element ratios as reliable predictors of SST in long-term
paleo-reconstructions, unless all trace element records are carefully scrutinized for
baseline shifts related to stress events. These findings should be tested on future coral
82
records, for example following the intense 2015/16 El Niño event. Nevertheless, the
sensitivity of some temperature proxies (e.g. Mg/Ca and U/Ca) to thermal stress
provides us with a new ‘fingerprint’ for identifying past stress events as well as a tool
for examining the biological mechanisms of calcification and trace element
incorporation. More importantly, we find that the Li/Mg temperature proxy proved
not only to be valid over a wide range of temperatures that include warm tropical
habitats, but also to be highly robust to the kind of climate-driven physiological
perturbations that will likely occur in the future, albeit once the perturbation passed.
This is likely due to Li and Mg exhibiting very similar mechanisms of incorporation into
the aragonite crystals during skeletal formation; thus, negating the vital effects that
such physiological disturbances would have on their respective abundances relative
to calcium.
Acknowledgments
We would like to acknowledge the team at KAUST for their support, knowledge and
hospitality while the coral cores were being retrieved. With special thanks to Anna
Roik for her input in this paper. We thank Dr Julie Trotter for undertaking the U-series
geochemical analyses reported in this study and the technical staff of AGFIOR, Dr Hans
Oskierski and Anne-Marin Nisumaa Comeauat, for all their laboratory expertise.
Further, research reported in this paper was supported by King Abdullah University of
Science and Technology (KAUST). Supporting data are included in the supplementary
information tables and datasets.
83
Chapter 4 RESPONSE OF CORAL CALCIFICATION AND CALCIFYING FLUID
COMPOSITION TO CLIMATE CHANGE AND THERMAL STRESS IN
THE RED SEA
Lucy Georgiou a, b, Jim Faltera, b, Juan Pablo D’Olivo Corderoa, b, Xabier Irigoienc, e,f,
Christian R. Voolstrac, and Cornelia Roderc.d and Malcolm McCullocha,b
aSchool of Earth Sciences and Oceans Institute, The University of Western Australia,
Crawley WA 6009
bARC Centre of Excellence for Coral Reef Studies, The University of Western Australia,
Crawley WA 6009
cKing Abdullah University of Science and Technology (KAUST), Red Sea Research
Center (RSRC), Biological and Environmental Sciences & Engineering Division (BESE),
Thuwal, 23955-6900, Saudi Arabia
d†Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Shelf Sea
System Ecology, 27498 Helgoland, Germany
e†AZTI - Marine Research, Herrera Kaia, Portualdea z/g – 20110 Pasaia (Gipuzkoa),
Spain
f †IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
Corresponding author: Lucy Georgiou ([email protected])
† Current addresses
84
ABSTRACT
The regulation of the carbonate chemistry (saturation state (Ωcf), pH (pHcf) and
dissolved inorganic carbon (DICcf) of the corals’ calcifying fluid (cf) is a highly important
physico-chemical mechanism, which directly controls calcification; the latter being
influenced by the symbiotic associated zooxanthellae. Rising temperatures and
changes to the carbonate chemistry of seawater may influence this regulation but the
controls on these mechanisms are unclear. Here, we use the newly developed 11B-
B/Ca proxy on massive Porites spp. collected from the central Red Sea region; a region
where temperatures often exceed those thought to accommodate coral growth, to
investigate long-term changes in the carbonate chemistry of the calcifying fluid and
growth against a backdrop of rising temperatures, abrupt episodes of warming and
reduced seawater pH. Seasonally resolved data spanning 20 years (1994-2013) reveal
pHcf has an opposing seasonal signal to DICcf and Ωcf. Additionally, the DICcf seasonal
signal is disrupted during a prolonged four-year period of summer heat stress (1998-
2002), probably due to a loss of the zooxanthellae reducing the availability of
metabolically derived carbon. This disruption in seasonality also extends to the trace
element (Li/Mg and Sr/Ca) signal and a reduction in growth. Annually resolved data
spanning 75 years (1938-2013) suggest DICcf and Ωcf are declining; however, the
regulatory control on pHcf is still strong and calcification rates appear to be increasing
over this period. These findings suggest a degree of temperature driven compensation
whereby pHcf continues to be enhanced to facilitate calcification based on mineral
precipitation kinetics.
85
4.1 Introduction
Coral calcification is thought to occur within a fluid in a partly closed but
semi-permeable space (43) between the cells of the coral and the actively growing
skeleton often referred to as the ‘sub-calicoblastic layer’ (43). Corals exert a strict
control over the biomineralisation process by increasing the pH of the sub-
calicoblastic or calcifying fluid (pHcf) in order to raise the aragonite saturation state,
thereby accelerating the kinetics of mineral precipitation (43, 111). This control of the
chemical conditions governing biomineralization via ‘pH up-regulation’ (45) is critical
to corals modulating their rates of growth as many reef habitats are subject to highly
variable seawater pH regimes on daily to seasonal time scales (223, 47). Although
pHcf has been measured in live coral using either electrodes or pH-sensitive dyes (135,
46), the isotopic composition of trace levels of boron in the deposited skeleton
provide a robust and accurate measurements of the pH under which the skeletal
material was deposited (45). The elevation of pHcf is thought to largely occur due to
the removal of H+ ions from the fluid via a Ca2+-ATPase enzyme (51, 45); however, this
is only one means by which coral achieve the more important end: to elevate the
concentration (or activity) of carbonate ions within the calcifying fluid ([CO32-]cf),
which is elevated in order to reach aragonite saturation states high enough for rapid
rates of precipitation to occur (88, 44). Until recently, the direct measurement of
dissolved carbon species such as carbonate ions in the calcifying fluid has proven
elusive, but the development of new geochemical methods (214) have already proven
successful at unravelling some of the nuances of the carbonate chemistry of the
calcifying fluid (48). For instance, it has recently been shown that B/Ca ratios
measured in the skeleton can be used to determine [CO32-]cf as the borate ion [B(OH)4
-
86
] competes with [CO32-] in the aragonite lattice (214). Using both B/Ca and 11B can
therefore provide an invaluable proxy to determine the carbonate chemistry of the
calcifying fluid (214). From these proxies it is now apparent that the regulation of the
calcifying fluid extends to carbon (via DICcf reflecting changes in the productivity of
the zooxanthellae). On the other side of this pHcf is seasonally regulated to vary
counter-cyclically with levels of DICcf which, not surprisingly, rise in summer and fall
in winter; likely in response to seasonally driven changes in photosynthetic carbon
supply. Thus, the coral is maintaining a compensatory mechanism in order to elevate
the saturations state of the calcifying fluid (Ωcf) for optimal calcification (48). This new
approach has thus shown us how highly controlled the carbonate chemistry of the
calcifying fluid is throughout a seasonal cycle resulting in aragonite saturation states
(Ω) that are up to 6 times higher in the calcifying fluid (Ωcf) than in the external
seawater (48).
The coral-symbiont association driving levels of DICcf is particularly
vulnerable under heat stress (23, 236) this is particularly pertinent as abrupt episodes
of ocean warming are increasing in prevalence and intensity (12) with two of the most
intense El Niño episodes (e.g. 1998 and 2016) recorded in the past two decades (165,
236). As the regulation of the corals internal carbonate chemistry, calcification and
symbiont relationship are tightly linked in maintaining the overall health of the coral
there is a need to distinguish how increasing sea temperatures and changes in
seawater carbonate chemistry will affect these processes. Given the high
temperatures and limited circulation the central Red Sea region is an ideal location to
explore the impact of bleaching events and high temperature stress on physiological
87
processes using long-term geochemical records. Over the last two decades, the
central Red Sea has experienced several bleaching events including the global 1998 El
Niño event (167) as well as another severe event in 2010 (23) and several lesser events
between 2007 to 2009 (217). Here we use 11B-B/Ca, in addition to calcification and
trace elements information, from coral cores collected from Porites sp. living in the
central Red Sea region to explore how climate-driven changes in temperature and pH
affect the physico-chemical mechanisms governing coral calcification on two time
scales: on seasonal time scales over a 20-year period and on annual time scales over
a 75-year period.
4.1.1. Site description
The Red Sea is a highly saline (>35) semi-enclosed water body where the exchange of
water with other water bodies and precipitation is limited (161). At nearly 2000 km long, the
Red Sea can be divided into three distinct regions 1) the northern region which is connected
to the Mediterranean Sea via the Suez Canal in the northwest and to the Gulf of Aqaba in the
northeast, 2) the central region (20 – 24o N) which is extremely arid with minimal rainfall and
riverine input, causing a nutrient poor environment, and 3) the southern region which is most
heavily influenced by the Indian Ocean via exchange through the narrow Bab-al-Mandeb
Strait (37). Our study site is in the central eastern Red Sea Region adjacent to the Saudi
Arabian coast (~22 N) (Fig. 4-1). In this region sea surface temperatures range from around
25 °C in February to about 31 °C in September, there is strong diurnal variability around reef
platforms which is attributed to wind-driven cooling and wave-driven circulation (22, 189,
186). Over the past two decades, the central Red Sea region has experienced several periods
of high sea surface temperatures culminating in documented bleaching events. One of the
most intense occurred in 1998 with the onset of the 1997/98 El Niño (167), this was part of a
88
longer term (1998-2000) intense ENSO cycle where SST summer anomalies were high for four
consecutive years (Fig. 4-2).
Figure 4-1. Location map of reef site Shi'b Nazar and adjacent coast. Insert is an overview of
the Red Sea region and Saudi Arabian coastline.
4.1.2 Satellite and Reconstructed SST data
Daily local satellite sea surface temperature data (1980-2013) derived from the
Pathfinder Advanced Very High Resolution Radiometer (AVHRR) according to
Reynolds et al. (2007) (186) were obtained online from the NASA THREDDS (v5.2) data
server (http://thredds.jpl.nasa.gov/thredds/catalog_oceantemperature.html) at a
spatial resolution of 25 km. Long-term annual SST data (1939-2013) was derived from
89
HadSST2 SST (238) (available at http://www.cru.uea.ac.uk) at a spatial resolution of
2°×2°.
Figure 4-2. Seasonal SST records (1994-2012) (a) Summer (JAS) mean SST and anomaly
climatological data for the central Red Sea Region. Data extracted from Pathfinder AVHRR
v5.2 (http://thredds.jpl.nasa.gov/thredds/catalog_oceantemperature.html). Maximum
monthly mean SST (MMM-SST) calculated from average August SST per year for the reference
years: after the onset of El Niño (1998-2014; black dashed line) and before the onset of El
Niño (1985-1997; blue dashed line) (194).
4.1.3 Records of seawater carbonate chemistry
Mean monthly levels of atmospheric CO2 from March 1958 to Feb 2016 were
taken from observations made at the Mauna Loa Observatory
(http://www.esrl.noaa.gov/gmd/ccgg/trends/). Reconstructed average annual levels
of atmospheric CO2 from 1910 to 2014 were taken from measurements of pCO2 made
on Law Dome ice cores (http://cdiac.ornl.gov/trends/co2/ice_core_co2.html) (237).
Seasonally averaged Total Alkalinity (TA) and salinity data obtained by KAUST over
90
two four-week period, at 10 minute intervals, during the summer and winter of 2014
at Shi’b Nazar reef (187). The average of summer and winter TA and salinity values
were used when constructing annual records of ambient seawater pH and DIC (pHsw
and DICsw) whereas seasonally (sinusoidal) varying TA based on maximum and
minimum in situ values were calculated to construct seasonal (monthly) records of
pHsw and DICsw. Prior studies have shown that the salinity of the Red Sea has remained
relatively stable over at least the past several thousand years (239). Given that the
site is located far offshore, relatively deep (>5 m) and well flushed by offshore water;
we expect that the TA of those waters to be mostly dictated by offshore salinities and,
therefore, relatively stable over the past century as well. Surface pHsw and DICsw were
calculated from the composite atmospheric CO2 record and TA records (Fig. S4-1 and
S4-2) with CO2SYS (ver. 2.1) using values for dissociation constants K1, K2 from
Mehrbach et al., 1973 (241) as refit by Dickson and Millero, 1987 (240), (144).
4.2 Materials and Methods
4.2.1 Coral core sampling
One coral cores was collected from Shi’b Nazar Reef on the outer continental
shelf ~25 km from shore (38°56'41.14" E, 22°19'1.94" N) (Fig. 4-1). Core sections (5
cm in diameter and up to 50 cm in length) were removed on April 2013 from a living
massive colony of Porites lutea with a pneumatic drill. The freshly removed core was
then cleaned with fresh water, dried and transported to the University of Western
Australia (UWA) for further processing and analysis. The core was first sliced to 6-7
mm thickness and the resulting slices were then bleached overnight in a 1:1 NaClO
91
solution, washed thoroughly in deionized water, ultrasonicated 3 times and finally
dried at ~50 °C in a laboratory oven. A set of slices from the middle of the coral core
was taken for X-radiography (Fig. 4-3) and the images processed using Clarity Pacs
(Ver 4.6, NOVA Southeastern University) software. Profiles of optical intensity along
each core were digitized and extracted using the software CoralXDS (Ver 4.6, NOVA
Southeastern University). From the analysis of the X-ray images, it was determined
that high density bands were laid during winter months and low-density bands were
deposited during the summer months. This agrees with other studies on Porites spp.
from the northern Red Sea (211). The selected slabs were cut following major growth
axis previously identified via X-ray images to reduce possible biases in trace elements
or 11B due to growth related effects (54, 68). Annual samples from 1940 - 1994 were
taken by milling skeleton with a computer controlled Zenbot drill from one high
density band to the next along the major growth axis. High-resolution powder
samples were taken at 1.25 mm intervals from 1994 – 2013, this sampling strategy
resolved 10-11 samples per year (during ‘normal’ growth years producing a dataset at
near monthly resolution). For the annual time series, data points from 1994 onwards
were averaged yearly from the seasonal data and are highlighted as orange data
points on all plots.
92
Figure 4-3. X-ray images of core (Shi'b Nazar) showing sampling tracks and primary corallite
growth axes (red lines) of annually resolved data from around 1940 to 2013. Yellow line
indicates areas producing anomalous 11B values. Blue lines indicate additional sampling
tracks to test the validity of the anomalous 11B values.
93
4.2.2 Geochemical analysis
Powdered core samples were processed in the AGFIOR (Advanced
Geochemical Facility for Indian Ocean Research) ultra-clean laboratory based at UWA.
All sample and standard solutions were prepared using sub-boiling distilled HNO3
(Savillex DST-1000) and 18.2 MΩ water (Millipore). Coral powder samples were
weighed (10 ± 0.2 mg) and dissolved in 0.56 2M HNO3 before being centrifuged. From
the original dissolution, an aliquot of 400 μl was taken from each sample for 11B
determination. The boron was quantitatively separated using the cation AG50W-X8
and anion AG-1-W8 ion-exchange resins according to McCulloch et al. (2014) (212).
Boron isotope data are reported in the permil (‰) notation relative to the NIST SRM
951 standard where:
--------------Eq. 1
where δ11Bsample represents the 11B to 10B ratio of coral samples and 11B/10Bstandard is
the value of the NIST standard.
One JCp-1 coral standard powder was processed along with every 30 samples
in order to assess the quality of our full chemical treatment (24.7 0.07 ‰; n=14).
Duplicated boron isotope analyses of each sample were measured using NU Plasma II
MC-ICP-MS to ensure analytical reproducibility (0.4 ‰). A number of samples (n=12)
including four anomalous 11B values samples (from 1970-1974) were re-analysed
(Fig. 4-7). Root mean square error (RMSE) of initially analysed values of anomalous
samples and re-analysis of these samples indicate similar numbers were obtained
d11Bcarb
= 11B / 10Bsample
/ 11B / 10Bstandard( ) -1é
ëùû x 1000
94
(RMSE; 0.14‰, n=4) with a similar outcome for all samples re-analysed (RMSE;
0.17‰, n=12).
4.2.3 Calculation of pH of the calcifying fluid
The pH of the calcifying fluid (pHcf) was derived from measured skeletal 11Bcarb
values according to the following equation (39):The pH of the calcifying fluid (pHcf)
was derived from measured skeletal 11Bcarb values according to the following
equation (39):
----- Eq. 2
Where pKB is the dissociation constant of boric acid dependent on temperature and
salinity (213), 11Bsw = 39.61‰ (108) and αB3-B4 is the boron fractionation factor for
the pH-dependent equilibrium of [B(OH)4-] and [B(OH)3] in seawater equal to 1.0272
(109).
4.2.4 Calculation of carbonate ion and DIC of the calcifying fluid
In order to resolve the carbonate chemistry of the coral at the time of
calcification, we applied the newly developed method of utilizing the combined B/Ca
and 11B geochemical proxies; this method was developed by Holcomb et al.2016
(214) under inorganic laboratory conditions. Borate [B(OH)4-] competes with
carbonate [CO32-] for a position in the aragonite crystal lattice, this is likely to occur
through the de-protonation of [B(OH)4-] and the reaction is thought to be sensitive to
the pH of the calcifying fluid (214, 48). The abiotic experiments conducted by Holcomb
3 4 3 4
11 11
11 11 1000 1B B B B
carbsw
carb sw
cf B
B B
B BpH pK log
95
et al. 2016 (214) provided evidence of the close relationship of [CO32-] and B/Ca as
described by the partition coefficient (KD: equation 3) which exhibits a weak
dependency on solution pH (48). KD values (KD = 0.00297exp (-0.0202 [H+]T) are
experimentally determined (214). Total boron [BT] in the calcifying fluid is assumed
to be the same as the ambient seawater and therefore a function of salinity.
3
2
3
4
x solD
CaCO
sol
COBK
CaB OH
------- Eq. 3
These parameters therefore provide us with the necessary components to
reconstruct [CO32-]cf (equation 4). Dissolved inorganic carbon (DICcf) concentrations
were then calculated using the pHcf calculation derived from 11Bcarb (equation 2) and
the reconstructed [CO32-]cf calculations derived from B/Ca measurements (48).
2
3 4 x /cf cf carbD
BCO K BOCa
------------Eq. 4
4.2.5 Modelled pHsw/pHcf and growth rates
To test our direct measurements of changes in pHcf derived from the δ11B of
the deposited coral skeleton with predictions of how pHcf should change according to
laboratory experiments with Porites sp., we calculated an expected pHcf according to
the relationship provided by Trotter et al. 2011 (91) (pHcfexp = 5.95*pHsw + 0.32). In
the case of the annual data, we evaluated the linear relationship between the δ11B-
pHcf and modelled-pHcf in two parts, 1) before the anomalous data appearance (1940
– 1969) and 2) after the ‘recovery’ of the anomalous data until recent (1975 – 2012).
96
Calcification rates (G) calculated using abiotic rate kinetics where k is the constant and
n is the order of the reaction both of which are temperature dependant (equation 5).
( 1)nG k --------- Eq. 5
4.3 Results
4.3.1 Seasonal Geochemical Signal and ENSO disruption
The seasonal chronology of the core was based on the Li/Mg data as this proxy
was the paleo-temperature proxy which exhibited the highest correlation with
observed SST (table 4-1) although seasonal Sr/Ca records were also used to further
verify the Li/Mg-derived chronology (215). Once the correct seasonal chronology had
been established, the seasonally resolved geochemical records were divided into
three distinct periods: 1) 1998 to 2002 (ENSO+) and 2) 2007 – 2010 a known period of
intermittent heat stress and 3) all other years. Here we use the term ‘ENSO+’ to
identify the time period over which trace element ratios appeared to unusually and
poorly correlated with seasonal changes in SST and each other; a period which
incorporates both the strong El Niño and La Niña phases of the ENSO cycle (215) as
well as lag time following the completion of the ENSO cycle over which the trace
element composition of the deposited skeleton appeared to remain highly atypical.
Analysis of the high-resolution (monthly) B/Ca, Sr/Ca and Li/Mg ratios revealed a
strong coherent seasonal signal (Fig. 4-4) that was highly correlated with SST during
times when no heat stress/bleaching episodes are reported (r2=0.62, 0.70 and 0.72
respectively; p <0.001 for all trace element pairs, table 4-1). However, these
relationships broke down at the onset of the 1998 El Niño (r2=0.00, p = 1.00 for B/Ca,
r2=0.20, p = 0.15 for Sr/Ca) though Li/Mg did show some level of resilience (r2 = 0.32,
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p= 0.02 for Li/Mg). During 2007 – 2010 when intermittent bleaching episodes and
periods of heat stress are prevalent in the region (time interval SP1+) Li/Mg and Sr/Ca
maintain their seasonality (Fig 4-4) (r2 = 0.69, p= <0.0001 for Sr/Ca and r2 = 0.70, p=
<0.0001 for Li/Mg). Though the seasonality of B/Ca appears to be reduced (r2 = 0.22,
p= 0.21). Periodicity in the monthly resolved 11B data appear to be episodically
coherent with seasonal changes in SST (Fig. 4) and weakly correlated with SST during
years of no stress indicators (r2 = 0.25, p = 0.01). During periods where stress in
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indicated any seasonality is severely reduced (ENSO+; r2 = 0.10 to 0.47 and SP1+;
r2=0.07; p=0.69) (Fig. 4-4; table 1).
Figure 4-4. Seasonal geochemical and SST records (AVHRR v5.2) from 1994 to 2013 (a) 11B
(b) B/Ca (c) Li/Mg and (d) Sr/Ca, from a coral core collected from Shi'b Nazar in the Red Sea.
Right y-axis on a, b, c and d show seasonally resolved SST. Shaded bars indicate periods over
which mass coral bleaching within the central eastern Red Sea was observed. ENSO+ affected
years last from the summer of 1998 through the summer of 2002. Note that the left y-axis
representing all the geochemical tracers are inverted.
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Table 4-1. Correlation of select seasonally resolved geochemical records (11B, Li/Mg, B/Ca
and Sr/Ca) with seasonal changes in temperature (SST) over different periods of interest.
Outer reef (Shi’b
Nazar)
11B Li/Mg B/Ca Sr/Ca
ENSO+ r2 0.10 0.32 0.00 0.20
n=53 p value 0.47 0.02 1.00 0.15
SP1+ r2 0.07 0.70 0.22 0.69
n=32 p value 0.69 <0.0001 0.21 <0.0001
All other years r2 0.25 0.72 0.62 0.70
n=101 p value 0.01 <0.0001 <0.0001 <0.001
4.3.2 Annual changes in coral skeletal chemistry from 1938 -
2013
Annually resolved geochemical records (11B, Li/Mg, B/Ca and Sr/Ca)
spanning 75 years are shown in Fig. 4-5. Changes in 11B signals show no significant
relationship with corresponding changes in SST over the past three-quarters of a
century (r2 = 0.00, p = 1.00, table 4-2; Fig. 4-5). In contrast with the seasonal data,
average annual trace element ratios were not correlated with annual SST variations
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(r2 = 0.14, p = 0.91 for Li/Mg and r2 = 0.18, p = 0.88 for B/Ca; table 3). A particularly
notable ~8 ‰ drop in 11B (28.38 to 20.17 ‰) equating to a ~0.3 unit drop in pHcf
occurred from 1969 to 1970 (Fig. 4-5) across a break in the coral core (between
sections) after which values remained relatively low (<21 ‰) for several years until
rising again around 1974.
Figure 4-5. Annual records of boron isotopes and sea surface temperature (a) Annual records
of 11B and sea surface temperature (grey line) from Shib’ Nazar. Open circles represent
anomalous data (1970-1974) (HadSST2 SST available at http://www.cru.uea.ac.uk). Black
crosses indicate adjacent sampled track (shown in Fig. 4-3) to test low 11B data points. Annual
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records of (b) B/Ca, (c) Li/Mg, and (d) Sr/Ca from 1940 – 2012. Orange data points (1994 –
2013) highlight annual data calculated from averaging over seasonally resolved data. Note
that geochemical axes are inverted.
Table 4-2. Correlation of select annually resolved geochemical tracers (11B, Li/Mg B/Ca and
Sr/Ca) with annual SST.
Shi’b Nazar
11B Li/Mg B/Ca
Sr/Ca
All years r2 0.00 0.14 0.18 0.05
n=72 p 1.00 0.91 0.88 0.67
1940-1968 r2 0.00 0.09 0.00 0.01
n=28 p 1.00 0.64 1.00 0.96
1975-2012 r2 0.08 0.22 0.28 0.03
n=37 p 0.63 0.18 0.13 0.85
4.3.3 Growth over the past 75 years (1940-2012)
Annual rates of linear extension and density and were not significantly
correlated (r2 = 0.18, p = 0.30). Calcification was not significantly correlated with
skeletal density (r2 = 0.14, p =0.52) though it was weakly correlated with linear
extension (r2 = 0.35, p = 0.10) (Fig. 4-6). Growth was compromised for four years
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following the 1998 bleaching event (Fig. 4-6) that defined the ENSO+ period. The
period of 1970 – 1974 when low 11B values were measured (Fig. 4-5) showed no
apparent effects on growth rates (Fig. 4-6). Overall, the annual rate of linear extension
in this coral has significantly increased over the past 75 years while its skeletal density
has remained stable over the same time period. Despite these disparate long-term
trends in extension rate and skeletal density, calcification rates (being the product of
linear extension rate and skeletal density) exhibited a slight increase over their 75-
year record length (Fig. 4-6).
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Figure 4-6. Growth characteristics of the massive Porites coral at Shib Nazar from 1940 to
2012 as represented by (a) annual linear extension measured from x-ray images and trace
element data and then (b) bin-averaged over the period of each skeletal density
measurement shown in (c). Calcification rates (d) calculate from rates of extension averaged
over the period of each density measurement. Grey bars indicate periods of low growth
during 1998-2002 ENSO-affected periods (right).
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4.4 Discussion
4.4.1 Regulation of internal carbonate chemistry and
environmental stress
Reconstructed DICcf/DICsw, cf and pHcf exhibited inconsistent seasonality with
respect to both their amplitude and phase (Fig.4-7). Nonetheless, we generally
observed a seasonal trend (r2=0.60; p <0.001) where pHcf was lower in summer
months than in winter months; a trend that was opposite in phase with DICcf and Ωcf
which exhibited higher levels during winter months (Fig. 4-7). These findings concur
with seasonal records of carbonate chemistry parameters described in other massive
Porites formations from the Great Barrier Reef and Western Australian (48). However,
the seasonality of DICcf/DICsw and cf appear to breakdown for several years on two
separate occasions between 1994-2013. The first breakdown occurred from the
summer of 1998, coinciding with the start of the El Niño when coral in the central Red
Sea experienced heating stress of 14 degree heating weeks (DHW) (Coral Reef Watch
NOAA, 2000) and temperatures were ~2 °C above the maximum monthly mean
(MMM) (Fig. 4-3). These summer temperature anomalies lasted for the next four
years, coinciding with a strong ENSO period (ENSO+) and with a breakdown in trace-
element-SST signal (Fig. 4-3). Despite the breakdown in trace element-SST
relationships, the coral appeared to have maintained its capacity to regulate pHcf
through the highly stressful ENSO+ period (Fig. 4-7; r2 =0.63; p < 0.005). However,
DICcf/DICsw showed significantly lower seasonal variance during ENSO+ (F=0.3835,99;
p<0.001) than during non-stressed time periods (Fig. 4-3). Both the trace element and
DICcf/DICsw seasonal signals returned in 2002 and remained unperturbed until the
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summer of 2007 a period where no reported bleaching episodes in these reefs have
been recorded. The second breakdown occurs during 2007 to 2009 (which we refer
to as ‘SP1+’) when the DICcf/DICsw variable again appears to be impacted (r2 = 0.16; p
= 0.44) when the seasonal variance is significantly lower (F23,,99 =0.18; p <0.001) then
non-stressed periods; this in contrast to the years when no stress is reported (table 4-
4). During this period in time the reefs around the central Red Sea experienced several
stressful events including extreme low tides (216), COTS outbreaks and high
temperatures causing bleaching and mortality (217). In the summer of 2010 during an
intense La Niña phase of ENSO the central Red Sea reefs experienced high SSTs and
thermal stress of 10-11 DHW that also caused severe coral bleaching (23). Although
the disruption to DICcf (Fig. 4-7 and 4-8) during 2007-2009 suggests that coral was
under some form of environmental stress, key paleo-temperature proxies were
nonetheless well-correlated with seasonal changes in temperature during this time
(Sr/Ca and Li/Mg) (Fig. 4-4). This could be due to the continued length and intensity
of the ENSO+ disruption during these four years indicated by the highest mean
monthly maximum temperatures on record (1985 -2013) (Fig. 4-3). Coral growing in
the Red Sea have maximum growth rates during spring (187) and higher density of
zooxanthellae during cooler winter months (218). This suggests summer
temperatures are already exceeding the maximum for optimum growth and therefore
the additional temperatures seen over consecutive summers during the ENSO+ period
take them beyond their threshold to maintain pHcf and the resulting loss of
zooxanthellae during bleaching episodes effect DICcf/DICsw maintenance. There may
also be a degree of acclimatisation to these events through, for example, replacement
of more resilient zooxanthellae clades and/or density of the associated symbiont (218,
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219). A marked decrease during the ENSO+ period in the otherwise relatively stable
record of linear extension further highlights the severity of the 1998-2002 event (Fig.
4-6).
Figure 4-7. Seasonal time series of corals internal carbonate chemistry for Porites coral from
Shi’b Nazar, Red Sea (a) pH of the calcifying fluid derived from 11B isotope measurements
from 1994 - 2013 (b) Saturation state (Ωcf) and temperature from mid-1994 -2013 (c) Internal
DICcf/DICsw from August 1994 to February 2013 and temperature (right y-axis –black line).
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Plots are divided into relevant time periods of no stress (indicated by blue symbols) and
periods of known stress (ENSO+ and SP1+). The grey regions represents bleaching events
and/or high temperature anomalies including the ENSO+ period of 1998-2002. Shaded
area/red dotted outline represents period of intermittent observed bleaching and COTS
outbreaks along Saudi Arabian coast during 2007 – 2009 (217, 220).
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Figure 4-8. Relationship of seasonally resolved sea surface temperature with carbonate
chemistry profile of coral from Shi’b Nazar (1994-2013) (a) pH of the calcifying fluid (pHcf) (b)
saturation state (Ωcf) and (c) dissolved inorganic carbon DICcf/DICsw divided according to
periods of no recorded environmental stress (indicated by blue symbols) and to periods of
observed environmental stress (ENSO+, red triangles and SP1+; green circles). Data from coral
cores collected at Davies Reef in the central Great Barrier Reef (GBR Davies-2, grey squares)
and from Coral Bay on the Ningaloo Reef Tract (Coral Bay-2 , grey circles) and analysed by (48)
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are provided for comparison. Values of the correlation coefficients for all relationships shown
are presented in table 4-4.
Table 4. Regression analysis (r2, slope, intercept and significance) of internal
carbonate chemistry parameters (pHcf, saturation state (Ωcf) and dissolved inorganic
carbon DICcf/DICsw) against temperature for time periods of both observed coral stress
(ENSO+: August 1998 – 2002, SP1+: August 2007 – August 2009) and all other non-
stress years between 1994 and 2013. Data from coral cores collected at Davies Reef
in the central Great Barrier Reef (GBR Davies-2) and from Coral Bay on the Ningaloo
Reef Tract (Coral Bay-2) and analysed by (48) are provided for comparison.
4.4.2 Annual changes in coral carbonate chemistry and growth
over the past 75 years (1938-2013)
The amplitude of expected pHcf from pHSW based on prior experimental results
(221, 91) was roughly one-fourth (23%) of the observed pHcf derived from the 11B
measured in the coral skeleton (0.08 vs. 0.35 pH units, respectively); thus, suggesting
that the coral was up-regulating pHcf according to its own physiological needs and not
pHcf vs. Temperature Ωcf vs. Temperature DICcf/DICsw vs. Temperature
r2 slope y-int p r2 slope y-int p r2 slope y-int p
ENSO+ 0.63 8.86 0.01 <0.005 0.01 24.18 0.25 0.95 0.02 39.33 4.53 0.9
SP1+ 0.52 8.64 0.01 < 0.05 0.51 7.48 1.96 < 0.005 0.16 49.76 9.33 0.44
All other years 0.60 8.78 0.02 < 0.001 0.41 9.83 0.3 < 0.005 0.41 1.21 0.04 < 0.005
Davies Reef-2 0.85 8.97 0.02 <0.001 0.86 1.71 1.38 <0.001 0.90 2.71 8.34 <0.001
Coral Bay-2 0.72 9.00 0.02 <0.001 0.68 13.05 2.42 < 0.001 0.55 9.27 4.3 < 0.001
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in response to seasonal changing pHsw (Fig. 9). At first interpretation, the anomalously
low 11B values from 1970-1974 would suggest a major breakdown of the coral to
physiologically control the chemistry of its calcifying fluid; especially in light of the
coral’s demonstrated ability to up-regulate pHcf through the physiologically disruptive
ENSO+ period. Nonetheless, this period of unusually low 11B (and inferred pHcf) did
not correspond with anomalous growth or density values nor did they correspond
with a seawater temperature anomaly or reduced correlations between various trace
element ratio and seawater temperature. Their physical proximity to the core break
in 1970 is particularly notable; however, at present we cannot propose a mechanism
for how this would have influenced our isotopic measurements. It is more likely that
the order of causality is reversed; that the core break occurred where calcification had
occurred under sub-optimal chemical conditions within the calcifying fluid (and whose
origins remain unclear); thus leading to the growth of structurally weak skeleton. The
opposing trends we see in pHcf and Ωcf on seasonal time scales may have underlying
implications on the structure of the calcium carbonate skeleton. It has been suggested
that low 11B levels within the nucleation sites, referred to as centres of calcification
(CoC) indicate low Ωcf as it was inferred these two parameter follow each others trend
(222). However, our data supports the hypothesis that on seasonal time scales, even
though both pHcf and Ωcf are elevated they are in opposing trends (210).
Similarly to the seasonal data changes in pHcf there is a long term decline of
~0.11 pH units over 1940-2013 (or 0.15 pH units per century) which is much higher
than the long-term modelled pHcf of 0.024 pH units (or 0.03 pH units per century) that
would be predicted from changes in seawater pH alone based on laboratory
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experiments (Fig.4- 9). The long-term decline in pHcf we observed is not unique to this
coral, similar long-term declines have been observed in in the pHcf of coral from the
Great Barrier Reef and the North Pacific (223, 224, 225, 226). That the difference in
pH between the calcifying fluid and the ambient seawater (ΔpH) (Fig. 4-9) is actually
increasing over the same 75-year period, while both pHcf and pHsw are decreasing in
an absolute sense, indicates that increased biologically driven up-regulation of pH at
the site of calcification is occurring in response to lowering pHsw. The long-term
increase in coral up-regulation has important implications for the future of corals as
it is at least partly offsetting long-term declines in seawater pH as a result of ocean
acidification (Fig. S4-1 and S4-2). Given the dissociation between the upward long-
term trend in rates of growth from the downward long-term trends in pHcf, DICcf, and
Ωcf; the long-term increase in temperature recorded for this area (1.46 °C in the last
decade compared to the 1950-1997 average (227) may, therefore, be the most
prevalent influence on calcification due to temperature-dependent mineral
precipitation kinetics (111, 128). In other words, the coral can grow faster with lower
pHcf, DICcf, and Ωcf because the growth kinetics are more than offset by the higher
temperatures as hypothesized by (128). For example, the abrupt decline in DICcf of
~0.4 umol/kg during the 1990s though we cannot be sure of the origin this does
coincide with an abrupt increase in temperatures in this region in the order of 0.7 oC
(227). This coupled with the disrupted seasonal DICcf infers that this parameter is the
most sensitive to changes in temperature and therefore a ‘red flag’ for coral health
and (in some cases) bleaching episodes.
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There is a noticeable difference in calculated growth (G) and measured
calcification rate (as a product of linear extension and density) (r2 = 0.11; p = 0.34)
with calculations of G based on the temperature, Ωcf, and known mineral precipitation
kinetics which predict both lower and more variable growth rates. The stable nature
of our measured growth variables may be an artefact of using the 3-year bulk average
density in order to calculate growth rates (Fig. 4-6). These measurements were made
on sub-sections of core that were ~50 mm wide whereas the transects from which
samples for annually resolved measurements of skeletal geochemistry were taken
were ~1 mm wide. Nonetheless, we doubt that this disparity would mask the kind of
long-term trends that were manifest over the entire core. It is more likely that we as
yet do not fully understand how temperature affects the growth kinetics of aragonite
growth mitigated by actively growing coral versus abiotically precipitated crystals.
The long-term temperature increase recorded for this area may, therefore, be the
most prevalent influence on calcification due to temperature dependent kinetics of
biogenic mineral formation (45, 111). For example, we would expect to see a
reduction in calcification of around 40% at the temperatures recorded given the
observed long-term decline in Ωcf (111). Instead, we observed an increase in
calcification of around 33% over the 75-year period (considering a 3-year binned
linear trend). Increasing rates of calcification in line with increasing temperature over
the effect of the reduced saturation variable have been noted elsewhere (228). Many
studies have reported both increasing temperatures and reductions in seawater pH
resulting in reduced growth rates (229, 230). However, there are often inconsistencies
between reef-building species for example massive Porites spp. showed no change in
calcification under temperature or CO2 manipulations experiments while the
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branching coral (Porites rus) grew 50% faster at higher temperature (29.3 °C
compared with 25.6 °C) but 28% slower under the higher CO2 incubations (155).
Figure 4-9. Annual changes in the carbonate chemistry of the calcifying fluid from 1938-2013
with additional future projections for the years 2014-2030 (dark grey) (a) pHcf derived from
boron isotope measurements. Red open data points indicate annual averages calculated from
seasonally resolved measurements (10-11 per year collected between 1994-2013). (b)
Difference between annually resolved pHcf and pHsw (ΔpH). (c) Aragonite saturation state of
the calcifying fluid (Ωcf) and (d) dissolved inorganic carbon of the calcifying fluid (DICcf) as
calculated from 11B-B/Ca measurements. Black dashed lines represent the best-fit linear
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regressions applied to the coral data. Projected values (shaded area; 2014-2030) calculated
using time series analysis (R v.3.3.2) on de-trended data.
4.5 Conclusions
The data from this massive Porites colony from the central Red Sea suggest there is a
strong ability to regulate the carbonate chemistry of their calcifying fluid on seasonal
time scales; at least when environmental stress is not a factor. The overall trend in
pHcf in this coral over the last 75 years is one of decline (in line with a decline in pHsw)
however, the strong biological control on internal pH-regulation is evident from the
increasing trend of ΔpH (pHcf – pHsw) over this time period. In contrast, DICcf and Ωcf
are also showing rates of decline over the past 75 years indicating changes in
environmental variables, either rising temperatures and/or decreasing pHsw are
influencing the physiology of chemical regulation of the calcifying fluid over this time
period. It is further evident that environmental stress can disrupt this regulation by
the coral considerably, especially regulation of DICcf:. We suspect this is probably due
to a loss in photosynthetic productivity of associated algal symbiont as direct result of
heat stress. Therefore, the combination of ocean acidification and periodic warming
could increasingly cause a significant reduction in the ability of the coral to self-
regulate and therefore increase mortality. What is encouraging is that firstly this coral
appears to recover between these periods of high stress; indicated by a return of the
corals internal carbonate system to follow seasonal temperature changes. Secondly,
the long-term elevations in average seawater temperature appear to be increasing
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rates of growth suggesting temperature driven kinetic effects or other temperature
dependent variables play an important role in coral calcification.
Acknowledgements
We would like to acknowledge the scientists and technical staff at (King Abdullah University
of Science and Technology) KAUST for their support, knowledge and hospitality while the coral
cores were being retrieved. We thank the technical staff of AGFIOR, Dr Kai R Rankenburg, Dr
Hans Oskierski and Anne-Marin Nisumaa Comeauat, for all their laboratory expertise. Further,
research reported in this paper was supported (KAUST). Supporting data are included in the
supplementary information tables and datasets. Thanks to Dr. Ron Kanters of Bulk Bill Xray &
Ultrasound for providing us with x-ray images of the coral cores pro bono.
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Chapter 5
5.0 DISCUSSION AND CONCLUSIONS
In this thesis we have examined the potential resilience of massive and branching
corals to the effects of climate change (ocean acidification and temperature increases)
using geochemical tracers as tools to help evaluate the mechanisms of calcification.
We have also evaluated the use of geochemical tracers as valid environmental proxies
in modern corals. Initially, boron isotope ratios were investigated by researchers as
potential proxies for seawater pH in marine carbonates due to the unique speciation
of boron in seawater under variable pH conditions and the partitioning of the boron
isotope in marine carbonates (105). The application of the 11B has proven useful in a
few organisms such as some species of foraminifera, where biological pH up-
regulation appears to be non-existent (231). However, within tropical coral the 11B
is heavily influenced by physiological processes (such as pH up-regulation) occurring
at the site of calcification (91, 147). Under static laboratory conditions there appeared
to be a neat linear relationship between pHcf and pHsw (91); however, this is not the
case in corals from natural reef environments where environmental systems are
highly dynamic (47). Therefore, the association between pHcf and pHsw in corals
grown in natural and dynamic environmental regimes do not appear to conform to a
linear model and corals are likely maintaining their pHcf at levels high enough to
ensure adequate rates of calcification under variable pHsw conditions (47). These
results show that it is further necessary to investigate the phenomena of pH up-
regulation more thoroughly in corals residing in less stable environments, as this is
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what the boron isotope is recording in these corals before attempting to use the
boron isotopic composition of the coral skeleton as an seawater pH proxy. The boron
isotope does however, provide us with a unique opportunity to delve into the
physiology of coral; an area of study that has proven difficult to constrain but holds
key information about the result of changing ocean acidification and temperature on
the future of coral organisms. Our boron isotope results from the Porites cylindrica
corals at Heron Island also indicate some corals are highly resilient to relatively low
pH environments. This resilience is most likely due to exposure to variable pH
conditions naturally acting on the activity of the Ca2+ATPase pump that regulates pHcf.
The extent of this resilience needs to be explored within other species and in other
environments and efforts should be concentrated on in situ studies that replicate
‘real-world’ conditions as much as possible, such as using FOCE technologies. Building
on the in situ experiment at Heron Island future work would entail transplanting
colonies from areas where the environmental parameters (such as pH and
temperature) are more stable and transplanting them to the dynamic system of the
reef flat (and vice versa) and monitoring their progress within the FOCE system before
assessing their geochemical signals over time. Testing the limitations of any resilience
by the performing the same experiments but incorporating the synergetic interactions
of other environmental variables such as rising SST and local pollution is also a
necessary step in fully understanding resilience and coral physiology as is taking a
careful look at symbiont association and density under stressful conditions.
In order to fully comprehend the calcification process in coral there is a need to
explore the whole internal carbonate chemistry profile (i.e. saturation state (Ωcf), pH
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(pHcf) and dissolved inorganic carbon (DICcf). Pairing skeletal boron isotope ratios and
B/Ca ratios are already presenting new opportunities to investigate the physico-
chemical mechanisms occurring at the site of calcification (46, 48). Physiological
processes such as the transport of ions and the species of DIC used for the facilitation
of calcification are still not well understood but geochemical proxies such as the boron
isotope give us potential windows into the current questions relating to the
calcification mechanism in coral and the relationship of the associated zooxanthellae
(48). The function and adaptability of these processes which drive calcification need
to be investigated in order to build a knowledge base on the effect ocean acidification
will have on different coral species and in all environments. For example, the 11B-
B/Ca records retrieved from the outer-core of the central Saudi Arabian Red Sea
suggest pHcf is indeed following a downward trend however, the intra-seasonal and
interdecadal variability confirms strong biological control of the corals internal
environment in relation to the external environment. Incorporating this variability
into modelling analysis may uncover more insightful predictions on the effects of
ocean acidification and temperature increases. The fact that calcification in this same
Red Sea coral has increased over the last few decades (except during abrupt thermal
stress event of the extreme 1998 El Niño) suggest temperature has an important role
in the calcification process in line with known kinetic effects (45, 111). Again the
question remains is there a tipping point in the capabilities of carbonate chemistry
regulation in these corals and will multi-stressor effects create environments
unsustainable for coral growth and or recovery from the increasing prevalence and
intensity of abrupt warming events.
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Evidence for the negative impact of rising sea temperatures on coral health and
survival has been well-documented for more than 20 years (232) although variations
in the severity of bleaching events are evident (233). This is especially true of abrupt
warming events such as those bought on by ENSO events as discussed in chapters 3
and 4. The prolonged hiatus in coral calcification and trace element incorporation
observed in these coral due to heat stress indicates the severity of these events on
coral reef health. The eventual recovery in growth of these coral however, suggest
even under the most severe and prolonged level of heat stress some coral have the
ability to recover and survive. The full extent of recovery (i.e. impacts on reproductive
health) still needs to be fully assessed to understand the true nature and long-term
effects of abrupt heat stress events on the coral ecosystem as a whole. A similar
pattern in trace element and growth disruption stemming from heat stress has been
observed in corals from the Great Barrier Reef (Clarke; unpublished and D’Olivio;
unpublished) and the Indian Ocean (234). As the cores from the Red Sea took 3-4 years
to recover this would indicate these corals are at their thermal tolerance limit during
the summer months and indeed corals from the Red Sea exhibit higher growth rates
during cooler spring months (187). As the intensity and frequency of El Niño/La Niña
events are likely to increase in the future, monitoring of corals worldwide during these
periods is essential as is further investigations into how the incorporation of
geochemical tracers are affected during these times and may become a valuable tool
in assessing past stress events where instrumental data is not available. The notable
calibration shift in the trace element-SST relationship observed here and in other
studies (77, 234) reflects the need for caution to be applied to using trace elements
in coral as SST proxies. These calibration shifts however can provide interesting
120
insights into large scale stress events and global climate phenomena that have
compromised the coral and provide further insights into coral physiological
mechanisms. Promising results using two similar elements (i.e. Li and Mg) to negate
the biological influence occurring during trace element incorporation should be fully
explored in a range of species.
To extend the work accounted in this thesis additional coral cores’ from the central
Red Sea region should be assessed to fully explore any differences in the calibration
of TE-SST. In order to ‘ground-truth’ trace element proxies and assess links in
environmental stress and physiological changes, there is a need for continuous
environmental in situ data gathering, which is currently only being undertaken in
some regions. Therefore, long-term monitoring programme are required to address
these needs.
Coral reefs are experiencing unprecedented rates of environmental change and
though some resilience appears to be occurring in some regions other reefs are facing
growing uncertainty. Mass coral bleaching events in response to high SST have
occurred recently (2015-16)
(http://www.coralreefwatch.noaa.gov/satellite/index.php) due to strong ENSO
conditions and are set to become more prevalent (235). Some corals may be able to
adapt to the predicted levels of reduced pH if the natural variability of the reef negates
these changes and corals have already been exposed to relatively low pH conditions.
This however may rely on other conditions of the reef to remain healthy. Pressures
associated with pollution, fishing and development may reduce the ability of corals to
adapt and/or recover from the impacts of climate change. Therefore, strategic
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local/regional solutions to these pressures must also be a priority along with tackling
global climate change in order to give coral reefs the best chance of maintaining
equilibrium. The problem of climate change is one that is global and often complex
but ultimately the goal needs to be reducing CO2 emissions in favour of clean energy,
which can only be done through investment of new emerging technologies.
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Appendices
Chapter 2: Supporting Information
Figure S2-1 Heron Island location map positioned off the coast of Queensland, East
Coast of Australia (insert). Main map shows the position of FOCE 2010 experiment
(red circle cross) in relation to the island (in black box). Location of NOAA mooring
(Harry’s Bommie) which provided temperature and CO2 data. Data available from
http://cdiac.ornl.gov/oceans/Moorings/Heron_Island.html. Also shown are mooring
locations for sea surface temperature loggers: AIMS logger HERFL1
(http://data.aims.gov.au/metadataviewer/faces/view.xhtml?uuid=446a0e73-7c30-
4712-9ddb-ba1fc29b8b9a) and IMOS Heron Island sensor float
(http://data.aims.gov.au/metadataviewer/faces/view.xhtml?uuid=20eaf8a6-817a-
450d-9a36-deb980d2daab).
123
Figure S2-2 Phase-averaged diurnal cycles of (a) ambient pH and (b) ambient water
temperature (oC) in summer (red) and winter (blue) and at Heron Island Lagoon in
early summer (mid-Nov to mid-Dec, blue) and winter (June, red). The error bars
correspond to ±1 standard error.
124
Figure S2-3. Image showing the location where high-resolution 0.5mg trace element
(red) and 2.5mg boron isotope (blue) sub-samples were collected along the primary
growth axis. Samples for trace element analysis were milled from one half of each
sliced nubbin whereas samples for boron isotope analysis were milled from the
opposing half.
125
Figure S2-4. Strontium/Calcium ratios measured from control (blue) and treatment
(red) colonies plotted against distance sample was taken from below the tissue zone.
Right y-axis is SST derived from Heron Island ambient SST calibrated against Sr/Ca
measurements. Black arrow indicates start of experiment (June sampling ambient),
red arrow indicates highest Sr/Ca (lowest SST), green arrow indicates mid-August
sampling (-0.15 pH unit offset, treatments only), yellow arrow indicates September to
beginning of October sampling (-0.25 pH unit offset, treatments only). Final sample
126
(November, when all flumes returned to ambient) taken from below tissue zone
(indicated by green vertical band).
Figure S2-5 (a). Representative diagram showing position of milled samples, exploring
boron isotopic composition within single nubbins. Nubbins sampled at three or four
points along the most recent growth horizon (November 2010). 0 represents summer
growth at central growth axis, off-axis samples taken from left (-1,-2) and right (1, 2)
of growth axis. (b). Boron isotopic composition of two representative nubbins
comparing the axis and off-axis samples to the right and left of the central growth axis
shown in (a).
127
Figure S2-6 (a) measured δ11B composition from 5mg samples from treatment and
control flumes versus the average measured pH seawater conditions at June and
November 2010 within control and treatment flumes during the FOCE experiment.
The boron isotopic compositions of all samples are elevated relative to the abiotic
curve from Klochko et al.2006 (b) pH of the calcifying fluid (pHcf) derived from the δ11B
shown in (a) using eq. 2 (see methods in main manuscript).
128
Figure S2-7 (a) measured δ11B composition of all colonies (2.75mg samples) from
treatment and control flumes versus the average measured pH seawater conditions
at June, August, September and November 2010 within control and treatment flumes
during the FOCE experiment. The boron isotopic compositions of all samples are
elevated relative to the abiotic curve from Klochko et al. 2006 (b) pH of the calcifying
fluid (pHcf) derived from the δ11B shown in (a) using eq. 2 (see methods). Plots as in
figure 2 of main manuscript but connecting lines track pHcf of the same nubbin over
the course of the experiment.
129
Table S2-1. Summary of the key environmental water quality parameters in the FOCE
flumes and ambient waters of Heron Island during periods corresponding to when
boron isotope measurements were made. Data on ambient temperature (T), salinity
(S), and total alkalinity (TA in µeq kg-1) are taken from Kline et al. 2015 describing
ambient seawater conditions during the experiment. All pH data shown were the
result of direct measurements within and outside FOCE flume systems over the course
of the study.
Month T (oC) Salinity (‰) TA pH
Ambient
June 22.0 (±0.02) 35.0 (±0.06) 2239 (±22) 8.24 (±0.002)
August 21.7 (±0.03) 35.4 (±0.07) 2251 (±23) 8.13 (±0.002)
September 22.4 (±0.02) 35.3 (±0.01) 2253 (±37) 8.02 (±0.002)
November 24.3 (±0.33) 34.6 (±0.01) 2226 (±32) 8.11 (±0.023)
Control
June 22.0 (±0.02) 35.0 (±0.06) 2239 (±22) 8.24 (±0.002)
August 21.7 (±0.03) 35.4 (±0.07) 2251 (±23) 8.11 (±0.002)
September 22.4 (±0.02) 35.3 (±0.01) 2253 (±37) 8.04 (±0.002)
November 24.3 (±0.33) 34.6 (±0.01) 2226 (±32) 8.11 (±0.016)
Treatment
June 22.0 (±0.02) 35.0 (±0.06) 2239 (±22) 8.24(±0.001)
August 21.7 (±0.03) 35.4 (±0.07) 2251 (±23) 7.95 (±0.002)
September 22.4 (±0.02) 35.3 (±0.01) 2253 (±37) 7.74(±0.002)
November 24.3 (±0.33) 34.6 (±0.01) 2226 (±32) 8.10 (±0.020)
130
Table S2-2. Physiological data from P. cylindrica nubbins used to calculate calcification
rates including the weight each nubbin in air (dry weight g) and in water (wet weight
g), the density of each coral nubbin calculated from the buoyant weights according to
Spencer Davies, 1989. The extension rate (cm-2 yr-1) as derived from the seasonal
maxima in Sr/Ca ratio (Figure S4), and the estimated calcification rate (g cm-3 yr-1).
The estimated errors for each recorded weight and the resulting calculated densities
are ±0.01 g and ±0.02 g cm-3, respectively. The individual errors in estimates of
extension rates and calcification rates are shown in parentheses.
FOCE dry weight
(g) wet weight
(g) Coral density
(±0.02)
ext. rate (cm-2 yr-1)
(±0.24)
Growth rate (g cm-3 yr-1)
Control
7.48 0.14 1.02 1.45 1.47 (±0.31)
2.44 0.20 1.08 1.69 1.83 (±0.28)
4.28 0.43 1.11 1.45 1.60 (±0.28)
3.63 0.55 1.18 1.08 1.28 (±0.29)
1.05 0.15 1.16 1.45 1.68 (±0.28)
Treatment
4.93 0.23 1.05 1.08 1.13 (±0.31)
3.49 0.14 1.04 1.32 1.38 (±0.32)
1.21 0.18 1.17 1.57 1.84 (±0.30)
2.38 0.14 1.06 1.57 1.66 (±0.27)
5.73 0.40 1.07 1.20 1.29 (±0.29)
131
Table S2-3. Results from a linear mixed effects model of pHcf versus pHsw grouping
measurements by colony including the best-fit slope cf swpH pH and intercept
due to fixed effects of pHsw on pHcf as well as the standard error in each parameter
estimate (Std. Err.), the significance of the calculated parameter value (p), and the
random effects attributed to each colony in units of pH (Rand. eff.). Preliminary
analysis revealed that the variance due to random effects associated with the
intercept (7.85 in pH units) were several orders of magnitude higher than the variance
associated with the random effects in the slope of pHcf versus pHsw. Therefore,
random effects associated with the slope were excluded from the mixed effects
model. Also shown to the right of the table are the total degrees of freedom (df) as
well as the root mean square error in model predictions based on individual colonies
(rmsei) and the whole population (rmsep). We had originally wanted to include time
as an independent fixed predictor given that the experiment lasted ~6 months from
mid-winter to early summer. However, there was a significant correlation between
pHsw and time due to the seasonal changes in pHsw as well as the progressive decline
in FOCE treatment pHsw over the course of the experimental period (r = -0.41, p <
0.01, n = 40); thus, preventing time from being considered an independent factor.
Nonetheless, when we ran a linear mixed model including time as a separate fixed
factor, the best-fit slope of changes in pHcf versus pHsw was only modestly higher, or
0.107 versus 0.067. An F-test showed that the residual variance in model predictions
was not significantly lower when including time as a separate factor versus not
including time (F35,36 = 1.05, p = 0.46); thus, giving us further cause to exclude time as
a separate independent factor.
132
Slope intercept colony Rand. eff.
Best fixed 0.067 7.95 B -0.014 df 36
Std. Err. 0.046 0.37 G 0.003 rmsei 0.047
p 0.078 <0.001 R -0.013 rmsep 0.054
W 0.025
133
Dataset S2-1. Strontium to Calcium ratios (Sr/Ca) from P. cylindrica nubbins from both
treatment and control FOCE flumes. Samples were taken along the primary growth
axis at a resolution of ~3 weeks. Standard errors are reported in parentheses where
replicate measurements were made.
Lab No.
Ref
Colony
Treatment/Control
Distance milled (mm below tissue zone)
Sr/Ca (±SEM)
E166 W2424 W Treatment 1 9.22
E167 W2424 W Treatment 2.5 9.33
E168 W2424 W Treatment 4.5 9.47
E169 W2424 W Treatment 5.5 9.47
E170 W2424 W Treatment 6.5 9.39
E171 W2424 W Treatment 7 9.39
E172 W2424 W Treatment 8 9.33 (0.03)
E977 W2427 W Control 1 9.18
E978 W2427 W Control 3 9.33
E979 W2427 W Control 4 9.33
E980 W2427 W Control 5 9.46
E981 W2427 W Control 6 9.47
E982 W2427 W Control 7.5 9.35 (0.04)
E937 W2425 W Treatment 1.25 9.30
E938 W2425 W Treatment 3.5 9.39
E939 W2425 W Treatment 5 9.52
E940 W2425 W Treatment 6.5 9.55
E941 W2425 W Treatment 8 9.54
E942 W2425 W Treatment 9.25 9.41(0.04)
E187 B0948 B Treatment 1.25 9.34
E188 B0948 B Treatment 3 9.23
E189 B0948 B Treatment 4.5 9.22
E190 B0948 B Treatment 5.5 9.35 (0.03)
E212 R0219 R Control 1.25 9.22
E213 R0219 R Control 3.5 9.35
E214 R0219 R Control 6.5 9.44
E215 R0219 R Control 7 9.45
E216 R0219 R Control 8.5 9.40
E217 R0219 R Control 9.5 9.35
E218 R0219 R Control 10 9.22(0.04)
D719 G1716 G Control 1.25 9.47
D720 G1716 G Control 3 9.40
D721 G1716 G Control 4.5 9.46
D722 G1716 G Control 6 9.47
D723 G1716 G Control 7.5 9.39
D724 G1716 G Control 8.5 9.34
D725 G1716 G Control 9.5 9.45
B991 R0220 R Treatment 1.25 9.32
B992 R0220 R Treatment 2.5 9.46
B996 R0220 R Treatment 3.5 9.51
B997 R0220 R Treatment 5 9.59
B998 R0220 R Treatment 6.5 9.56
134
Dataset S2-2. Record of measured boron isotope compositions for each month
sampled from each colony. Also shown are boron isotope derived pH values (pHcf).
Lab. No. Reference Colony
Month Flume ᵟ11B
(carb) (‰)
ᵟ11B-derived pH
F808 B0948 B November (rep) Treatment 23.89 8.47
F807 B0948 B June (rep) Treatment 24.83 8.58
I489 B0948 B November Treatment 23.29 8.48
I519 B0948 B September Treatment 23.02 8.50
I495 B0948 B August Treatment 22.49 8.41
I496 B0948 B June Treatment 23.54 8.45
I506 B1379 B November Control 22.53 8.41
I524 B1379 B September Control 24.04 8.51
I507 B1379 B August Control 24.46 8.56
I508 B1379 B June Control 22.42 8.42
G477 G1716 G November (rep) Control 24.77 8.53
G478 G1716 G June (rep) Control 22.51 8.43
I503 G1716 G November Control 24.47 8.54
I523 G1716 G September Control 24.00 8.51
I504 G1716 G August Control 23.37 8.49
I505 G1716 G June Control 24.09 8.53
G476 G1722 G November (rep) Treatment 23.46 8.44
G479 G1722 G June (rep) Treatment 23.02 8.46
I490 G1722 G November Treatment 24.10 8.52
I520 G1722 G September Treatment 22.65 8.42
I497 G1722 G August Treatment 23.45 8.49
I498 G1722 G June Treatment 23.17 8.47
G488 R0216 R November Control 23.71 8.46
G489 R0216 R June (rep) Control 24.57 8.56
I515 R0216 R November Control 23.05 8.45
I527 R0216 R September Control 23.12 8.47
I517 R0216 R August Control 23.21 8.48
I516 R0216 R June Control 24.82 8.56
G480 R0220 R November (rep) Treatment 24.49 8.51
G485 R0220 R June (rep) Treatment 23.68 8.51
I492 R0220 R November Treatment 24.01 8.51
I522 R0220 R September Treatment 22.37 8.40
I501 R0220 R August Treatment 23.08 8.47
I502 R0220 R June Treatment 22.30 8.42
I509 W2422 W November Control 24.59 8.55
I525 W2422 W Sep Control 22.85 8.44
I510 W2422 W August Control 23.86 8.52
I511 W2422 W June Control 23.60 8.50
F810 w2424 W November (rep) Treatment 25.92 8.65
135
F809 w2424 W June (rep) Treatment 25.42 8.57
I491 W2424 W November Treatment 25.13 8.58
I521 W2424 W September Treatment 22.59 8.42
I499 W2424 W June Treatment 23.80 8.51
I500 W2424 W August Treatment 23.79 8.51
F806 W2425 W November (rep) Treatment 24.30 8.50
F805 W2425 W June (rep) Treatment 24.97 8.54
I488 W2425 W November Treatment 24.77 8.56
I529 W2425 W September Treatment 24.73 8.56
I493 W2425 W August Treatment 24.81 8.58
I494 W2425 W June Treatment 24.38 8.55
F808 W2427 W November (rep) Control 25.74 8.64
F807 W2427 W June (rep) Control 25.19 8.55
I512 w2427 W November Control 23.95 8.53
I526 w2427 W September Control 24.63 8.48
I527 w2427 W August Control 24.55 8.56
I514 w2427 W June Control 23.58 8.55
Mean 23.88 8.50
136
Chapter 3: Supporting Information
S3-1 Figure. Comparison of offshore reef temperature (blue) and nearshore reef (black) daily
mean temperature data over 12 months (mid Sep 2012 to mid Sep 2013). Temperature data
distilled from 1-hour reef sampling data provided by KAUST (187).
S3-1 Table 230Th/232Th data table for two points along each of the inner and outer core
UWA Code SampleSampling
date
Date of
chemistry230Th (nmol/g)232Th (nmol/g) 238U (nmol/g) Utot (ppm) (230Th/238U)
234U(0)(230Th/232Th)
initial (1)
(230Th/232Th)
initial (2)
Calendar Age
(kyr)
uncorrected
Calendar Age
corrected (1)
Calendar Age
corrected (2)
R2-2 Red Sea-inner 2006.1 2016.5 2.217E-08 1.45E-04 10.101 0.002 2.422 0.0001297 5.3E-06 28 145.40 0.77 4.4 4.4 2004.18 2006.10 2006.10
R2-4 Red Sea-inner 1998.1 2016.5 3.692E-08 1.51E-04 11.866 0.004 2.845 0.0001839 3.9E-06 45 145.42 0.77 4.4 0.0 1999.03 2000.73 1999.03
R4-2 Red Sea -outer 2003.1 2016.5 2.772E-08 3.97E-04 8.914 0.007 2.138 0.0001838 7.6E-06 13 145.12 0.77 3.1 3.1 1999.04 2003.22 2003.22
R4-4 Red Sea -outer 1998.1 2016.5 3.983E-08 3.52E-04 9.103 0.001 2.183 0.0002587 6.7E-06 21 145.16 0.77 3.1 5.2 1991.93 1995.57 1998.03
137
S3-2 Table. Comparison of annual linear extension rates (cm yr-1) between the three
time periods divided by 1998 El Niño event and ENSO affected years. p-values
represent results from Kruskal-Wallis One Way Analysis of Variance on Ranks pairwise
multiple comparison (Holm-Sidak).
Outer-shelf
Comparison
Diff of
Means t p
Post vs. ENSO affected 7.244 9.775 <0.001
Pre vs. ENSO affected 6.313 7.414 <0.001
Pre vs. Post 0.932 1.361 0.191
Inner-shelf
Comparison
Diff of
Means t p
Post vs. ENSO affected 4.688 6.928 <0.001
Pre vs. ENSO affected 5.625 7.026 <0.001
Pre vs. Post 0.938 1.549 0.141
138
S3-3 Table. Linear regression statistics for the trace element comparisons: coefficient of
determination (r2), slope and intercept and p value for inner-shelf core, Abu Shosha (top) and
outer-shelf core (Shi’b Nazar). Trace element ratios divided into time intervals, pre 1998 ENSO
years, post ENSO affected, and ENSO affected years.
Inner-shelf Pre-ENSO ENSO+ Post-ENSO+
r2
slope
y-int
p value r2
slope
y-int
p value r2
slope
y-int
p value
Sr/Ca vs. Li/Mg
0.89 -6.62 0.9 <0.001 0.8 -6.2 0.9 <0.001 0.8 -7.1 0.9 <0.001
Sr/Ca vs. Mg/Ca
0.47 11.7 -0.8 <0.001 0.3 10.0
-0.7 0.249 0.4 12.3
-0.9 <0.001
Sr/Ca vs. U/Ca 0.77 -1.96 0.3 <0.001 0.4 1.2 -0.0 0.307 0.4 -4.1 0.6 0.002
Li/Mg vs. U/Ca 0.28 0.52 0.4 <0.001 0.1 0.5 0.3 0.434 0.5 0.6 0.3 <0.001
Mg/Ca vs. Li/Mg
0.52 5.61 -0.9 <0.001 0.3 5.2 -0.6 0.877 0.4 5.7 -0.8 <0.001
U/Ca vs. Mg/Ca
0.32 5.16 -0.8 <0.001 0.0 4.6 -0.1 0.251 0.6 7.1 -2.5 <0.001
Outer-shelf Pre-ENSO ENSO+ Post-ENSO+
r2
slope
y-int
p value r2
slope
y-int
p value r2
slope
y-int
p value
Sr/Ca vs. Li/Mg
0.96 -11.4 1.4 <0.001 0.6 -6.9 0.9 0.002 0.8 9.6 1.2 <0.001
Sr/Ca vs Mg/Ca
0.56 -30.6 2.8 <0.001 0.7 -34 3.3 <0.001 0.8 39.4
3.9 <0.001
Sr/Ca vs U/Ca 0.86 -3.85 0.5 <0.001 0.6 -4.2 0.6 <0.001 0.4 2.3 0.4 <0.001
Li/Mg vs U/Ca 0.88 -0.48 0.3 <0.001 0.5 0.4 0.4 0.064 0.4 0.6 0.3 <0.001
Mg/Ca vs Li/Mg
0.91 -8.58 2.7 <0.001 0.6 8.6 2.8 0.013 0.4 8.2 2.3 <0.001
U/Ca vs Mg/Ca
0.94 -1.63 0.1 <0.001 0.6 1.6 0.1 <0.001 0.7 1.6 0.1 <0.001
139
Chapter 4: Supporting Information
Fig. S4-1. Long-term records of (a) atmospheric CO2 concentrations data calculated from Law
Dome Ice core data (blue) and Mauna Loa data (red) (b) Seawater pH (pHsw) over time (1940
– 2012) calculated from atmospheric records using CO2SYS. Average seasonal total alkalinity
data from recent in situ measurements taken at Shi’b Nazar reef.
140
Fig. S4-2 Atmospheric CO2 emissions from 1992-2012 with regression and 95%
confidence interval. Data downloaded from Mauna Loa monthly mean data, available
at http://www.esrl.noaa.gov/gmd/ccgg/trends/.
141
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142
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