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Nearshore seagrass and reef condition in Yankalilla Bay
Report to the Adelaide and Mount Lofty Ranges Natural Resources
Management Board
Simon Bryars
2014
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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This publication should be cited as:
Bryars, S. (2014) Nearshore seagrass and reef condition in Yankalilla Bay. Report to the Adelaide and
Mount Lofty Ranges Natural Resources Management Board. Dr Simon Richard Bryars, Adelaide.
Cover photo: Seagrass habitat (right of picture) with erosion scarp adjacent to bare rock and algal
habitat (left of picture) off Normanville Beach. Photo: S. Bryars, November 2013.
Disclaimer
The findings and opinions expressed in this publication are those of the author and do not necessarily reflect those of the Adelaide and Mount Lofty Ranges Natural Resources Management Board. While reasonable efforts have been made to ensure the contents of this report are factually correct, the Adelaide and Mount Lofty Ranges Natural Resources Management Board and the author do not accept responsibility for the accuracy and completeness of the contents. The author does not accept liability for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this report.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Acknowledgements The Adelaide and Mount Lofty Ranges Natural Resources Management Board provided funding for
the study.
Thanks to Tony Flaherty (AMLRNRM, Manager, Coast and Marine Services) for his continued support
of marine investigations in Yankalilla Bay.
Thanks to James Brook (J Diversity Pty Ltd) for field assistance, technical support and useful feedback
on the project.
Thank you to the many people who provided their local knowledge of Yankalilla Bay, including John
Croser, James Brook, David Muirhead, Darcy Wedd, Steve Roberts, Graham McAinsh, and Ian James.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Contents Acknowledgements ................................................................................................................................. 3
Non-technical summary .......................................................................................................................... 5
Introduction ............................................................................................................................................ 6
Methods .................................................................................................................................................. 9
Kayak survey ....................................................................................................................................... 9
Diver survey ...................................................................................................................................... 11
Desktop survey .................................................................................................................................. 11
Threat assessment ............................................................................................................................ 11
Results and discussion .......................................................................................................................... 13
Kayak survey ..................................................................................................................................... 13
Diver survey ...................................................................................................................................... 22
Desktop survey .................................................................................................................................. 28
Habitat surveys ............................................................................................................................. 28
Historical information ................................................................................................................... 28
General discussion ................................................................................................................................ 33
Threat assessment ................................................................................................................................ 37
Historical threats to seagrass ............................................................................................................ 37
Current threats to seagrass............................................................................................................... 37
Conclusions and recommendations ...................................................................................................... 38
References ............................................................................................................................................ 39
Appendix 1 ............................................................................................................................................ 40
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Non-technical summary Previous investigations had indicated that nearshore habitats within Yankalilla Bay may be
under stress with evidence of unhealthy seagrass and reef adjacent to Normanville.
An investigation was subsequently undertaken to examine broad-scale patterns of habitat
distribution across Yankalilla Bay and to examine fine-scale patterns of seagrass and reef
composition in relation to the three river discharges in Yankalilla Bay: the Bungala River,
Yankalilla River and Carrickalinga Creek. It was hypothesized that seagrass and reef condition
may be related to the three freshwater outflows.
Across the nearshore (<500m offshore) of Yankalilla Bay between Carrickalinga Creek and
Yankalilla River there was a mosaic of benthic habitats comprised of seagrass, reef and sand.
The composition of habitats was indicative of a dynamic system in which erosion of seagrass
meadows plays a major role.
Seagrass habitat was dominated by Posidonia angustifolia/sinuosa and Amphibolis
antarctica/griffithii. Reef habitat composition varied between sites but was indicative of
nutrient/sediment impacts adjacent to the Yankalilla River and Bungala River. Sand habitat
was comprised of an inshore strip of beach as well as sand holes amongst seagrass and reef
scattered across Yankalilla Bay.
Unhealthy Amphibolis was observed at numerous locations across Yankalilla Bay and it is
possible that selective historical loss of Amphibolis has occurred and is continuing to occur.
However, there was no evidence of selective loss of Amphibolis in relation to the three river
outflows with healthy Amphibolis present at all three river locations. Nonetheless, this
pattern does not preclude the river outflows as a cause of historical Amphibolis dieback as it
is possible that the outflows are affecting the entire inshore area from Yankalilla River to
Carrickalinga Creek. In addition, it is also possible that groundwater freshwater upwellings
are having an impact on seagrasses in Yankalilla Bay.
Blowouts and/or erosion scarps amongst seagrass meadows were common across the Bay
and in many locations (particularly off Normanville) the inshore edge of the seagrass
meadow had an erosion scarp with deeper water shoreward of the scarp.
In many cases the blowout or inshore area of seagrass appears to have eroded to the
underlying hard substrate which has then been colonised by macroalgae to form reef;
termed ‘emergent climax reef’.
Historical investigations suggest that habitat changes have occurred adjacent to the
Yankalilla River, the Bungala River and Carrickalinga Creek.
Further investigations are required to confirm that historical habitat changes have occurred
in Yankalilla Bay and that seagrass erosion is an ongoing issue.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Introduction Yankalilla Bay lies within the boundaries of the Adelaide and Mount Lofty Ranges Natural Resources
Management (AMLRNRM) region in the south-eastern part of Gulf St Vincent (Figure 1). Bryars
(2013) identified that three of the main catchments adjacent to Yankalilla Bay (viz. Yankalilla River,
Bungala River and Carrickalinga Creek, Figures 2, 3) posed a potential threat to the nearshore
seagrass and reef habitats. Brook and Bryars (2014) undertook reef condition assessments during
2012/13 of two sites within Yankalilla Bay; Normanville Beach and Carrickalinga Reef. The
Carrickalinga Reef site that was surveyed appeared in reasonable condition but anecdotal
observations of mussels and sedimentation on reef closer to shore were indicative of impacts from
nutrients and sediments. The Normanville Beach reef site had not been surveyed previously and the
presence of mussels and sedimentation was also indicative of a reef in poor condition. In addition to
the reef assessments, anecdotal observations of Amphibolis seagrass in poor condition at
Normanville Beach were made during November 2012 (S. Bryars, pers. obs.). It was then proposed to
the AMLRNRM Board that further targeted surveys of the condition of nearshore seagrass and reefs
in Yankalilla Bay were warranted. The aim of the current report was to investigate the condition and
distribution of inshore habitats in the Yankalilla Bay region and to specifically look for:
Evidence of a correlation between reef/seagrass condition and the three river discharges.
Evidence of selective loss of Amphibolis adjacent to the three river discharges.
Evidence of inshore loss of seagrass at Normanville.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 1. Map showing the Adelaide and Mount Lofty Ranges Natural Resources Management
(AMLRNRM) region with the focus area of Yankalilla Bay including adjacent catchments
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 2. The three main catchments discharging freshwater into Yankalilla Bay between Lady Bay
and Carrickalinga Head. Photos: S. Bryars.
Yankalilla River
Bungala River
Carrickalinga Creek
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Methods
Kayak survey A total of 97 preset sites were distributed across the survey area which extended from the area
adjacent to Yankalilla Bay in the south to the area adjacent to Carrickalinga Creek in the north
(Figure 3) (note that a few of the inshore preset sites had to be moved slightly offshore as they were
too shallow to sample). The density of sites was greater adjacent to the three rivers to enable
detection of patterns possibly related to freshwater outflows (Figure 3).
At each site, underwater video footage of the seabed was recorded to determine the broad-scale
distribution of seagrass, reef and sand habitats across the inshore (<500m from low tide) of
Yankalilla Bay. While surveying the preset sites, observations were also made for the presence of
unhealthy Amphibolis (plants with few or no leaves and plants with heavy epiphyte cover) and
erosion scarps on the edge of seagrass meadows; these occurrences were documented for the
preset sites and also for other sites during transitions between the preset sites. A high definition
video camera oriented horizontally to the seabed was used for collecting data. The camera set-up
consisted of a Panasonic LUMIX DMC-FT3 mounted inside a waterproof housing and which was set
to the widest lens angle with an INON 28mm wide angle lens attached to the housing. The
camera/housing was lowered on a frame from the surface (aboard a sea kayak – see below) using a
graduated rope (with 1m increments) to about 0.5–1m height above the seabed. The camera was
then drifted with the prevailing current for about 30 seconds which was estimated (using Track
Manager on a GARMIN GPSMAP78) to cover about 10–30 linear metres of the seabed depending on
sea conditions; although this was not precisely measured for every individual site. At the specified
height from the seabed, the camera had a field of view of approximately 2 x 1.5 m of the seabed.
The kayak survey consisted of two components: (1) broad-scale distribution of habitat, and (2)
distribution of seagrass erosion scarps off Normanville.
During November 2013, the 97 pre-sites were videoed in the manner described above from aboard a
4.1 m sit-on-top sea kayak (Wilderness Systems Tarpon 120) fitted with a GPS (GARMIN GPSMAP78)
loaded with pre-determined site coordinates. Operations were commenced at each site once a site
was located using the GPS. During each drift, the seabed and camera were monitored using an
underwater viewer; this enabled the height of the camera to be adjusted, for additional ‘live’ results
to be noted on a datasheet and for some seabed ‘scale’ to be gauged in relation to the known length
of the kayak (see Data Analysis below). The depth of the water was estimated using the graduated
rope to the nearest 0.5m. At the shallower sites (<3m depth) one to three still photographs oriented
directly downwards to the seabed from just below the surface were also recorded. The entire length
of video footage from each site was later viewed on a computer screen to document the occurrence
of four dominant habitat types: Amphibolis (including mixed Amphibolis/Posidonia), Posidonia
(including mixed Amphibolis/Posidonia), Macroalgae and Sand. Each habitat was scored only if there
was a continuous section of at least 1 linear metre of habitat.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 3. Map showing pre-set sites used in the kayak survey
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Pre-set sites
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Diver survey A diver survey was undertaken during December 2013 and March 2014 at a number of sites across
the area to characterise seagrass and reef composition in relation to the three river outlets; five sites
for seagrass (Yankalilla River, South Normanville, Bungala River, North Normanville, Carrickalinga
Creek) and four sites for reef (Yankalilla River, Bungala River, North Normanville, Carrickalinga Creek;
Figure 4, see Appendix 1 for coordinates) (Note that suitable reef could not be located at South
Normanville). The diver survey also enabled more detailed examination of erosion scarps, unhealthy
Amphibolis and other features. While an attempt was made to select sites that were comparable
from a physical perspective this was not possible with varying distance from shore (Figure 4) and
depth:
Yankalilla River (1–2m depth)
South Normanville (1m depth)
Bungala River (1–2m depth)
North Normanville (2m depth)
Carrickalinga Creek (3–4m depth)
At each site three 20m transect lines about 10-20 m apart were laid perpendicular to shore from the
inshore edge of the seagrass or reef habitat. A point-intercept (PIT) method was then used every
0.2m to characterise the habitat using a combination of life form and species categories. A photo
was also taken every 2.5m along the transect line as per Reef Life Survey methodology. PIT data
were used to calculate % cover of different life forms. Photos were used to provide visual records of
the different life forms, including unhealthy Amphibolis.
Desktop survey To assist with interpretation of results from the kayak and diver surveys, a desktop survey was
undertaken to gather further information on the current and historical condition/distribution of
inshore habitats in the Yankalilla Bay and Normanville areas. The desktop survey utilised previous
reports on marine habitats (Murray-Jones et al. 2009, Tanner et al. 2012), historical information and
imagery (including Collins 2010, www.mapfinder.sa.gov.au), and verbal interviews of various people
with historical anecdotes.
Threat assessment Based upon the outcomes from the kayak, diver and desktop surveys, an update of the Bryars (2013)
threat analysis was undertaken to inform future management decisions.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 4. Map showing sites used for the diver survey
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Dive sites
Yankalilla River
– Reef & Seagrass
North Normanville - Reef
Bungala River - Reef
Carrickalinga Creek - Reef
Carrickalinga Creek - Seagrass
Bungala River - Seagrass
North Normanville - Seagrass
South Normanville - Seagrass
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Results and discussion
Kayak survey The depth of sites surveyed varied from 0.5 to 6m (Figure 5). In general, there was a pattern of
increased depth from the south to the north for both the innermost and outermost sites (Figure 5).
The water adjacent to Carrickalinga Creek was the deepest in the surveyed area. The pattern of
depth correlates with wave energy across the survey area with larger waves reaching the shore in
the Carrickalinga area (author, pers. obs.). Between the Yankalilla River and the Bungala River there
were some very shallow areas close to shore such that some of the pre-sites had to be relocated
further offshore to enable data collection from the kayak. This shallow water also precluded having a
reef site in the area for the diver survey as the reef was essentially intertidal and too shallow to be
comparable with the other sites (see Diver survey later).
The survey area was characterised by a mosaic of seagrass, macroalgae and sand habitats. Seagrass
was present across the entire survey area (Figures 6, 7). Amphibolis (as dense Amphibolis and/or
Amphibolis/Posidonia meadows) was present across the survey area with no evidence of selective
loss adjacent to the three main rivers/creeks (Figure 6). Posidonia (as P. sinuosa and/or P.
angustifolia) was mainly present in the southern and central parts of the survey area and was rare in
the northern part adjacent to Carrickalinga Creek (Figure 7); this pattern may be related to wave
energy with Amphibolis being more resilient to wave energy than P. sinuosa/angustifolia.
Macroalgae was most prevalent in the central and northern parts of the survey area where hard
substrate was most available (Figure 8).
In general seagrass meadows appeared in good condition with a dense canopy cover and low to
moderate epiphyte load (note that epiphyte load was not quantified). However, there were several
locations where ‘unhealthy’ Amphibolis was present being characterised by bare stems or stems
with few leaves and sometimes high epiphyte loads also (Figures 9, 11). The spatial distribution of
unhealthy Amphibolis observations did not correlate with the three main discharge points;
observations were made at some sites in the central part of the study region at south Normanville
between the Yankalilla and Bungala Rivers and numerous sites just to the north of the Bungala River
at Normanville (Figure 9).
Noticeable erosion scarps (greater than about 0.3m) were most prevalent in the southern and
central parts of the area (Figures 10, 12). In the southern part these were usually associated with
blowouts filled with sand while in the central part adjacent to Normanville they were associated with
an inshore erosion scarp that defined the seagrass ‘blue-line’ (Figure 13) and also by blowouts that
were filled with sand and/or hard substrate colonised by macroalgae. Repeated traverses of the area
shoreward of the erosion scarp off Normanville revealed that it was dominated by macroalgae and
sand with an absence of seagrass (Figure 13; see also Diver Survey later and Brook and Bryars 2014).
The seagrass erosion scarp adjacent to the Bungala River was up to 150m offshore (Figure 13).
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 5. Depth of pre-set sites from the kayak survey across Yankalilla Bay (note that a few of the
inshore sites off Carrickalinga Creek are missing data)
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Depth (m)
0.500000 - 1.000000
1.000001 - 2.000000
2.000001 - 2.500000
2.500001 - 4.000000
4.000001 - 6.000000
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 6. Distribution of Amphibolis (as dense Amphibolis and/or dense mixed Amphibolis/Posidonia)
at pre-set survey sites across Yankalilla Bay
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Amphibolis present
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 7. Distribution of Posidonia (as dense Posidonia and/or dense mixed Amphibolis/Posidonia) at
pre-set survey sites across Yankalilla Bay
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Posidonia present
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 8. Distribution of macroalgae at pre-set survey sites across Yankalilla Bay
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Macroalgae present
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 9. Distribution of unhealthy Amphibolis at pre-set and additional survey sites across Yankalilla
Bay
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Unhealthy Amphibolis present
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 10. Distribution of erosion scarps at pre-set survey sites across Yankalilla Bay
/
0 0.5 1 1.5 20.25
km
Carrickalinga
Normanville
Carrickalinga Creek
Bungala River
Yankalilla River
Yankalilla Bay
Legend
Erosion scarp present
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 11. Unhealthy Amphibolis in 3m depth to the north of the Bungala River. Photo: S. Bryars, 26-
Nov-2013
Figure 12. Blowout with erosion scarp at the edge of a Posidonia meadow in 1m depth to the south
of the Yankalilla River. Photo: S. Bryars, 14-Nov-2013.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 13. Aerial photograph from 2010 of the Normanville foreshore area showing the seagrass
erosion scarp (marked by dots) as mapped using a sea kayak during November 2013. The 2010
imagery (rather than 2014 imagery) was used as it clearly shows the colour difference between the
inshore macroalgae habitat and the offshore seagrass habitat. (Aerial image: Google Earth)
Bungala River
Normanville jetty
Macroalgae
Macroalgae
Erosion scarp
Seagrass
Bare sand
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Diver survey Each of the five seagrass sites had a distinct species composition that was possibly related to depth,
wave exposure and the river outlets (Figure 14). The Yankalilla River site was dominated by
Amphibolis antarctica with significant amounts of unhealthy Amphibolis. The South Normanville site
was characterised by a mixture of Posidonia meadows (P. sinuosa/angustifolia) and A. antarctica
meadows, while the Bungala River site had mainly Posidonia (P. sinuosa/angustifolia). The North
Normanville site had a mixture of A. antarctica and A. griffithii with very little Posidonia, while the
Carrickalinga Creek site was predominantly A. griffithii. Of note was that Amphibolis was detected in
substantial amounts adjacent to all three of the river outlets and there was no clear pattern of
selective loss that might be related to proximity to each river outlet. The change in seagrass species
composition from south to north may be partly related to wave energy and depth with greatest
energy off Carrickalinga Creek; further investigations north of this area during the kayak survey
revealed patches of Posidonia coriacea which is known to be tolerant of higher wave energy than P.
sinuosa/angustifolia. However, it is possible that the inshore margin of the seagrass meadows off
the Bungala River has been affected by historical seagrass loss via offshore migration of an erosion
scarp offshore (see later).
Figure 14. Composition of seagrass meadows at the five sites sampled in Yankalilla Bay during the
diver survey.
Each of the four reef sites had a distinct species/group composition that was possibly related to the
profile of the reef, exposure to wave energy, and nutrients and sediments (Figure 15). The Yankalilla
River, Bungala River and South Normanville sites were all low profile reefs while the Carrickalinga
Creek site was a medium profile reef. Brown branching algae were most prevalent at the two
northernmost sites which have increased wave energy, while Ecklonia was only present at the
Carrickalinga Creek site where the reef has a medium profile and wave energy is greatest. Turfing
algae were prevalent at the Yankalilla River and Bungala River sites while mussels were also
prevalent at the Bungala River site; these life forms may be indicative of increased nutrients and/or
sediments.
0
20
40
60
80
100
120
T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3
Yankalilla River South Normanville
Bungala River North Normanville
Carrickalinga Creek
% c
ove
r
Amphibolis antarctica
Amphibolis griffithii
Amphibolis antarctica / A. griffithii
Amphibolis antarctica / Posidonia
Unhealthy Amphibolis
Posidonia
Brown branching algae
Sand
Mussels
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 15. Composition of reefs at the four sites sampled in Yankalilla Bay during the diver survey.
The diver surveys also allowed closer examination of seagrass and reef habitats. Figures 16 and 17
show a section of the erosion scarp off the Bungala River where Amphibolis seedlings have attached
to the exposed rhizome matte; clearly this habitat is undergoing some form of disturbance. Dense
epiphytes were noted in some places (e.g. Figures 18, 19) but did not appear to be related to
unhealthy seagrass. Unhealthy Amphibolis (as bare stems or stems with few leaves) was observed at
the Yankalilla River site (Figures 20, 21). Figures 22 and 23 show the brown branching algae and rock
typical of the reef habitat off the Bungala River.
0
10
20
30
40
50
60
70
80
90
100
T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3
Yankalilla River Bungala River North Normanville Carrickalinga Creek
% c
ove
r
Amphibolis antarctica
Brown branching algae
Brown understorey algae
Ecklonia
Heterozostera
Mussels
Posidonia
Red encrusting algae
Red foliose algae
Rock
Sand
Turfing algae
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 16. Erosion scarp off Bungala River showing a diver for scale and Amphibolis seedlings
colonising the exposed seagrass rhizome matte. Photo: S. Bryars, 2-Dec-2013.
Figure 17. Amphibolis seedlings colonising the exposed rhizome matte of an erosion scarp off
Bungala River. Photo: S. Bryars, 2-Dec-2013.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 18. Dense cover of epiphytes on Amphibolis griffithii in 3m depth off Carrickalinga Creek.
Photo: S. Bryars, 2-Dec-2013.
Figure 19. Dense cover of epiphytes on Amphibolis griffithii in 3m depth off Carrickalinga Creek.
Photo: S. Bryars, 2-Dec-2013.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 20. Unhealthy Amphibolis antarctica in 2m depth off Yankalilla River. Photo: S. Bryars, 4-Mar-
2014.
Figure 21. Unhealthy (left) versus healthy (right) Amphibolis antarctica in 2m depth off Yankalilla
River. Photo: S. Bryars, 4-Mar-2014.
Healthy Amphibolis
Unhealthy Amphibolis
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 22. Macroalgae reef habitat adjacent to the Bungala River. Photo: S. Bryars
Figure 23. Rock reef habitat adjacent to the Bungala River. Photo: S. Bryars
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Desktop survey
Habitat surveys
Murray-Jones et al. (2009) undertook a series of underwater video transect lines in Yankalilla Bay
and concluded that seagrasses generally appeared to be in good condition with little indication that
the local catchments were impacting on nearshore seagrasses. Tanner et al. (2012) reanalysed the
data from Murray-Jones et al. (2009) for epiphyte cover and found no correlation between epiphyte
cover and proximity to the mouth of either the Bungala River or Carrickalinga Creek. However, the
transect lines of Murray-Jones et al. (2009) only went to within about 300m of the shoreline and
they did not appear to reach the inshore seagrass blue-line. Thus they did not cover some of the
area included in the current survey.
Irving (2009) in a diver survey concluded that outflow from the Yankalilla River and Bungala River did
not appear to be affecting the reproduction, recruitment or growth of the seagrass, Amphibolis
antarctica, during 2009; although the rivers were apparently not flowing at the time of the study
(Bryars 2013).
A Reef Health survey conducted on the subtidal reef in 5m depth at Carrickalinga in 2005 rated the
condition as ‘Good’ with no evidence of sedimentation (Turner et al. 2007). A more recent survey of
Carrickalinga Reef in 2012/13 indicated a similar condition to 2005 (Brook and Bryars 2014).
However, these surveys were conducted further offshore than the current survey.
Historical information
From the accounts in Collins (2010) it is apparent that the inshore area of Normanville has been
subjected to some level of physical disturbance from the construction (and subsequent destruction)
of three different jetties. The first jetty was built in the 1850s just to the north of the Bungala River
mouth but it was soon destroyed by flood waters from the Bungala River when severe erosion of the
seabed occurred (Collins 2010). The second jetty was also built in the 1850s but details of its exact
location and demise appear less clear, but it no longer remains. The third jetty was constructed in
the 1860s to the north of the Bungala River mouth and the shoreward section (approximately half of
its original length of 128m) still remains today (Collins 2010). Unfortunately there is no mention of
seagrass habitat in the historical accounts of jetty construction for the Normanville area (Neville
Collins, pers. comm.). Interviews with people who frequented the area as children suggest that
seagrass once existed at the end of the third jetty when it was full length (circa. 1940s) and that
seagrass once extended further inshore than it currently does in the area between the existing jetty
and the Bungala River.
Short (2001) in his book on beaches of SA reported that “seagrass grows to within 50 m of the
shoreline” at Normanville Beach and provided a habitat map showing a continuous seagrass
meadow between Lady Bay and Haycock Point to the north of Carrickalinga Creek. However, it is
unclear where (and when) this habitat information was gathered from, but it is apparent from the
present study that this area is currently mainly a mixture of seagrass and reef habitats.
Comparison of historical and contemporary aerial photos indicates that there may once have been
seagrass closer to shore in the area adjacent to the end of the old jetty and to the south of the jetty
(Figures 24, 25). The 1949 aerial image indicates fragmented inshore habitat adjacent to the Bungala
River that may have contained seagrass (dark patches) and seagrass adjacent to the end of the jetty
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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(dark patch, Figure 24). However, as of 1949 there was clearly a large area of bare sand around the
jetty structure and no evidence that seagrass was growing around the majority of the jetty structure
at that time. Nonetheless, the 1949 imagery is inconclusive and a proper remote sensing
investigation is required to determine historical habitat distributions.
Comparison of historical and contemporary aerial photos from the areas adjacent to the Yankalilla
River and Carrickalinga Creek indicate that there may have been some habitat changes since 1949
(Figures 26, 27), but again a proper remote sensing investigation is required to determine if habitat
changes have indeed occurred.
Figure 24. Aerial photograph from 5-April-1949 of the Normanville foreshore area showing the jetty
when most of the seaward half was intact. The darkest areas indicated are possibly seagrass, while
the ‘fragmented habitat’ is possibly a mixture of seagrass (darkest patches), sand (lightest patches)
and other habitats (macroalgae on rock). (Aerial image: Department of Environment, Water and
Natural Resources)
Fragmented habitat
Possibly seagrass
Bungala River
Normanville jetty
Possibly seagrass
Possibly seagrass
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 25. Aerial image from 2010 showing the original length of the Normanville jetty and various
habitat features identified from the present study. The dots indicate the edge of the erosion scarp as
mapped from the kayak survey. Note that if the original jetty were still present it would now lie on
bare sand with the end of the jetty barely touching an area covered with macroalgae. (Aerial image:
Google Earth)
Erosion scarp
Original length of Normanville jetty
Macroalgae
Seagrass
Bare sand
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
31
Figure 26. 1949 and 2010 aerial images of the area adjacent to Yankalilla River. The possible habitat
change indicated is related to an area that appears to have an increase of sand (light shade)
between 1949 and 2010. (1949 aerial image: Department of Environment, Water and Natural
Resources, 2010 aerial image: Google Earth)
Possible habitat change
20101949
Yankalilla RiverYankalilla River
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
32
Figure 27. 1949 and 2010 aerial images of the area adjacent to Carrickalinga Creek. The possible
habitat change indicated is related to an area that appears to have an increase of sand (light shade)
between 1949 and 2010. (1949 aerial image: Department of Environment, Water and Natural
Resources, 2010 aerial image: Google Earth)
Possible habitat change
20101949
Carrickalinga Creek
Carrickalinga Creek
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
33
General discussion Current patterns of seagrass, reef, sand and blowouts across Yankalilla Bay suggest a dynamic
system in which erosion and seagrass loss play a key role. It is proposed that the current pattern of
habitats has been affected in some part by the following sequence of events: Healthy seagrass
meadow (= original habitat state) Seagrass dieback Erosion of underlying rhizome matte and
sand Exposure of hard substrate (= transitional habitat state) and deepening of seabed
Colonisation of hard substrate by macroalgae and other reef biota (= final habitat state) (see Figures
28–32). The final habitat state is referred to here as ‘emergent climax reef’ to indicate that the reef
emerged from a major transition and that it is the final state in the transition. The proposed process
implies that some factor (possibly anthropogenic such as polluted catchment water) caused the
seagrass to dieback prior to erosion (e.g. Figure 32), however, it appears that erosion can occur in a
healthy seagrass meadow even without seagrass dieback as severe erosion was observed in
apparently healthy seagrass meadows (Figure 31). The fact that emergent climax reef is not a
‘natural’ habitat for a particular location has implications for coastal management decisions and
assessing conservation value. Due to the nature of the change, it would also be virtually impossible
to reverse the transformation from emergent climax reef back to the original habitat state of a
seagrass meadow.
The case for emergent climax reef appears most compelling for the inshore area adjacent to
Normanville where there is a clear erosion scarp that delineates the offshore seagrass meadows
(blue-line) from inshore macroalgal-covered low profile reef with a thin band of bare reef in
between (Figures 28, 30). The bare reef appears to be the transitional habitat before becoming
colonised by macroalgae to become emergent climax reef. In the case of Normanville the seagrass
blue-line is up to 150 m offshore but it is possible that it was originally much closer to shore (see
earlier). For example, the diver survey site to the south of the Bungala River (South Normanville)
may be representative of what conditions were once like adjacent to the Bungala River; the blue-line
is very close to shore (around 20–30 m from low tide mark) and the depth is shallow (around 1m).
The apparent nearshore seagrass loss at Normanville aligns with the model for parts of the Adelaide
coast (e.g. off Somerton Beach) where there has been a transformation from seagrass meadow to
low profile macroalgal-covered reef (see Bryars 2013). In addition to Normanville, there are
significant sections of inshore low-profile reef across Yankalilla Bay and also in areas offshore
associated with blowouts within seagrass meadows. It is possible that some of these areas of reef
have arisen due to seagrass and substrate loss.
Current patterns of seagrass distribution across Yankalilla Bay are not indicative of selective loss of
Amphibolis linked to the three river discharges as healthy Amphibolis was observed adjacent to all
three rivers. However, a number of observations suggest that it is possible that loss of Amphibolis
has occurred (and is continuing to occur) on a Bay-wide scale:
The current patterns of seagrass distribution adjacent to the rivers could in fact be a result
of historical processes whereby seagrass from further inshore has already been lost (as
appears to have occurred at Normanville).
Unhealthy Amphibolis (but not unhealthy Posidonia) was observed at a number of locations
across the Bay including adjacent to the Yankalilla River and Bungala River.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
34
At several locations unhealthy Amphibolis was observed at the edge of blowouts which is
suggestive that Amphibolis had already been lost from the areas where the blowouts
occurred (e.g. Figure 32).
Figure 28. Apparent transition of habitat types adjacent to the Bungala River with healthy seagrass
to the right (seaward direction) an erosion scarp with adjacent bare rock and rock colonised by
macroalgae to the left (landward direction).
Erosion scarp
Healthy Posidonia and
Amphibolis
Colonising macroalgae
Bare rock
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 29. Apparent transition of habitat types adjacent to the Bungala River with healthy seagrass
to seaward, an exposed section of seagrass rhizome matte presumably after seagrass dieback, an
erosion scarp, and an area of rock below the erosion scarp that has been colonised by macroalgae
Figure 30. Apparent migration of erosion scarp from inshore to offshore with transition from
seagrass meadow to bare rock to macroalgae reef at an area to the north of the Normanville jetty.
Erosion scarp
Healthy seagrass
Colonising macroalgae
Colonising macroalgae
Exposed seagrass rhizome matte
Erosion scarp
Healthy seagrass
Colonising macroalgae
Bare rock
Direction of erosion scarp migration
Inshore Offshore
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Figure 31. Severe erosion of Posidonia seagrass meadow in 2 to 3m depth north of the Bungala
River. Note the scarp (shadow area) and macroalgae colonising the rock substrate adjacent to the
seagrass meadow. Note also that despite the Posidonia appearing to be healthy the erosion is still
under-mining the meadow.
Figure 32. Dead or dying Amphibolis covered with epiphytes (on right) adjacent to an erosion scarp
with sand, rock and macroalgae (on left) in 2 to 3m depth north of the Bungala River. The direction
of the land is to the left of the photo.
Erosion scarp
Healthy Posidonia
Colonising macroalgae
Erosion scarp
Unhealthy AmphibolisColonising
macroalgae
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Threat assessment Bryars (2013) undertook a semi-quantitative threat analysis of the reef and seagrass habitats in
Yankalilla Bay based upon spatial coverage within different ‘coastal cells’. However, based upon the
results of the present study it is unclear what proportions of the present-day coverage of seagrass
and reef habitats are related to historical changes. Thus it was deemed inappropriate to attempt a
similar threat analysis to that done by Bryars (2013). Rather the present report will discuss possible
historical and current threats to seagrass. Reef is not discussed in this context as it appears that the
extent of reef (viz. emergent climax reef) has actually expanded due to seagrass loss.
Historical threats to seagrass Assuming that seagrass has indeed been lost from the inshore area of Normanville, there appear to
be a number of possible triggers for the loss:
Freshwater discharge from the Bungala River (and possibly the Yankalilla River and
Carrickalinga Creek). Nutrients and sediments from these three catchments have been
identified in several previous studies as threats to the nearshore habitats of Yankalilla Bay
(e.g. Bryars 2013). However, there are still limited data directly linking habitat condition with
the discharges (including from the current study).
Construction (and destruction) of three jetties at Normanville. It is apparent that disturbance
of the seafloor associated with the three jetties has occurred in the past. It is plausible that
this could have triggered erosion processes in seagrass meadows (if indeed seagrass once
occurred closer to shore).
Boat launching at Normanville Beach. Trailer boats have been launched from the beach
adjacent to the Bungala River since the 1960’s. Scouring from motor boats can cause
damage to seagrass meadows but in this case it appears that the advent of beach launching
occurred after the seagrass began regressing (see below).
The proposed loss of seagrass and habitat transformation in the Normanville inshore area possibly
commenced many decades ago during the 1800s. There are several lines of evidence for this
reasoning:
Significant land clearing for cropping in the region had already occurred by 1850 (e.g. Collins
2010) and thus the impacts of sediments/nutrients via the river discharges was possibly
already occurring.
Construction and loss of the first jetty (and thus disturbance of the seafloor) occurred in the
1850s.
Aerial imagery from 1949 indicates that the inshore habitats at the time were fragmented
with a possible mix of seagrass, sand and macroalgae on rock (Figure 24).
Current threats to seagrass Based upon observations from the current study and previous studies, the existing seagrass
meadows in Yankalilla Bay appear to be facing a number of threats:
Freshwater discharges from the Bungala River, Yankalilla River and Carrickalinga Creek. As
noted above the three discharges are well recognised as posing a potential threat to inshore
habitats and management actions are underway to mitigate this threat (see Bryars 2013).
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Freshwater upwelling from inland aquifers. Freshwater upwellings from the seabed were
noted at several locations during the current study and it is possible this water is having a
negative impact on seagrass condition. These upwellings could be related to the apparently
random occurrence of patches of unhealthy Amphibolis at locations away from the river
discharges (e.g. Figure 9). Further work is required to investigate this possible link.
Ongoing physical erosion from wave energy. Regardless of whether initial seagrass loss is
related to anthropogenic activities, there is strong evidence that erosion is causing ongoing
seagrass loss in Yankalilla Bay.
Conclusions and recommendations The nearshore area of Yankalilla Bay is characterised by a mosaic of reef, seagrass and sand habitats.
There are multiple lines of evidence that habitat changes have occurred (and are continuing to
occur) in the nearshore zone of Yankalilla Bay:
Erosion scarps and apparent seaward migration of the inshore seagrass margin (or blue-line)
that is exposing underlying rock substrate.
Erosion scarps further offshore and associated seagrass loss (blowouts).
Apparent colonisation of exposed rock substrate by macroalgae to form reef (‘emergent
climax reef’) that was not there previously.
Observations of unhealthy Amphibolis at numerous locations across the Bay. While there
was no evidence of selective loss of Amphibolis adjacent to the three river discharges, this
pattern does not mean that Amphibolis loss has not already occurred further inshore or is
continuing to occur. For example, unhealthy Amphibolis was recorded on the inshore
sections of the three transects adjacent to Yankalilla River and it is possible that this
Amphibolis will disappear.
Anecdotal evidence by various people that seagrass once grew closer to shore adjacent to
the Bungala River and Normanville jetty.
It is likely that ongoing discharges from the three catchments adjacent to Yankalilla Bay are having
some impact on the condition of seagrass and reef habitats. Nonetheless, freshwater upwellings
were noted at several locations across the survey area and the possibility cannot be discounted that
groundwater from inland is contributing to unhealthy Amphibolis and seagrass dieback at locations
well away from the river discharges.
In order to better understand processes driving habitat change in the Yankalilla Bay inshore area and
to direct management actions the following recommendations are made:
GIS mapping of inshore habitats using historical aerial imagery to more clearly define what
changes have occurred (note however that historical imagery may not be available as far
back as required for detecting post-European settlement changes to inshore habitats).
Monitoring of erosion and unhealthy Amphibolis using permanent markers and photopoints.
Monitoring of ongoing seagrass loss and habitat change using remote sensing techniques.
Investigation of sources and water quality of apparent freshwater groundwater seepage.
Implementation of management actions aimed at improving discharge water quality from
adjacent catchments in Yankalilla Bay.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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References Brook, J. and Bryars, S. (2014) Condition status of selected subtidal reefs on the Fleurieu Peninsula. Report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board. J Diversity Pty, Adelaide.
Bryars, S. (2013) Nearshore marine habitats of the Adelaide and Mount Lofty Ranges NRM region: values, threats and actions. Report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board, Dr Simon Richard Bryars, Adelaide.
Collins, N. (2010) The Jetties of South Australia – Past and Present. Revised and Expanded Edition.
Irving, A.D. (2009) Reproduction, recruitment, and growth of the seagrass Amphibolis antarctica near the Bungala and Yankalilla Rivers, South Australia. Final report prepared for the Coastal Management Branch of the Department for Environment and Heritage SA and the Adelaide and Mount Lofty Ranges Natural Resources Management Board. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. 18 pp. SARDI Publication Number F2009/000468-1.
Murray-Jones, S., Irving, A. and Dupavillon, J. (2009) Seagrass Condition Monitoring: A report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board. Department for Environment and Heritage, Coastal Management Branch. Adelaide.
Short, A.D. (2001) Beaches of the South Australian Coast and Kangaroo Island: A guide to their nature, characteristics, surf and safety. Australian Beach Safety and Management Program, Sydney.
Tanner, J.E., M. Theil and D. Fotheringham (2012) Seagrass Condition Monitoring: Yankalilla Bay, Light River and Encounter Bay. Final report prepared for the Adelaide and Mount Lofty Ranges Natural Resources Management Board. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. SARDI Publication No. F2012/000139-1. SARDI Research Report Series No. 653.
Turner D., Kildea T. & Westphalen G. (2007) Examining the health of subtidal reef environments in SA. 2. Status of selected reefs based on the results of the 2005 surveys. SARDI Publication No. RD 03/0252-6. SARDI, Adelaide.
Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay
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Appendix 1
Coordinates of sites used for the diver survey
Site Latitude (°S) Longitude (°E)
Bungala River - Reef 35.4465 138.3057
Bungala River - Seagrass 35.447 138.3044
Carrickalinga Creek - Reef 35.4284 138.3171
Carrickalinga Creek - Seagrass 35.4296 138.3166
North Normanville - Reef 35.438 138.3113
North Normanville - Seagrass 35.436 138.3125
South Normanville - Seagrass 35.454 138.3011
Yankalilla River - Reef 35.4663 138.2937
Yankalilla River - Seagrass 35.4663 138.2937