does biological community structure influence biogeochemical fluxes? jgofs says yes!

Does Biological Community Structure Influence Biogeochemical Fluxes?

JGOFS Says Yes!

Anthony F. MichaelsAnthony F. MichaelsUniversity of Southern CaliforniaUniversity of Southern California

Wrigley Institute for Environmental StudiesWrigley Institute for Environmental Studies

RoadmapRoadmap

• What do we mean by community structure?

• Selected survey of community impacts on global carbon cycle– HNLC– Particulate Inorganic Carbon– Nitrogen Fixation– Remineralization length-scales

Tension and BalanceTension and Balance

• Biologist’s love of the details of life

• Biogeochemist’s need to simplify in order to model global dynamics

What do we need to What do we need to get a food web?get a food web?

Phytoplankton

Zooplankton

NutrientsSinking

Particles

SinksStays Suspended

What do we need to What do we need to get a food web?get a food web?

Phytoplankton

Zooplankton

NutrientsSinking

Particles

SinksStays Suspended

Bacteria

(Fasham et al., 1990)

Time

Nitrate

Phyto

Zoop

P-Z-N Dynamics: Populations Change through TimeP-Z-N Dynamics: Populations Change through Time

Nitrate

PhytoZoop


Nitrate

Phyto

Zoop


So, what are we really asking?

Nitrate

Phyto

Zoop


Phyto

Does it matter what biology is hidden within each box?Does it matter what biology is hidden within each box?

Phyto

DiatomsPrasinophytesPrymnesiophytes

ProchlorococcusSynechococcus

Is this level of details necessary Is this level of details necessary to understand the carbon cycle?to understand the carbon cycle?

Rephrase the QuestionRephrase the Question

Do we need more structure thanP-Z-N-B to capture the important

carbon dynamics for global scales?

(e.g., more phytoplankton, more zooplankton, viruses, marine snow, etc.)

Pico-Phyto

Nano-Phyto

Micro-Phyto

Community Structure and Flux: Community Structure and Flux: Circa 1988Circa 1988

Start with big size range of plantsStart with big size range of plants

<2 µm 2-20 µm 20-200 µm

Nutrients

Pico-Phyto

Nano-Phyto

Micro-Phyto

Pico-Phyto

Nano-Phyto

Micro-Phyto

Nano-Nano-ZooZoo

Micro-Micro-ZooZoo

Meso-Meso-ZooZoo

Nutrients

Bacteria & Nutrients

Sinking Paricles

Stays Suspended

Pico-Phyto

Nano-Phyto

Micro-Phyto

Nano-Nano-ZooZoo

Micro-Micro-ZooZoo

Meso-Meso-ZooZoo


Sinking Particle

s

Stays Suspended

Pico-Phyto

Nano-Phyto

Micro-Phyto

Nano-Nano-ZooZoo

Micro-Micro-ZooZoo

Meso-Meso-ZooZoo

Foodwebs and Flux - 1988Foodwebs and Flux - 1988

• Focus on f-ratio and the amount of export from surface waters

• “Ryther-esque” outcome (# trophic steps, transfer efficiency)

• Imply that removal of carbon from surface is directly related to exchange with atmosphere

Ocean biology maintains a vertical DIC gradient:Ocean biology maintains a vertical DIC gradient:Balance of biology and physicsBalance of biology and physics

Nitrate+Nitrite

Dissolved Inorganic Carbon

What Processes Have Significant What Processes Have Significant Affects on Air-Sea Partitioning of Affects on Air-Sea Partitioning of

Carbon Dioxide?Carbon Dioxide?

1. Incomplete Nutrient Utilization (HNLC)

2. Particulate Inorganic Carbon:Organic Carbon Ratio

3. Changes in Nitrogen fixation:Denitrification balance (LNLC)

4. Changes in Remineralization Length-scales

Do food webs matter for any of these?

1.1. Incomplete Nutrient Utilization in Incomplete Nutrient Utilization in the Surface Waters (HNLC)the Surface Waters (HNLC)

Century residence times

Decadal residence times

Millenial residence times Dissolved Inorganic Carbon

Nitrate+Nitrite

Most of the ocean shows Most of the ocean shows near-complete nutrient utilizationnear-complete nutrient utilization

Surface Nitrate (µmoles/kg)

Low Nutrient AreasLow Nutrient Areas

If nutrient levels stay near detection and if C:N:P stoichiometry is constant,

then P-Z-N might be enough for the global carbon models

– Those are some big “ifs”

– There is more of interest on this planet than just global carbon

HNLC RegionsHNLC Regions

Changes in net utilization of surface nutrients Changes in net utilization of surface nutrients changes DIC gradient and air-sea partitioning changes DIC gradient and air-sea partitioning

(within limits)(within limits)

• High-nutrient, low chlorophyll areas (HNLC) in Southern Ocean, Equatorial Pacific, North Pacific and some coastal zones

• Trace nutrient limitation (Fe), Diatom blooms (Si)

• Reduce HNLC area - 15-100 ppm reduction in atm CO2

• Amenable to direct manipulation (political hot potato)

Nutrient Cycling in the Modern Southern OceanNutrient Cycling in the Modern Southern Ocean

(Anderson et al., 2002)


Nutrient Cycling in the Glacial Southern OceanNutrient Cycling in the Glacial Southern Ocean

1.1. Incomplete Nutrient Utilization in Incomplete Nutrient Utilization in the Surface Waters (HNLC)the Surface Waters (HNLC)

Influence of Community Structure?Influence of Community Structure?

• Taxa-specific responses to Fe• Si+N requirements in diatoms, N

requirements in other phytoplankton• Taxa-specific grazing responses• Ballasting issues

2. Changes in2. Changes inParticulate Inorganic Carbon FluxParticulate Inorganic Carbon Flux

• Archer and Maier-Reimer, 1994

• Skeletons of coccolithophorids, foraminifera and pteropods

• Alkalinity change! Increase in PIC flux -> increase in pCO2

• Net effect on atmosphere depends on PIC:POC ratio

• Community structure effects analogous to remineralization length scale

(Sarmiento et al., 2002)

Carbonate Flux Linked toCarbonate Flux Linked to

Organic Carbon Flux via BallastingOrganic Carbon Flux via Ballasting (a la Armstrong et al, 2002)(a la Armstrong et al, 2002)

(Klaas and Archer, 2002)

2.2. Changes inChanges inParticulate Inorganic Carbon FluxParticulate Inorganic Carbon Flux


• Carbonate skeletons found in a subset of taxa: coccolithophorids, foraminifera and pteropods

• Ecosystem dynamics will determine relative contributions of these taxa

• Mix of autotrophs and heterotrophs - must be mix of top-down and bottom-up controls

• Some of these taxa susceptible to creating blooms

• Ballasting link to organic carbon fluxes, stronger for carbonate fluxes than opal (correlation - causation?)

3. Changes in Nitrogen Fixation - Denitrification Balance



Millenial residence times

Lower DIC

Nitrate+Nitrite


Higher DIC

Extra Nitrogen Fixation

3. Changes in Nitrogen Fixation - Denitrification Balance



Millenial residence times

Lower DIC

Nitrate+Nitrite


Higher DICExtraDenitrification

Trichodesmium spp.Trichodesmium spp.Best Known Planktonic DiazotrophBest Known Planktonic Diazotroph

Excess Nitrate (µmoles/kg)

Dep

th (

m)

1 Year

4 Years7 Years

7.5 Years

7.3 Years

9 Years

8 Years

11 Years

39 Years

29 Years

19 Years

15 Years

12 Years

0 4 50

200

400

600

800

1000

1200

Excess Nitrate = NO3 - 16*PO4

Data from BATS

(Gruber and Sarmiento, 1997)

N* in the Atlantic Ocean

(Deutsch et al., 2001)

N* in the Pacific Ocean

(Husar et al., 1997)

Key Assumptions and Dynamics:Key Assumptions and Dynamics:

• Iron from dust limits nitrogen fixation

• Reasonable flexibility in N:P ratios and nutrient controls on N-fixation

• Carbon Modeling

– NCAR Ocean GCM (Doney)

– Multi-compartment box model (Sigman)

– Add adequate N fixation for 1 Gt/y C fixation

– Restrict N fixation to 10-40° (N and S)

– C:N=6.6, Assume flexible N:P within N* range

– 100-300 year runs

(Doney, Moore in prep)

Nitrogen Fixation Feedback Cycle Hypothesis(as an example)

Two Additional Biological Two Additional Biological Characteristics:Characteristics:

• Diazotrophs form large surface blooms

Two Additional Biological Two Additional Biological Characteristics:Characteristics:

• Diazotrophs form large surface blooms

• Nitrogen fixation dynamics seem to change on decadal time-scales (at least in the Pacific Ocean)

Dave Karl and co-workers

3.Changes inNitrogen Fixation - Denitrification Balance


• Diazotrophy found in a distinct subset of all prokaryotic taxa

• Chemical defenses against grazing in some• Some taxa symbiotic in eukaryotes (esp.

diatoms, creates silica requirement)• Denitrification - special environmental

conditions, specific prokaryotes

4. Changes in remineralization length-4. Changes in remineralization length-scales and C-N-P stoichiometryscales and C-N-P stoichiometry



Millenial residence times Dissolved Inorganic Carbon

Nitrate+Nitrite

Changes in remineralization length-scalesChanges in remineralization length-scales

• Depends on the depth horizon and ventilation time-scale:

– Annual: 10-20 Gt C/y – Multi-annual (>200 m): 5-10 Gt C/y– Multi-Decadal: 2-4 Gt C/y– > Centennial: ~1-2 Gt C/y

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10 12 14

b=1.5b=1b=0.7

De

pth

(m

)

POC Rain (mmolC/m2d)

1-3 GtC/yr

Export ~5-15 GtC/yr

Global PP~50 GtC/yr

Larger b value, more POC is recycled in upper oceanSmaller b value, more POC gets to deep ocean

Small b

Large b

Fz = F100 *(z/100)-b

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25

NABEEQPACASSO

Exp

on

ent,

b

POC Export at 100 m (mmolC m-2 d-1)

US-JGOFS Open-Ocean SitesUS-JGOFS Open-Ocean Sites

(Berelson, 2001)

(Martin et al., 1987)

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25

JGOFSMartin OceanMarginMartin Margin

Exp

onen

t, b

POC Export at 100 m (mmolC m-2 d-1)Berelson (in prep)

Slope’s always positive---

best explanation-

Reactivity POC > Reactivity bSi

0

1000

2000

3000

4000

50000 0.5 1 1.5 2 2.5 3

Arabian SeaEqPac Central Cal.NABEUSJGOFS SO (NACC)Guinea BasinWalvis RidgeOS Papa

Wat

er D

epth

(m

)

bSi/Corg ratio (molar)Berelson (in prep)

Can bSi content slow net particle settling rates?(Hypothesis: Yes)

Higher bSi Content

SteeperSlope

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5

Sediment Trap Data

Slo

pe

(1/m

) x1

0^5

Si/Corg

at 3000 m

Berelson (in prep)

0

50

100

150

200

Eq. (880-2284) Eq. (2284-3618)) 5°N (1191-2091)

El Nino Non-El Nino

Set

tlin

g V

eloc

ity

(m/d

ay)

Location (depth, m)

US-JGOFS EqPac—1992US-JGOFS EqPac—1992

Low Opal

Hi Opal

Berelson (2001)

1 mm

salp

euphausiid

copepod

Fecal Pellets


Sinking Particle

s

Stays Suspended

Pico-Phyto

Nano-Phyto

Micro-Phyto

Nano-Nano-ZooZoo

Micro-Micro-ZooZoo

Meso-Meso-ZooZoo


Pico-Phyto

Nano-Phyto

Micro-Phyto

Nano-Nano-ZooZoo

Micro-Micro-ZooZoo

Sinking Particles

SALPSSALPS

Twice in a decade!Twice in a decade!

Enormous Blooms!

Asexual reproduction by budding

The Outer LimitsThe Outer Limits

~ 0.5 Gt C

~ 1 Gt C

Ecosystems and Ecosystems and Plankton BloomsPlankton Blooms

• Ecosystems are complex-dynamical systems

• Bloom dynamics are poorly understood and hard to study

• Top-down vs bottom-up, a debate that is sorely lacking in ocean science

• Blooms created and controlled by internal dynamics of ecosystem, rarely bounded by external controls like nutrients

4.4. Changes in remineralization length-Changes in remineralization length-scales and C-N-P stoichiometryscales and C-N-P stoichiometry


• Mix of biological and physical sources to different types of sinking particles

• Ballasting signals (mechanisms?)

• Diatom blooms may have non-intuitive impact on remineralization length-scale

• Bloom forming organisms create unique time-space scale issues for remineralization and for science.

ConclusionsConclusions

• Community structure matters for the partitioning of carbon between ocean and atmosphere.

• JGOFS has clarified some simple issues that now allow us to ask much more sophisticated questions.

• We still lack tools to study these processes on the time, space and taxonomic levels that are required.

• When community structure is important, the outcome is often emergent from internal dynamics.

Future DirectionsFuture Directions

• Community structure where it matters (HNLC, nitrogen fixation, etc.)

• The “twilight zone” and below

• Bloom dynamics (when they occur and when they matter)

• Complex systems approaches

• Be ready for surprises (viruses, archea, who knows what else).

Thank you!Thank you!

does biological community structure influence biogeochemical fluxes? jgofs says yes!

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