CO2 Chemistry Effects on Benthic Calcifying Communities

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CO2 Chemistry Effects on Benthic Calcifying
Communities

• Chris Langdon
• Rosenstiel School of Marine and Atmospheric
  Science
• Uni. of Miami

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How will rising CO2 impact benthic communities?

• pCO2 has increased by 32 between 1880 and 2000
  (280 vs. 370 uatm) Houghton et al., 2002.
• Sea surface temperatures have risen by 0.6C over
  the same period (Sheppard and Rioja-Nieto, 2005).
• Coral reef ecosystems are negatively affected by
  the increase of both temperature and pCO2.
• Increased temperature leads to loss of
  zooxanthellae (bleaching)
• Increased pCO2 leads to reduced calcification of
  corals and algae

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What are the concerns?

• Reduced geographic range As pCO2 rises, regions
  with a saturation state sufficient to support
  vigorous coral growth will shrink.
• Reduced tolerance to other environmental
  fluctuations rising pCO2 may reduce thermal
  optimum for growth (Reynaud et al. 2003).
• Reduced rate of recovery following disturbance
• Reduced skeletal growth to repair damage done by
  storms, predators and humans
• Reduced fecundity
• Reduced survivorship of early life stages
• Accelerated phase shift from coral to algal
  dominance

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Modes of manipulation

1. Constant TA. Adjust DIC with CO2 gas. (Simulates
   natural situation)
2. Constant DIC. Adjust TA with acid or base.
3. Constant pH. Adjust TA and DIC.

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Natural situation adding CO2 by diffusion
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Artificial situation changing TA by addition of
acid or base without changing DIC
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Artificial situation adding Na2HCO3 both TA and
DIC increase
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What happens to the photosynthesis and
calcification of a coral or alga when the
carbonate chemistry is altered?
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Borowitzka and Larkum 1976 looked at effect of
increasing DIC on the photosynthesis and
calcification of the green calcareous alga
Halimeda tuna by adding Na2HCO3
Ptns
Calcif
Both photosynthesis and calcification increased
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Borowitzka and Larkum 1976 also varied pH while
holding DIC constant, mimicking the natural
situation
Calcif
Ptns
Conclusion Ptns using CO2 aq and calcif. using
CO32-
pH
Photosynthesis increased and calcification
decreased!
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Reynaud et al. 2003 looked at the effects of
temperature and CO2 on the photosynthesis,
respiration and calcification of the coral
Stylophora pistillata. Corals were grow for 5
weeks at each condition.

• Elevated pCO2 caused slight reduction in net
  photosynthesis.
• Net photosynthesis increased with temperature as
  expected for this species.
• Cell specific density was 24 higher at elevated
  pCO2 suggesting some disruption in the balance of
  growth rates of the algal and animal cells.
• Dark respiration not effected by elevated pCO2 or
  temperature.

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Interesting interactions of temperature and CO2
on coral calcification

• Elevated pCO2 caused no significant change in
  calcification at 25C but a 50 reduction at
  28C.
• The reduction in calcification was immediate and
  persisted unchanged over the 5 wk experiment.
• At normal pCO2, the increase in temperature
  caused an increase in calcification but at
  elevated pCO2 the increase in temperature caused
  a 34 reduction in calcification.
• One interpretation is that elevated pCO2 reduced
  the thermal optimum for this species.

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Experiment in an outdoor flowing seawater flume

• 200 closely packed colonies of corals forming a
  patch 2.2 m2 in area simulating a patch of reef
  with 100 coral cover.
• Flowing seawater duplicates turbulent boundary
  conditions in the field.
• Receiving full natural sunlight
• Carbonate chemistry manipulated by addition of
  HCl or NaOH.

Langdon, C., and M.J. Atkinson, Effect of
elevated pCO2 on photosynthesis and calcification
of corals and interactions with seasonal change
in temperature/irradiance and nutrient
enrichment, J. Geophysical Res., in press.
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Effect of CO2
January 2000 23.4C 19 E m-2 d-1
August 1999 27.3C 37 E m-2 d-1
Langdon and Atkinson, in press
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Coral net carbon production may benefit from rise
in pCO2
Decrease in coral calcification is not due to an
adverse effect of acidification on the
zooxanthellae.
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Coral calcification decreases with decreasing
saturation state
G (81)(Wa-1) r20.87
First-order rate law explains 87 of variability
in calcification of this coral assemblage
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Aragonite saturation state

• Wa Ca2CO32-/Ksp
• where Ksp is the solubility product for the
  particular mineral phase of carbonate of
  interest, i.e. calcite, aragonite or high
  Mg-calcite
• has been found to be useful predictor of the
  rate of calcification in inorganic systems. The
  rate law Rk(W-1)n gives a good fit to many data
  sets.

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Is it pH or Wa?
TA DIC pCO2 pH sw Wa
832 697 190 7.91 0.81
1223 1068 311 7.92 1.16
1504 1343 427 7.90 1.37
2013 1792 491 7.91 2.06
3562 3222 863 7.96 3.82
4774 4270 950 8.01 5.91
6195 5591 1311 7.97 7.41
Calcification varied 3-fold at constant pH
indicating that change in seawater pH not
required to explain decrease in calcification.
Langdon unpublished
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Agegian 1985 found a linear relationship
between linear extension of the red coralline
alga Porolithon and saturation state
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Red Sea coral Stylophora pistillata
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Pacific and Caribbean branching corals Porites
compressa/P. porites/Montipora capitata
G16.1Wa21.7 R20.82
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Pacific massive corals Porites lutea/Fungia sp.
G24.6Wa-11.7 R20.81
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Assortment of Red Sea corals of branching and
foliose structure
Varied TA
G7.6Wa65.2 R20.96
Marubini et al. 2003
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Summary of coral and reef community response to
saturation state
2065
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Effect of a doubling in pCO2 on
calcificationWide range of sensitivity
Species Source decline by 2065
S. pistillata Gattuso et al. 1998 -3
P. compressa Marubini et al. 2001 -16
G. fascicularis Marubini et al. 2002 -11
P. cactus -13
T. reniformis -9
A. verweyi -13
S. pistillata (25C) Reynaud et al. 2003 7
(28C) -57
P. lutea Ohde and Hossain 2004 -38
P. compressa/M. capitata Langdon and Atkinson in press -41
P. lutea Hossain and Ohde, in press -33
Fungia sp. -60
-8
-46
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Predictions based on pCO2 alone are probably
underestimates because we also need to take the
temperature increase into account

• There is evidence that many corals are currently
  at or slightly above their thermal optimum.
• This means that any increase in the average
  annual temperature will result in reduced
  calcification.
• Estimating the temperature effect is complicated
  because some species possess the ability to
  acclimate to new temperature regime while others
  do not.

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Temperature dependence of coral calcification
Data for Pacific corals
Bleaching threshold
Optimum temperature for calcification is at or
below current peak summer temperatures for many
species.
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• Cellular mechanism underlying the response of
  coral calcification to an elevation of pCO2 in
  the external environment is poorly understood.
• Calcification is known to occur within a membrane
  enclosed space. Ca2 and HCO3- ions are thought
  to be actively transported across the membrane
  and into the calcifying space.
• In this scenario it is not obvious how changes in
  external pH or CO32- would influence the rate
  of calcification.
• The explanation may be that the calcifying space
  (CS) is leaky and some Ca2, HCO3- and CO32- ions
  may arrive via leakage of seawater into the CS.
• In this scheme corals that have a tight CS would
  exhibit little sensitivity to change in the
  chemistry of the external environment and corals
  with a leakier CS would exhibit more sensitivity.

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Role in the global carbon cycle

• Calcification (shallow water and pelagic) and
  volcanism are the main sources of CO2 to the
  atmosphere that counter-balance the removal of
  CO2 via weathering of silicate rock and burial of
  organic matter in deep sea sediments.
• As atmospheric CO2 rises the magnitude of the CO2
  flux from calcification is going to diminish and
  at some point it will switch and become a sink as
  carbonate deposits start to dissolve.
• If we lose calcifying organisms we will also lose
  a negative feedback control on atmospheric CO2.

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Response of Biosphere 2 coral reef mesocosm
Dissolution
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Role of coral reefs as a source of CO2 could
reverse
Unpublished data from Biosphere 2 experiment
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Conclusions
We need to understand the the temporal and
spatial changes of the carbon system in the
global oceans and their impacts on biological
communities and ecosystems.

• There is a need for longer term experiments to
  see if marine
• calcifying organisms are able to acclimate to
  elevated CO2
• and/or temperature if given sufficient time.

• There is a need to understand why certain species
  are able
• to adapt to life in low saturation state
  water.

• There is a need for manipulative experiments to
  look at the
• effects of high CO2 on coral calcification,
  reproduction,
• settlement, and reattachment of fragments.

• Need to know about the effect of high CO2 on the
  processes
• that recycle the reef framework, i.e.
  bioerosion and dissolution.

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Fossil fuel emissions are acidifying the ocean
AAAS Annual Meeting Washington, D.C., 2005
After Turley et al., 2005
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Observations at the Hawaiian Ocean Time Series
(HOTS) station confirming ocean acidification
-0.030.01 units per decade
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Changes in CO2 chemistry based on IPCC Business
as usual(percent change from pre-industrial)
Glacial Pre-industrial Present day Future 2XCO2 Future 3XCO2
pCO2 180 (-56) 280 380 (36) 560 (100) 840 (200)
CO2 7 (-29) 9 13 (44) 18 (100) 25 (178)
HCO3- 1666 (-4) 1739 1837 (5) 1925 (11) 2004 (15)
CO32- 279 (-20) 222 186 (-16) 146 (-34) 115 (-48)
pH sws 8.32 8.16 8.05 7.91 7.76
Wa 4.26 (-19) 3.44 2.90 (-16) 2.29 (-33) 1.81 (-47)
Modified from Feely et al., (2001)
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Why are some corals more sensitive to changes in
external CO32-?
Seawater reaches the calcifying space via
diffusion thru porous skeleton, junctions between
cells or exocytosis of vacuoles.
Light-activated Ca-ATPase pumps Ca2 into the
calcifying space (CS) during the day. However,
its main role is to transport H out of the CS
thereby maintaining a pH favorable to the
conversion of CO2 to CO32-.
CS
Corals with strong Ca-ATPase activity would be
predicted to be less sensitive to a decrease in
ambient CO32- while corals depending more on
passive transport would be more sensitive.
Cohen and McConnaughey 2003