CO 2 Chemistry Effects on Benthic Calcifying Communities

1. CO2 Chemistry Effects on Benthic Calcifying Communities Chris Langdon Rosenstiel School of Marine and Atmospheric Science Uni. of Miami

2. 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 0.6°C 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

3. 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

4. Modes of manipulation • Constant TA. Adjust DIC with CO2 gas. (Simulates natural situation) • Constant DIC. Adjust TA with acid or base. • Constant pH. Adjust TA and DIC.

5. Natural situation: adding CO2 by diffusion

6. Artificial situation: changing TA by addition of acid or base without changing DIC

7. Artificial situation: adding Na2HCO3 both TA and DIC increase

8. What happens to the photosynthesis and calcification of a coral or alga when the carbonate chemistry is altered?

9. 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

10. 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!

11. 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.

12. Interesting interactions of temperature and CO2 on coral calcification • Elevated pCO2 caused no significant change in calcification at 25°C but a 50% reduction at 28°C. • 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.

13. 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.

14. Effect of CO2 January 2000 23.4°C 19 E m-2 d-1 August 1999 27.3°C 37 E m-2 d-1 Langdon and Atkinson, in press

15. 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.

16. Coral calcification decreases with decreasing saturation state G = (8±1)(Wa-1) r2=0.87 First-order rate law explains 87% of variability in calcification of this coral assemblage

17. Aragonite saturation state Wa = [Ca2+][CO32-]/K’sp where K’sp 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 R=k(W-1)n gives a good fit to many data sets.

18. Is it pH or Wa? Calcification varied 3-fold at constant pH indicating that change in seawater pH not required to explain decrease in calcification. Langdon unpublished

19. Agegian 1985 found a linear relationship between linear extension of the red coralline alga Porolithon and saturation state

20. Red Sea coral Stylophora pistillata

21. Pacific and Caribbean branching corals Porites compressa/P. porites/Montipora capitata G=16.1Wa+21.7 R2=0.82

22. Pacific massive corals Porites lutea/Fungia sp. G=24.6Wa-11.7 R2=0.81

23. Assortment of Red Sea corals of branching and foliose structure Varied TA G=7.6Wa+65.2 R2=0.96 Marubini et al. 2003

24. Summary of coral and reef community response to saturation state 2065

25. Effect of a doubling in pCO2 on calcificationWide range of sensitivity -8% -46%

26. Predictions based on pCO2 aloneare 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.

27. 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.

28. 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.

29. 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.

30. Response of Biosphere 2 coral reef mesocosm Dissolution

31. Role of coral reefs as a source of CO2 could reverse Unpublished data from Biosphere 2 experiment

32. 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.

34. Fossil fuel emissions are acidifying the ocean AAAS Annual Meeting Washington, D.C., 2005 After Turley et al., 2005

35. Observations at the Hawaiian Ocean Time Series (HOTS) station confirming ocean acidification -0.03±0.01 units per decade

36. Changes in CO2 chemistry based on IPCC “Business as usual”(percent change from pre-industrial) Modified from Feely et al., (2001)

37. 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