How does rising atmospheric CO2 affect marine organisms?

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Does the Calcium Carbonate Saturation State of Seawater Absolutely Constrain Coral Calcification Rates?
Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H. and Atkinson, M.J.  2000.  Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef.  Global Biogeochemical Cycles 14: 639-654.

What was done
The authors manipulated the calcium carbonate saturation state of the water in the artificial coral reef mesocosm at the Biosphere-2 facility over a 3.8-year time period via periodic additions of NaHCO3, Na2CO3 and CaCl2 - which they used to bring about changes in water chemistry believed to be analogous to those resulting from increases in the atmosphere's CO2 concentration - to determine the effects of the historical (and predicted) rise in the air's CO2 content on the mesocosm's net community calcification rate.

What was learned
Net community calcification rate responded to manipulations in the concentrations of Ca2+ and CO32- in such a way that the calcification rate was well described by a linear function of the ion concentration product [Ca2+]0.69[CO32-].

What it means
What are the implications of this finding for the world's coral reef ecosystems?  In the words of the authors, "the predicted decrease in coral reef calcification between the years 1880 and 2065 A.D. based on our long-term results is 40%."  There are, however, a number of reasons for not putting too much stock in this dismal conclusion.

First of all, the authors note that "mechanisms of calcification are very different in calcareous algae and corals," and they state in the introduction to their paper that "caution should be exercised in extrapolating from Ca2+ control in corals and CO32- control in coralline algae to the assumption that corals and coralline alga respond in the same way to the ion concentration product."  This warning takes on added significance when one reads near the end of their paper that "the Biosphere coral reef is algal dominated," which means that the relationship they derived is also algal dominated.  In fact, it may be so algal dominated that nothing can truly be said about the sought-after coral relationship.

Second, the authors state that their experiments "demonstrate that changes in the concentration of Ca2+ and CO32- of the bulk water have a large impact on the rate of calcification of the Biosphere-2 coral reef biome."  Again, this demonstration says nothing about the truly coral components of the biome, nor does it prove that an increase in atmospheric CO2 concentration will have such an effect.  The authors appear to understand this latter fact and indicate that "more natural" experiments - presumably based on actual manipulations of the air's CO2 content - may be conducted in the future.

Third, the authors acknowledge that much of their data was derived from "unnatural rapid increases in saturation state," which may have induced the calcifying organisms "into showing a response to saturation state that would not occur if the change was more gradual."

Fourth, they observed that the increase in community calcification rate due to one of these unnatural increases in water saturation state was sustained for a period of two months, which they took as evidence for the validity of the assumption that it would continue to remain at that high level indefinitely.  Evidence obtained from experiments with terrestrial plants, however, indicates there is typically a large initial increase in growth response to atmospheric CO2 enrichment - lasting as much as two months in new sour orange tree branches (Idso et al., 2000) and a year or more for the entire organism (Idso, 1999) - which drops off considerably as time progresses further.  Hence, the Biosphere-2 experiment is not conclusive on this last point either.

In conclusion, the authors' experiments do indeed demonstrate certain important points, but they have yet to nail down their primary contention, i.e., that "globally increasing atmospheric CO2 has a negative impact on coral reef ecosystems."  Until that is done - which we truly believe never will be - it is much more logical to believe that coral reef ecosystems, like their terrestrial counterparts, are actually benefited by the ongoing rise in the air's CO2 content.

Idso, S.B. 1999.  The long-term response of trees to atmospheric CO2 enrichment.  Global Change Biology 5: 493-495.

Idso, C.D. Idso, S.B., Kimball, B.A., Park, H.-S., Hoober, J.K., and Balling Jr., R.C.  2000.  Ultra-enhanced spring branch growth in CO2-enriched trees: Can it alter the phase of the atmosphere's seasonal CO2 cycle?  Environmental and Experimental Botany 43: 91-100.

Reviewed 6 September 2000