From laboratory manipulations to earth system models: predicting pelagic calcification and its consequences

Abstract

The variation in pH-dependent calcification responses of coccolithophores paint a highly incoherent picture, particularly for the most commonly cultured "species", Emiliania huxleyi. The disparity between magnitude and even sign of the calcification change at higher CO2 (lower pH), raises challenges to quantifying future carbon cycle changes and feedbacks, by introducing significant uncertainty in parameterizations used for global models. Putting aside the possibility of methodological differences that introduce an experimental bias, we highlight two pertinent observations that can help resolve conflicting interpretations: (1) a calcification "optimum" in environmental conditions (pH) has been observed in other coccolithophore species, and (2) there exists an unambiguous direction to the CO2-calcification response across mesocosm and shipboard incubations. We propose that an equivalence can be drawn between integrated ecosystem calcification as a function of pH (or other carbonate system parameter such as calcite saturation state) and a widely used description of plankton growth rate vs. temperature – the "Eppley curve". This provides a conceptual framework for reconciling available experimental manipulations as well as a quasi-empirical relationship for ocean acidification impacts on carbonate production that can be incorporated into models. By analogy to the Eppley curve temperature vs. growth rate relationship, progressive ocean acidification in the future may drive a relatively smooth ecosystem response through transition in dominance from more to less heavily calcified coccolithophores in addition to species-specific calcification changes. However, regardless of the model parameterization employed, on a century time-scale, the CO2-calcification effect is a minor control of atmospheric CO2 compared to other C cycle feedbacks or to fossil fuel emissions.