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Published: April 1, 2009
First published in IGBP's Global Change Newsletter Issue 73, April 2009

From the lab to models:
algal calcification and ocean acidification

Andy Ridgwell
School of Geographical Sciences
University of Bristol
Bristol, UK

Email: andy@seao2.org
Features |
The Ocean in a High-CO2 World: Science highlights from the symposium
Global modellers are called upon to predict the future uptake of fossil fuel carbon dioxide (CO2) by the ocean. The impact of ocean acidification on marine calcifiers may be important in this calculation, so we construct model parameterizations for plankton carbonate production based on biological experiments. However, for a major carbonate producer in the open ocean, algae called coccolithophores, no consistent calcification response to acidification (pH) is apparent in laboratory studies. This gives us a real headache – how to write a single equation for wildly differing experimental responses? Which, if any, is the “correct” response? Two clues may help: firstly, a peak in calcification is observed in some experiments, hinting at an environmental pH “optimum” for this process. Secondly, in manipulations of more complete ecosystems such as mesocosms (large partly submersed bags) and shipboard incubations of seawater samples, calcification is consistently lower in more acidic conditions (higher CO2). The existence of pH optima for calcification would allow the use of the same quasi-empirical trick as we already employ for modelling the response of algal growth rate to temperature – the “Eppley curve”. Marine ecosystems may then be expected to respond to future acidification in an analogous way to increasing temperature – by gradual transition in dominance from more to less heavily calcified coccolithophores and progressively reduced carbonate production globally, as illustrated below.

See the full article in Ridgwell et al. (2009) Biogeosciences 6 (2): 3455-3480.

Potential ecosystem level response of carbonate production with increasing acidity – a simplification based on individual pH optimum curves for different (hypothetical) species of calcifying phytoplankton.
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