Predicting Coral Reef Responses to Ocean Acidification: New Insights from the Laboratory and the Field

Rates of calcium carbonate (CaCO₃) production by reef calcifying organisms (predominantly scleractinian corals and coralline algae) must exceed rates of CaCO₃ loss via dissolution and sediment transport, if coral reefs are to persist and grow. Historically, constructive and destructive processes have been closely balanced – with net reef accumulation slightly higher than net reef loss. But ocean acidification, caused by rising levels of atmospheric CO₂, threatens to tip this balance by slowing calcification, enhancing bio-erosion and reducing rates of abiogenic cementation.  One study predicts that coral reefs globally could shift from net accumulating to net eroding structures within the next few decades.  We are investigating the effects of ocean acidification on calcification by massive corals using a combination of laboratory CO₂ manipulation experiments and data generated from field corals across sites representing a natural range in seawater pCO₂ and Ω_ar. In experiments, calcification generally declines with increasing seawater pCO₂ and decreasing carbonate ion concentration [CO₃^2-] but there is significant variability that currently limits our ability to make accurate predictions of the impact of future ocean acidification on coral reef calcification on regional or local scales.  Observations made on corals under both experimental and natural conditions suggest that interactions amongst temperature, light, nutrients, and Ω_ar, species-specific sensitivities to these parameters, the existence of threshold responses, the exaggerated impacts on spawning females, the nutritional and energetic status of the coral host, and phenotypic plasticity, will all play a key role in predicting the responses of individual reefs and reef assemblages to ocean acidification.