Artificially increasing the acidity of seawater flowing over a natural coral reef community in the Great Barrier Reef reduces coral calcification by about one-third, reports a paper published in Nature this week. Ocean acidification poses a significant threat to tropical coral reef ecosystems by reducing the saturation state of the aragonite mineral of which coral skeletons are primarily composed, and by lowering the concentration of carbonate ions required to build the reef. The findings suggest that near-future reductions in the aragonite saturation state of the ocean will compromise coral-reef function.
Coral reefs feed millions of people worldwide, provide coastal protection, and generate billions of US dollars annually in tourism revenue. Past research on the effects of ocean acidification on coral reefs has focused on the reactions of individual species instead of evaluating more complex, community-level responses. The ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses to environmental changes.
Rebecca Albright and colleagues exposed a 400-m2 area of natural coral reef community in the southern Great Barrier Reef to decreased aragonite saturation states, simulating the levels that are expected to occur later this century if deep cuts to global CO2 emissions are not made. In contrast to laboratory settings, CO2 enrichment in the natural environment allowed the authors to monitor the calcification response of an entire coral reef community to acidification under natural temperature, light, and nutrient conditions. The authors observed that net community calcification was reduced by 34% in the acidified reef, which has the potential to significantly compromise the ecosystem.