Carbon dioxide addition to coral reef waters suppresses net community calcification

In situ carbon dioxide enrichment experiments show that ocean acidification poses a threat to coral reefs by reducing the saturation state of aragonite and the concentration of carbonate ions and that this impairs community calcification. Ocean acidification impairs coral calcification and poses a substantial threat to tropical coral reef ecosystems. Rebecca Albright and colleagues exposed a natural coral reef community in the southern Great Barrier Reef to levels of ocean acidification that are expected to occur later this century unless deep carbon emissions cuts are made, and monitored calcification. Net community calcification was reduced by 34% in the acidified reef. The findings suggest that acidification of the ocean will compromise coral reef function in the near future. Coral reefs feed millions of people worldwide, provide coastal protection and generate billions of dollars annually in tourism revenue1. The underlying architecture of a reef is a biogenic carbonate structure that accretes over many years of active biomineralization by calcifying organisms, including corals and algae2. Ocean acidification poses a chronic threat to coral reefs by reducing the saturation state of the aragonite mineral of which coral skeletons are primarily composed, and lowering the concentration of carbonate ions required to maintain the carbonate reef. Reduced calcification, coupled with increased bioerosion and dissolution3, may drive reefs into a state of net loss this century4. Our ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses. Past research has primarily focused on the responses of individual species rather than evaluating more complex, community-level responses. Here we use an in situ carbon dioxide enrichment experiment to quantify the net calcification response of a coral reef flat to acidification. We present an estimate of community-scale calcification sensitivity to ocean acidification that is, to our knowledge, the first to be based on a controlled experiment in the natural environment. This estimate provides evidence that near-future reductions in the aragonite saturation state will compromise the ecosystem function of coral reefs.