Alkalinity Enrichment Enhances Calcification of a Coral Reef Flat

Ocean acidification is projected to shift reefs from a state of net accretion to one of net dissolution sometime this century. While retrospective studies show large-scale changes in coral calcification over the last several decades, determining the contribution of ocean acidification to these changes is difficult due to the confounding factors of temperature and other environmental parameters. Here, we quantified the calcification response of a coral reef flat to alkalinity enrichment to test whether reef calcification increases when ocean chemistry is restored closer to pre-industrial conditions. We used sodium hydroxide (NaOH) to increase the total alkalinity of seawater flowing over a reef flat, with the aim of increasing carbonate ion concentrations [CO₃^2-] and the aragonite saturation state (Ω_arag) to values that would have been attained under pre-industrial atmospheric pCO₂ levels. We developed a dual tracer regression method to estimate alkalinity uptake (i.e., calcification) in response to alkalinity enrichment. This approach uses the change in ratios between a non-conservative tracer (alkalinity) and a conservative tracer (a non-reactive dye, Rhodamine WT) to assess the fraction of added alkalinity that is taken up by the reef as a result of an induced increase in calcification rate. Using this method, we estimate that an average of 17.3% ± 2.3% of the added alkalinity was taken up by the reef community, inferring a 6.9 ± 0.9% increase in net community calcification. The magnitude of the calcification response is in agreement with the theoretical increase expected from earlier laboratory and mesocosm studies. In providing results from the first seawater chemistry manipulation experiment performed on a natural coral reef community (without artificial confinement), we demonstrate that, upon increase of [CO₃^2-] and Ω_arag closer to pre-industrial values, net reef calcification increases. Thus, we conclude that ocean acidification is already impairing coral reef growth. This work is the culmination of years of work in the Caldeira lab at the Carnegie Institution for Science, involving many people including Jack Silverman, Kenny Schneider, and Jana Maclaren.