Reversal of ocean acidification enhances net coral reef calcification

Chair: Jean-Pierre Gattuso

Rebecca Albright(1), Ken Caldeira(2)

1 Carnegie Institution for Science, Stanford, CA, 94305, USA
2 Carnegie Institution for Science, Stanford, CA, 94305, USA

Background
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.

Methods
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 [CO32-] and the aragonite saturation state (Ωarag) closer 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., net calcification) in response to alkalinity addition. This approach uses the change in ratios between a non-conservative tracer (alkalinity) and a conservative tracer (non-reactive dye, Rhodamine WT) to assess the fraction of added alkalinity that is taken up by the reef.

Findings
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.

Conclusions
In providing results from the first seawater chemistry manipulation experiment performed on a natural (i.e., unconfined) coral reef community, we demonstrate that, upon increase of [CO32-] and Ωarag closer to pre-industrial values, net reef calcification increases. Thus, we conclude that ocean acidification may already be 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.