Abstract
The Anthropocene climate has largely been defined by a rapid increase in atmospheric CO2, causing global climate change (warming) and ocean acidification (OA, a reduction in oceanic pH). OA is of particular concern for coral reefs, as the associated reduction in carbonate ion availability impairs biogenic calcification and promotes dissolution of carbonate substrata. While these trends ultimately affect ecosystem calcification, scaling experimental analyses of the response of organisms to OA to consider the response of ecosystems to OA has proved difficult. The benchmark of ecosystem-level experiments to study the effects of OA is provided through Free Ocean CO2 Enrichment (FOCE), which we use in the present analyses for a 21-d experiment on the back reef of Mo’orea, French Polynesia. Two natural coral reef communities were incubated in situ, with one exposed to ambient pCO2 (393 µatm), and one to high pCO2 (949 µatm). Our results show a decrease in 24-h net community calcification (NCC) under high pCO2, and a reduction in nighttime NCC that attenuated and eventually reversed over 21-d. This effect was not observed in daytime NCC, and it occurred without any effect of high pCO2 on net community production (NCP). These results contribute to previous studies on ecosystem-level responses of coral reefs to the OA conditions projected for the end of the century, and they highlight potential attenuation of high pCO2 effects on nighttime net community calcification.
Highlights
Tropical coral reefs provide coastal protection, livelihoods, and food to millions of people[1]
A second project was conducted on Heron Island, Australia, in which deployment of technology described as a “coral proto-Free Ocean CO2 Enrichment (FOCE)” showed decreases in coral calcification[21], and alteration in the boron isotopic composition of coral skeletons[22]
Our study presents the first community metabolism results of the deployment of a FOCE experiment on a shallow, back reef community, and it describes the response of this community to high pCO2 under ecologically relevant environmental conditions[23]
Summary
Tropical coral reefs provide coastal protection, livelihoods, and food to millions of people[1]. OA is expected to negatively affect organisms which produce calcareous shells and skeletons more than non-calcifying taxa[6,7,8] These effects will be experienced acutely by coral reefs, where the physical structure of the calcareous substratum is built by scleractinian corals and calcified algae[7]. OA is predicted to severely impact the underlying biogenic substrata of coral reefs through increased dissolution in response to acidifying seawater conditions[9,10]. We significantly expand on these previous experiments by conducting a FOCE experiment to test the effects of predicted end-of-century OA conditions on NCC and Net Community Production (NCP) of a natural back reef community in Mo’orea, French Polynesia. Our results show how the ecosystem function of coral reefs (NCC and NCP) will be altered by OA during the day and night, resulting in improved accuracy of predictions of the effects of OA on coral reef metabolism
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