Changes in coral reef community structure in response to year-long incubations under contrasting pCO2 regimes

Coral reefs are threatened by ocean acidification (OA), which depresses net calcification of corals, calcified algae, and coral reef communities. These effects have been quantified for many organisms, but most experiments last weeks-to-months, and do not test for effects on community structure. Here, the effects of OA on back reef communities from Mo'orea, French Polynesia (17.492 S, 149.826W), were tested from 12 November 2015 to 16 November 2016 in outdoor flumes maintained at mean pCO(2) levels of 364 mu atm, 564 mu atm, 761 mu atm, and 1067 mu atm. The communities consisted of four corals and two calcified algae, with change in mass (G(net), a combination of gross accretion and dissolution) and percent cover recorded monthly. For massive Porites and Montipora spp., G(net) differed among treatments, and at 1067 mu atm (relative to ambient) was reduced and still positive; for Porolithon onkodes, all of which died, G(net) was negative at high pCO(2), revealing dissolution (sample sizes were too small for analysis of G(net) for other taxa). Growth rates (% cover month(-1)) were unaffected by pCO(2) for Montipora spp., P. rus, Pocillopora verrucosa, and Lithophyllum kotschyanum, but were depressed for massive Porites at 564 mu atm. Multivariate community structure changed among seasons, and the variation under all elevated pCO(2) treatments differed from that recorded at 364 mu atm, and was greatest under 564 mu atm and 761 mu atm pCO(2). Temporal variation in multivariate community structure could not be attributed solely to the effects of OA on the chemical and physical properties of seawater. Together, these results suggest that coral reef community structure may be more resilient to OA than suggested by the negative effects of high pCO(2) on G(net) of their component organisms.