Seawater carbonate chemistry and 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, 149.826 W), were tested from 12 November 2015 to 16 November 2016 in outdoor flumes maintained at mean pCO2 levels of 364 µatm, 564 µatm, 761 µatm, and 1067 µatm. The communities consisted of four corals and two calcified algae, with change in mass (Gnet, a combination of gross accretion and dissolution) and percent cover recorded monthly. For massive Porites and Montipora spp., Gnet differed among treatments, and at 1067 µatm (relative to ambient) was reduced and still positive; for Porolithon onkodes, all of which died, Gnet was negative at high pCO2, revealing dissolution (sample sizes were too small for analysis of Gnet for other taxa). Growth rates (% cover month−1) were unaffected by pCO2 for Montipora spp., P. rus, Pocillopora verrucosa, and Lithophyllum kotschyanum, but were depressed for massive Porites at 564 µatm. Multivariate community structure changed among seasons, and the variation under all elevated pCO2 treatments differed from that recorded at 364 µatm, and was greatest under 564 µatm and 761 µatm pCO2. 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 pCO2 on Gnet of their component organisms.