Coral community metabolism during year-long exposure to experimental elevated CO2 conditions

The implications of ocean acidification are acute for calcifying
organisms, notably tropical reef corals, for which accretion generally is
depressed and dissolution enhanced at reduced seawater pH. We describe
year-long experiments in which back reef and fore reef (17-m depth)
communities from Moorea, French Polynesia, were incubated outdoors under
pCO2 regimes reflecting endpoints of representative concentration pathways
(RCPs) expected by the end the century. Incubations were completed in 3–4
flumes (5.0 × 0.3 m, 500 L) in which seawater was refreshed and circulated
at 0.1 m s-1, and the response of the communities was evaluated monthly by
measurements of net community calcification (NCC) and net community
photosynthesis (NCP). For both communities, NCC (but not NCP) was affected
by treatments and time, with NCC declining with increasing pCO2, and for
the fore reef, becoming negative (i.e., dissolution was occurring) at the
highest pCO2 (1067–1433 µatm, RCP8.5). There was scant evidence of
community adjustment to reduce the negative effects of ocean
acidification, and inhibition of NCC intensified in the back reef as the
abundance of massive Porites spp. declined. These results highlight the
risks of dissolution under ocean acidification for coral reefs and suggest
these effects will be most acute in fore reef habitats. Without signs of
amelioration of the negative effects of ocean acidification during
year-long experiments, it is reasonable to expect that the future of coral
reefs in acidic seas can be predicted from their current known
susceptibility to ocean acidification.