Responses of coral reef community metabolism in flumes to ocean acidification

Much of the research on the effects of ocean acidification on tropical coral reefs has focused on the calcification rates of individual coral colonies, and less attention has been given to carbonate production and dissolution at the community scale. Using flumes (5.0 x 0.3 x 0.3 m) located outdoors in Moorea, French Polynesia, we assembled local back reef communities with similar to 25% coral cover, and tested their response to pCO(2) levels of 344, 633, 870 and 1146 mu atm. Incubations began in late Austral spring (November 2015), and net community calcification (G(net)) and net community primary production (P-net) were measured prior to treatments, 24 h after treatment began, and biweekly or monthly thereafter until early Austral autumn (March 2016). G(net) was depressed under elevated pCO(2) over 4 months, although the magnitude of the response varied over time. The proportional decline in G(net) as a function of saturation state of aragonite (Omega(ar)) depended on the initial Omega(ar), but was 24% for a decline in Omega(ar) from 4.0 to 3.0, which is nearly twice as sensitive to variation in Omega(ar) than the previously published values for the net calcification of ex situ coral colonies. However, community G(net) was less sensitive to Omega(ar) than coral reefs that have been analyzed in situ. P-net was unaffected by pCO(2), but P-net and G(net) expressed on a hourly time base were positively associated, thus revealing the tight coupling between these metabolic processes. The high sensitivity of Gnet to pCO(2) for the back reef of Moorea, versus lower sensitivity of individual coral colony calcification to pCO(2), underscores the challenges of scaling-up experimental results on the effects of pCO(2) from coral reef organisms to coral reef communities.