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 × 0.3 × 0.3 m) located outdoors in Moorea, French Polynesia, we assembled local back reef communities with ~ 25% coral cover, and tested their response to pCO2 levels of 344, 633, 870 and 1146 μatm. Incubations began in late Austral spring (November 2015), and net community calcification (Gnet) and net community primary production (Pnet) were measured prior to treatments, 24 h after treatment began, and biweekly or monthly thereafter until early Austral autumn (March 2016). Gnet was depressed under elevated pCO2 over 4 months, although the magnitude of the response varied over time. The proportional decline in Gnet as a function of saturation state of aragonite (Ωar) depended on the initial Ωar, but was 24% for a decline in Ωar from 4.0 to 3.0, which is nearly twice as sensitive to variation in Ωar than the previously published values for the net calcification of ex situ coral colonies. However, community Gnet was less sensitive to Ωar than coral reefs that have been analyzed in situ. Pnet was unaffected by pCO2, but Pnet and Gnet expressed on a hourly time base were positively associated, thus revealing the tight coupling between these metabolic processes. The high sensitivity of Gnet to pCO2 for the back reef of Moorea, versus lower sensitivity of individual coral colony calcification to pCO2, underscores the challenges of scaling-up experimental results on the effects of pCO2 from coral reef organisms to coral reef communities.
Funding for this study was provided by the National Science Foundation grant OCE 14-15268, and benefitted from support through the Moorea Coral Reef LTER program (NSF OCE 12-36905) and gifts from the Gordon and Betty Moore Foundation. Research was completed under permits issues by the Haut-commissariat de la République en Polynésie Francaise (DRRT) (Protocole d’Accueil 2015–2016). We thank J. Manning, J. Bergman, S. Merolla, B. Shayka, S. Comeau, V. Moriarty, A. Siravo, D. Sternberg and S. Zimmerman for assistance with maintaining the flumes over 4 months. We also are grateful to three reviewers whose comments improved the manuscript. This is contribution number 268 of the CSUN Marine Biology Program.
Compliance with ethical standards
This study was funded by the National Science Foundation (grant OCE-1415268). All authors declare that they have no conflict of interest. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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