Ocean acidification on coral reefs: the effects of community composition on benthic metabolism and seawater carbonate chemistry
The ability of many coral reefs to maintain community composition (i.e., high coral cover), ecosystem structure, and reef functions have diminished as they face regional perturbations and global climate change. Future anthropogenic ocean acidification (OA) from oceanic uptake of carbon dioxide (CO2) may further contribute to declining global reef health. However, the magnitude of OA on coral reefs will be compounded by natural temporal and spatial variability which is influenced by a suite of biological and physical oceanographic properties including benthic metabolism by reef communities. In mesocosm experiments, calcifying communities experienced relatively static seawater pH and saturation state with respect to aragonite (Ωa) due to counteracting effects of net organic carbon production and net calcification on these parameters. In contrast, fleshy macroalgae elevated daytime seawater pH and Ωa due to high rates of net organic carbon production; a mixed community also elevated seawater pH and Ωa albeit to a lesser extent. Under short-term OA, daytime net organic carbon production by fleshy macroalgae and nighttime calcium carbonate dissolution buffered against acidification. In additional experiments, net community calcification and net respiration scaled proportionally to coral cover; however, daytime net organic carbon production did not. This differential scaling of benthic metabolism caused increasing acidification with increasing coral cover at night and similar seawater chemistry during the day. Thus, areas with higher coral cover experience larger diurnal seawater pH and CO2 chemistry as observed in mesocosms and on reefs in Bermuda and Hawaii. Although community composition influences in situ temporal and spatial variability in seawater CO2 chemistry on a reef flat in Kaneohe Bay, Hawaii, the strongest drivers relate to seawater depth, which reflects water volume to benthic biomass ratio, and circulation, which relates to the age of seawater that has been continuously modified by biogeochemical processes. Thus, to more accurately predict the magnitude and potential effects of future OA on coral reefs, experimental results should be bridged to natural settings which often are more complex and have co-varying biological (i.e., community composition and benthic metabolism) and physical (i.e. seawater depth and circulation) properties that influence seawater CO2 variability across space and time.