Ocean acidification (OA) and organic matter (OM) enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22-h incubations in flume aquaria with captive sediment communities to measure the combined effect of elevated pCO(2) (representing Ocean Acidification) and particulate organic carbon (representing coastal eutrophication) on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (G(net)). Relative to control sediment communities, both OA (pCO(2) similar to 1,000 mu atm) and OM enrichment (similar to +40 mu mol C L-1) significantly decreased rates of sediment G(net) by 1.16 and 0.18 mmol CaCO3 m(-2) h(-1), respectively, but the mechanism behind this decrease differed. The OA-mediated transition to net dissolution was physiochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (omega(arg)) of the overlying water column and the physical factors governing the pore water exchange rate with this overlying water column. In contrast, the OM-mediated decline in G(net) was due to a decline in the overlying seawater omega(arg) due to the increased respiratory addition of CO2. The decrease in G(net) in response to a combination of both stressors was additive (-0.09 mmol CaCO3 m(-2) h(-1) relative to OA alone), but this decrease did not significantly differ from the individual effect of either stressor. In this study OA was the primary driver of future carbonate sediment dissolution, but longer-term experiments with chronic organic matter enrichment are required.
