The interaction between current flow and topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment-water solute exchange relative to fluid shear along a flat bottom. It is currently unknown how this effect from surface ripples will modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-hour incubations in flume aquaria with permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on carbonate sediment metabolism and dissolution. According to dissolved oxygen optode image analysis, the presence of surface ripples increased the oxygenated area below the sediment surface by roughly 295% relative to flat sediments. This was reflected in the sediment-to-water column fluxes of dissolved oxygen, where rippled sediments exhibited rates of respiration (R) and gross primary production (GPP) that were 43% and 52% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying (Gnet > 0) to net dissolving (Gnet < 0), an effect that was amplified an additional 59% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment Gnet may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

Supplement to: Lantz, Coulson A; Kessler, A; Schulz, Kai Georg; Eyre, Bradley D (in review): Sediment topography enhances the response of coral reef carbonate sediment dissolution to ocean acidification. Journal of Geophysical Research: Biogeosciences