Benthic microbial metabolism in seagrass meadows along a carbonate gradient in Sulawesi, Indonesia

Sediment carbon and nitrogen cycling were examined in tropical seagrass meadows across a gradient of increasing carbonate content during wet and dry seasons in Awerange Bay, Sulawesi, Indonesia. Coincident with an increase in seagrass and microalgal productivity, increases in rates of sulfate reduction, ammonification, O-2 consumption, nitrogen fixation, CH4 release, carbonate dissolution, and pore water concentrations and fluxes of H2S were observed across the gradient. These increases occurred despite a decline in particulate nutrient and dissolved iron concentrations in the sediment. Sulfate reduction appeared to be the dominant decomposition pathway, with rates ranging from 27-68 mmol S m(-2) d(-1) inshore to 53-98 mmol S m(-2) d(-1) in the outer bay. Rapid rates of carbon oxidation were matched by rapid rates of N transformation, with ratios of C and N mineralization rates close to the sediment C:N ratio. Denitrification (range: 1022 to 1897 mu mol N m(-2) d(-1)) equated to similar to 25% of ammonification, with the proportion decreasing from 19-53% inshore to 0.5-17% offshore. The patterns of increase in C and N cycling were attributed to greater water clarity along the gradient, allowing greater light levels to foster higher rates of seagrass productivity and, in turn, increased rates of mineralization and carbonate dissolution.