Enhancement of coupled nitrification-denitrification by benthic photosynthesis in shallow estuarine sediments

Net sediment-water interface fluxes of oxygen, dinitrogen, and dissolved inorganic carbon and nitrogen were determined in shallow, subtidal estuarine sediments (Galveston Bay, Texas) using in situ benthic chamber incubations. Diel variability in the sediment oxygen demand and in the flux of dinitrogen were attributable to the occurrence of benthic photosynthesis. Daytime denitrification rates exceeded nighttime rates. During daytime chamber incubations, small changes in dissolved oxygen concentration were observed. At night, oxygen concentrations decreased, as consumption reactions dominated the oxygen flux. Oxygen production by benthic microalgae enhanced rates of, and coupling between, nitrification and denitrification when ammonium was not limiting, resulting in significant loss of nitrogen from sediments as dinitrogen gas. A transient three box model was used to assess the effect of benthic primary production on sediment nitrogen cycling. Model results demonstrate increased rates of nitrification and denitrification in the presence of benthic photosynthesis and agree well with observed patterns of dissolved dinitrogen- and oxygen-flux data obtained during benthic chamber incubations. When no photosynthesis was permitted during the model runs, rates of nitrification and denitrification decreased significantly. Together, our results suggest that denitrification estimates for shallow estuaries could be underestimated if the influence of benthic primary production on denitrification is not considered.