Temporal variability of denitrification in estuarine sediments

Sediment denitrification rates and fluxes of nitrous oxide, nitrate, nitrite and ammonium were determined at two intertidal sites in the Tamar estuary (S.W. England). High sediment nitrate uptake rates were recorded throughout the year, whereas the nitrite and ammonium fluxes were positive (from sediment to water column), with the former resulting from nitrification. Nitrous oxide flux was also positive, being largely attributable to denitrification with some contribution from nitrification or nitrification-denitrification coupling. No relationship was apparent between denitrification rate and nitrate concentration in the overlying water, invalidating the notion that denitrification automatically regulates nitrate during periods of elevated ambient concentration. However, denitrification exhibited a strong covariance with the degree of sediment bioturbation (Nereis diversicolor), which was considered to be attributable to increased transport and supply of nitrate via Nereis burrows. Denitrification accounted for 8·5% of the annual total nitrate loading to the Tamar estuary, although a maximum of 100% was observed in summer when the phytoplankton nutrient requirement would also be highest.