Sediment–water inorganic nutrient exchange and nitrogen budgets in the Colne Estuary, UK

Abstract

The exchange of dissolved inorganic nutrients (NO3 - , NO2 - NH4 + , PO4 3- and SiO3 - ) was measured across the sediment-water interface at 4 sites along the Colne Estuary at monthly intervals between August 1996 and March 1998 in the dark and under illumination. Fluxes across the sedi- ment-water interface were scaled up to calculate nutrient budgets for the whole estuary, accounting for the area of sediment exposed to illumination. Sediments were a net sink of -16.28 Mmol N yr -1 for oxidized inorganic nitrogen (NO3 - + NO2 - ). Mean nitrate fluxes across the sediment-water interface were -329 and -91 µmol m -2 h -1 in the dark and light respectively. Colne sediments were a source of ammonium in the dark (12.4 Mmol N yr -1 ). Illumination reduced ammonium flux into the overlying water or affected uptake due to assimilation by the microphytobenthos and a stimulation of nitrifi- cation. Mean ammonium fluxes across the sediment-water interface were 399 and -169 µmol m -2 h -1 in the dark and light, respectively. Consequently, sediments were a net sink for ammonium in the light (-0.31 Mmol N yr -1 ). Similarly, illumination affected silicate exchange across the sediment- water interface, with silicate flux into the overlying water reduced or reversed under illumination compared to the dark. The estuarine sediments were a net source of silicate (6.09 Mmol yr -1 ) and soluble reactive phosphate (1.91 Mmol yr -1 ). Uptake of inorganic oxidized nitrogen was approxi- mately balanced by adding together ammonium flux and denitrification supported by nitrate in the overlying water. This mass balance indicates that dissimilatory nitrate reduction to ammonium (DNRA) was a significant process in the sediments of the Colne Estuary. Fluxes in the light accounted for less than 6% of the total sediment-water exchange of both ammonium and nitrate. However, a large proportion of the -15.49 Mmol N yr -1 of oxidized inorganic nitrogen taken up by the sediment in the dark was cancelled out by a flux of 12.4 Mmol N yr -1 into the overlying water as ammonium. Net dissolved inorganic nitrogen (DIN) fluxes were -3.09 Mmol N yr -1 in the dark and -1.17 Mmol Ny r -1 in the light, as fluxes of all nitrogen species were negative under illumination. Therefore fluxes under illumination accounted for 28% of the net uptake of DIN by estuarine sediments in the Colne Estuary, despite the short time periods and small area of sediment under illumination. In macrotidal estuaries such as the Colne, where microphytobenthos are abundant, light plays an important role in moderating sediment-water nutrient exchange.