Use of NOx- microsensors to estimate the activity of sediment nitrification and NOx- consumption along an estuarine salinity, nitrate, and light gradient

Abstract

Microprofiles of nitrate plus nitrite (NOx-) were measured in sediment cores sampled at 6 stations along a nitrate and salinity gradient and a depth transect in Randers Fjord, Denmark. Rates of NOx- production and consumption were calculated from the concentration profiles to describe the variation of these processes under in situ conditions. A microscale biosensor for NOx- was used to obtain NOx- profiles in darkness and under in situ light conditions. This new tool for nitrate plus nitrite measurements in saline environments made it possible to measure NOx- profiles with submicromolar resolution and without interference from chemical species other than nitrous oxide. The NOx- concentration in the inner and middle part of the estuary was very high (55 to 220 μM). The range of NOx- consumption rates found here (47 to 577 μmol N m-2 h-1) were within the range of denitrification rates found in similar environments (measured by 15N isotope techniques). The NOx- concentration in the water was low at the outermost part of the estuary, and all NOx- diffusing into the sediment was consumed in the oxic zone (top 1 mm) of the sediment. In this study, NOx- consumption was not closely coupled to nitrification and depended mostly on NOx- (mainly nitrate) from the overlying water. The NOx- consumption rates were correlated with NOx- concentration in the overlying water, but the sediment was a sink for NOx- only at the highest NOx- concentrations. The effect of changing light conditions on NOx- consumption was significant at the station with the highest NOx- level but not at other stations.