AME 26:181-193 (2001)  -  doi:10.3354/ame026181

ABSTRACT: Microprofiles of nitrate plus nitrite (NO_x^-) 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 NO_x^- production and consumption were calculated from the concentration profiles to describe the variation of these processes under in situ conditions. A microscale biosensor for NO_x^- was used to obtain NO_x^- 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 NO_x^- profiles with sub-micromolar resolution and without interference from chemical species other than nitrous oxide. The NO_x^- concentration in the inner and middle part of the estuary was very high (55 to 220 μM). The range of NO_x^- 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 NO_x^- concentration in the water was low at the outermost part of the estuary, and all NO_x^- diffusing into the sediment was consumed in the oxic zone (top 1 mm) of the sediment. In this study, NO_x^- consumption was not closely coupled to nitrification and depended mostly on NO_x^- (mainly nitrate) from the overlying water. The NO_x^- consumption rates were correlated with NO_x^- concentration in the overlying water, but the sediment was a sink for NO_x^- only at the highest NO_x^- concentrations. The effect of changing light conditions on NO_x^- consumption was significant at the station with the highest NO_x^- level but not at other stations.

KEY WORDS: Nitrate biosensor · Nitrogen transformation