Nitrogen removal in coastal sediments of the German Wadden Sea

Abstract

Although sediments of the German Wadden Sea are suspected to eliminate a considerable share of nitrate delivered to the SE North Sea, their denitrification rates have not been systematically assessed. We determined N2 production rates over seasonal cycles (February 2009–April 2010) at two locations with two sediments types each, the first site (Meldorf Bight) receiving nitrate during all seasons from the Elbe river plume, and a second site on the island of Sylt, where nitrate is depleted during summer months. In sediments from the Sylt site, N2 production ranged from 15 to 32 μmol N2 m−2 h−1 in the fine sand station and from 7 to 13 μmol N2 m−2 h−1 in the coarse sand station; N2 production was not detected when nitrate was depleted in May and July of 2009. N2 production in the Meldorf Bight sediments were consistently detected at higher rates (58–130 μmol N2 m−2 h−1 in the very fine sand station and between 14 and 30 μmol N2 m−2 h−1 in the medium sand station). Analysis of ancillary parameters suggests that major factors controlling N2 production in coastal sediments of the German Wadden Sea are the nitrate concentrations in the overlying water, the ambient temperature, and the organic matter content of the sediment. Extrapolating our spot measurements to the zone of nitrate availability and sediment types, we estimate an annual nitrogen removal rate around 16 kt N year−1 for the entire northern sector of the German Wadden Sea area. This corresponds to 14% of the annual Elbe river nitrogen load.