N-2 fluxes in sediments of the Elbe Estuary and adjacent coastal zones

The conversion of reactive nitrogen species to N-2 during denitrification in sediments may be one of the most valuable ecosystem services provided by estuarine and intertidal environments near river discharge areas. To quantify the rates and limiting factors of denitrification in the estuary of the Elbe River and adjacent Wadden Sea (SE North Sea), we measured sediment N2 fluxes across subtidal and intertidal sediments, and along gradients in nitrate and organic matter concentrations. We conducted 2 sampling campaigns, in March and September 2009, during which N2 fluxes were quantified by N-2/Ar measurements in sediment core incubations, and compared to isotope pairing results in September 2009. At ambient nitrate concentrations, sediments in the inner Elbe Estuary produced N2 fluxes of up to 156 mu mol N m(-2) h(-1). In September, nitrate concentration in bottom water and organic matter content in sediments limited N-2 production; such limitations were not observed in March. We extrapolated the estuarine sediment nitrogen removal of March and September to the present-day area of intertidal and subtidal sediments in the Elbe Estuary between the port of Hamburg and the transition to the North Sea. Our estimate suggests that 3.3 +/- 1.2 kt nitrate-N are removed in sediments in this region in spring and summer. This implies that reactive nitrogen removal in the inner Elbe Estuary reduces the spring/summer load of the Elbe River (47 kt N) by around 7%, a reduction that is much lower than commonly assumed for estuaries, and significantly lower than nitrate removal in the Elbe was in historical times