Description: Denitrification on continental margins and in coastal sediments is a major sink of reactive N in the present nitrogen cycle and a major ecosystem service of eutrophied coastal waters. We analysed the nitrate removal in surface sediments of the Elbe estuary, Wadden Sea, and adjacent German Bight (SE North Sea) during two seasons (spring and summer) along a eutrophication gradient ranging from a high riverine nitrate concentrations at the Elbe Estuary to offshore areas with low nitrate concentrations. The gradient encompassed the full range of sediment types and organic carbon concentrations of the southern North Sea. Based on nitrate penetration depth and concentration gradient in the porewater we estimated benthic nitrate consumption rates assuming either diffusive transport in cohesive sediments or advective transport in permeable sediments. For the latter we derived a mechanistic model of porewater flow. During the peak nitrate discharge of the river Elbe in March, the highest rates of diffusive nitrate uptake were observed in muddy sediments (up to 2.8 mmol/m**2/d). The highest advective uptake rate in that period was observed in permeable sediment and was tenfold higher (up to 32 mmol/m**2/d). The intensity of both diffusive and advective nitrate consumption dropped with the nitrate availability and thus decreased from the Elbe estuary towards offshore stations, and were further decreased during late summer (minimum nitrate discharge) compared to late winter (maximum nitrate discharge). In summary, our rate measurements indicate that the permeable sediment accounts for up to 90 % of the total benthic reactive nitrogen consumption in the study area due to the high efficiency of advective nitrate transport into permeable sediment. Extrapolating the averaged nitrate consumption of different sediment classes to the areas of Elbe Estuary, Wadden Sea and eastern German Bight amounts to an N-loss of 3.1 * 10**6 mol N/d from impermeable, diffusion-controlled sediment, and 5.2 * 10**7 mol N/d from permeable sediment with porewater advection.

Description: As the determinant of solute and particle fluxes through sediments, quantifying sediment permeability is vital step in understanding of the exchange phenomena between the water column and sediment as permeability determines the mode and intensity of solute and particle fluxes. Reliable estimates of sediment permeability are therefore a constraint on the accurate implementation of benthic biogeochemical models. This is particularly true for the North Sea, as field data are scarce and available grain-size-based models fail to represent the full range of sediment types. In this study, we combine measurements of sediment permeability and grain size analysis with a generic permeability model to establish a high-resolution permeability map of the sediment in the German Bight (North Sea). Our results show a good agreement between model-based prediction and measurements of permeability, even for a wide range of permeability values.