We investigated microbial pathways of nitrogen transformation in highly permeable sediments from the German Bight (South‐East North Sea) by incubating sediment cores percolated with ¹⁵N‐labeled substrates under near in situ conditions. In incubations with added , production of occurred while the sediment was oxic, indicating ammonia oxidation. Similarly, production during incubations indicated nitrite oxidation. Taken together these findings provide direct evidence of high nitrification rates within German Bight sands. The production of ¹⁵N‐N₂ on addition of revealed high denitrification rates within the sediment under oxic and anoxic conditions. Denitrification rates were strongly and positively correlated with oxygen consumption rates, suggesting that denitrification is controlled by organic matter availability. Nitrification and denitrification rates were of the same magnitude and the rapid production of ¹⁵N‐N₂ in incubations with added confirmed close coupling of the two processes. Areal rates of N‐transformation were estimated taking advective transport of substrates into account and integrating volumetric rates over modeled oxygen and nitrate penetration depths, these ranged between 22 μmol N m⁻² h⁻¹ and 94 μmol N m⁻² h⁻¹. Furthermore, results from the ¹⁵N‐labeling experiments show that these subtidal permeable sediments are, in sharp contrast to common belief, a substantial source of N₂O. Our combined results show that nitrification fuels denitrification by providing an additional source of nitrate, and as such masks true N‐losses from these highly eutrophic sediments. Given the widespread occurrence of anthropogenically influenced permeable sediments, coupled benthic nitrification–denitrification might have an important but so far neglected role in N‐loss from shelf sediments.

The Coastal Observing System for Northern and Arctic Seas (COSYNA) was established in order to better understand the complex interdisciplinary processes of northern seas and the Arctic coasts in a changing environment. Particular focus is given to the German Bight in the North Sea as a prime example of a heavily used coastal area, and Svalbard as an example of an Arctic coast that is under strong pressure due to global change.The COSYNA automated observing and modelling system is designed to monitor real-time conditions and provide short-term forecasts, data, and data products to help assess the impact of anthropogenically induced change. Observations are carried out by combining satellite and radar remote sensing with various in situ platforms. Novel sensors, instruments, and algorithms are developed to further improve the understanding of the interdisciplinary interactions between physics, biogeochemistry, and the ecology of coastal seas. New modelling and data assimilation techniques are used to integrate observations and models in a quasi-operational system providing descriptions and forecasts of key hydrographic variables. Data and data products are publicly available free of charge and in real time. They are used by multiple interest groups in science, agencies, politics, industry, and the public.

A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent. Here we present a laboratory investigation where we systematically varied the dune lee slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee slopes of 10°, 20°, and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high‐frequency, vertical profiles collected with a Laser Doppler Velocimeter. We show that the temporal and spatial occurrence of flow separation decreases with dune lee slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes. Aperiodic, strong ejection events dominate the shear layer but decrease in strength and frequency for low‐angle dunes. Flow resistance of dunes decreases with lee slope; the transition being nonlinear. Over the 10°, 20°, and 30° dunes, shear stress is 8%, 33%, and 90% greater than a flat bed, respectively. Our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance.