The role of retention processes in the coastal filter of the Oder River

Riverine nutrient export significantly impacts coastal and shelf seas, causing eutrophication. However, nutrient export can be mitigated by the coastal filter function, potentially resulting in permanent removal of nutrients through denitrification. Besides removal, nutrients can also be retained in the coastal zone through assimilation, remineralisation or nitrification, with repeated recycling prolonging their availability to organisms. While removal processes have been extensively studied, their effect on the riverine dissolved inorganic nitrogen (DIN) loads reaching the coast may be small. There are only a few accurate estimates that include all processes of the coastal filter. In this study, nutrient concentrations and net autotrophic uptake rates in the water column, together with benthic NO₃⁻ reduction rates and solute fluxes across the sediment-water interface, were measured. We compared a lagoon with the open coast within the outflow area of the Oder River, one of the largest inflows to the southern Baltic Sea, over a seasonal cycle. Particle drift experiments using hydrodynamic model simulations of the Oder estuary yielded water residence times (WRT) along the outflow. In the lagoon, DIN concentrations, primary production and DIN uptake rates were higher than at the coastal station. The data suggested efficient DIN retention, as the majority of riverine DIN was assimilated (85 %), with a preference for NH₄⁺ over NO₃⁻ uptake. High retention in the lagoon was supported by long WRT of up to 180 days. In the open coast, productivity was lower due to continuous dilution of riverine nutrients by transport processes. The sediment served as a periodic source of nutrients for water column processes. Our data suggest that intense recycling in the water column and strong benthic-pelagic coupling promote a tightly coupled nitrification-denitrification, resulting in vital N removal rates in the sediment. Nevertheless, strong predominance of N retention processes, and specifically assimilation over N removal processes, illustrates that the former is an important component of the coastal filter function, as it provides substrates for nutrient removal (organic matter and NO₃⁻). These results extend the assessment of the effectiveness of the coastal filter and should be taken into account in further studies.