Controls of inputs of reactive nitrogen into the German Bight in the main Estuaries: No two are alike

Estuaries act as biogeochemical filters for organic matter and nutrients transported from rivers into coastal waters, with the balance of turnover processes such as remineralization and nitrification determining whether these are retained, transformed or exported into the coastal ocean. In the German Bight, three main river systems (Ems, Weser and Elbe) provide water and matter inputs. These rivers and their estuaries are heavily impacted by human activities, including dredging, damming and intensive nutrient inputs causing eutrophication, which may substantially alter their biogeochemical filter function. We aim to assess how differing anthropogenic pressures may influence nitrogen transformation processes and, consequently, the efficiency of estuaries as biogeochemical filters.

During an early autumn 2024 cruise on the RV Heincke (HE647), we measured parameters such as salinity, turbidity, oxygen and chlorophyll-a-fluorescence in all three estuaries and sampled nutrients focusing on dissolved inorganic nitrogen (ammonium, nitrite and nitrate) and dual stable isotopes of nitrate. Additionally, nitrification and ammonium uptake rates were determined in the Elbe and Ems estuaries.

All three estuaries were characterized by high nitrate input to coastal waters. However, ammonium uptake and nitrification rates differed substantially among the systems, with the highest uptake observed during a phytoplankton bloom in the coastal outer waters of the Ems Estuary. Our results indicate that suspended matter concentration, oxygen availability and chlorophyll-a-fluorescence are the main factors driving the remineralization and retention of reactive nitrogen in estuarine and coastal waters.