Transformation processes in the Oder Lagoon as seen from a model perspective

Abstract. The Oder/Szczecin Lagoon is one of the largest Baltic Sea lagoons and is subject to very high nutrient loads from the Oder/Odra River. For our study, we employ a modified, high-resolution 3D ecosystem model specifically adapted for this shallow lagoon. The model demonstrates stable and reliable performance over 25 years of simulation (1995–2019), enabling a detailed assessment of lagoon processes under various scenarios. Our model simulations indicate that changes in riverine nutrient inputs have an immediate impact on the lagoon’s water quality, affecting parameters such as phytoplankton biomass and water transparency. Hypoxia is a prevalent phenomenon, affecting most parts of the lagoon which promotes internal eutrophication. On average, the lagoon retains 12 % (253 t yr−1) of the phosphorus and 40 % (17 278 t yr−1) of the nitrogen riverine inputs. The primary sink process for phosphorus is sediment burial, and for nitrogen, it is denitrification. Nitrogen retention decreases with increasing riverine loads, dropping to around 30 % during years with exceptionally high inputs. In contrast, phosphorus retention is independent on loads. The nutrient retention capacity of the lagoon has significant implications for Baltic Sea eutrophication but is not currently accounted for in major policies and Baltic Sea models. Although recent nutrient loads from the Oder River comply with policy targets, such as the Baltic Sea Action Plan’s maximum allowable inputs and Germany’s river targets, these levels are insufficient to improve the lagoon’s ecological state sufficiently. The Oder Lagoon remains in a highly eutrophic condition, making the achievement of a good ecological status unlikely under the current management targets.