Long-term eutrophication history and ecosystem changes in a large Baltic river basin - estuarine system

1 - We apply a spatial integrative approach to analyse the eutrophication history and functional changes of the Odra river basin, the Szczecin Lagoon and its coastal waters in the southern Baltic region (area of about 150,000 km2) between early 1960s and 2000s. For this purpose, a river basin model was linked to a coastal and marine 3D-hydrodynamic ecosystem model. 2 - Annual riverine nutrient loads increase from the 1960s until the mid 1980s to 110,000 t nitrogen (N) and 15,000 t phosphorus (P) and decrease afterwards, but show a strong inter-annual variability. The contribution and location of emission sources in the catchment change significantly during the investigated 40 years. 3 - Nutrient and chlorophyll concentrations in the estuary follow changes in riverine loads quickly. Strong spatial gradients in the estuary are maintained. Nutrient availability, N/P ratios as well as processes like denitrification and N fixation show spatial and temporal changes. 4 - In the lagoon, N fixation does not play an important role and riverine N load reductions are not compensated by increased N fixation. 5 - The lagoon’s quantitative N retention is stable over the 40 years. However, in the early 1960s, about 26% of the total N load into the lagoon is removed by denitrification, compared to only about 15% during 1999–2002. 6 - River basin management measures should focus on a load reduction of P and on N