Eutrophication of the shallow Szczecin Lagoon (Baltic Sea): modelling, management and the impact of weather

The Oder estuary, especially the large, shallow Szczecin (Oder) Lagoon (687 W, average depth 3.8 m) suffers from severe and ongoing eutrophication due to heavy loads, mainly by the Oder river. Poor water quality nowadays is a main obstacle for further touristic development around the lagoon. Long-term nutrient concentrations show a high interannual variability and a decline during recent years. Using a simple eutrophication box model and comparing dry, warm years (1989-1991) with colder, wet years (1986-1988) we analyse the impact of interannual and short-term weather conditions on the eutrophication process. Internal nutrient cycling processes in the lagoon are mainly driven by shortterm weather conditions. During rare and short calm summer periods a stratification and oxygen depletion above the sediment is hkely. Coarse model-based estimations indicate an anoxic P-release fiom sedirnents of up to 10 pm01 P m-3d-' or up to 400-600 t P for the entire lagoon. These situations are restricted to several days and occur only in a few years. Wind with a daily average velocity above 2-3 mis cause mixing and put an end to anoxic P-release. Compared to a monthly summer load of 100150 t P by the Oder river, internal eutrophication in the lagoon is important, but has no pronounced effect on biology. In wet years the P and N load with the Oder river can be up to twice as high compared to dry years. Discharge and load by rivers control the nutnent dynamic in the Szczecin Lagoon, to a high degree. The observed reduction in nutrient concentrations in early 90's is an effect of the warm, dry years and cannot be attributed to anthropogenic nutrient load reductions. Management implications are discussed. Transactions on the Built Environment vol 58 © 2001 WIT Press, www.witpress.com, ISSN 1743-3509