Eutrophication Of The Shallow Szczecin Lagoon (Baltic Sea): Modelling,Management And The Impact Of Weather

Abstract

The Oder estuary, especially the large, shallow Szczecin (Oder) Lagoon (687 km2, 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 likely. Coarse model-based estimations indicate an anoxic P-release from sediments of up to 10 μmol P md 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 m/s cause mixing and put an end to anoxic P-release. Compared to a monthly summer load of 100-150 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 nutrient 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. 1 Problems and management challenges in the Oder estuary The large Oder-catchments is located at the German-Polish border. With a surface area of 120,000 km and a population of about 13 Mio inhabitants it is responsible for the heavy nitrogen and phosphorous load of the Oder river. In recent years, about 63,000 t total N/a and 3,500 t total P/a were transported with the Oder towards the Baltic Sea. The river therefore is the most important source of eutrophication and pollution for the south-western Baltic Sea. The respective coastal zones, especially the Szczecin (Oder) Lagoon, suffer from severe eutrophication, heavy algal blooms of potentially toxic species, high water turbidity and even hygienic quality problems [1]. The Szczecin Lagoon is a key element of the Oder river estuary. It is a large (687 km2) and shallow (average depth 3.8 m) coastal flow lake. The lagoon consists of two main parts Kleines Haff on the German side and Wielki Zalew located on Polish territory (Fig. 1). The Wielki Zalew comprises about 60 % of the lagoon area and volume. A theoretical water exchange time of the entire water body is about 2 months, but its western part (Kleines Haff) receives only between 10 % and 20 % of the Oder water [2]. Germany Sweden Finland Poland Germany Poland Dziwna