Spatial impact of the Oder river plume on water quality along the south-western Baltic coast

The Oder (Odra) river is the most important nutrient source and pollutant for the south-western Baltic Sea. Adjacent German-Polish coastal waters, the Oder (Szczecin) Lagoon and the Oder (Pomeranian) Bight therefore suffer from severe eutrophication and water quality problems. At the same time, summer (bathing) tourism is the most important economical factor in this coastal zone, especially on the islands of Usedom and Wolin. On the basis of model simulations and remote sensing data we analysed the spatial extent and variability of the Oder river plume in the lagoon and the Balic Sea in common summer situations and during the extreme Oder flood in August 1997. Water quality shows pronounced gradients between coastal waters and open Baltic Sea. In the lagoon, it usually takes more than 6 weeks until Oder water enters the large western bay, the Kleines Haff. During transport, degradation, transformation and sedimentation processes alter the water quality and prevent the inner coast of Usedom from direct impact of polluted Oder water. Ongoing nutrient supply promotes intensive algal proliferation in all parts of the lagoon and contributes to the low water transparency. Oder water passing the lagoon and entering the Baltic Sea is transported over long distances in narrow bands along the shore. Under easterly winds the water quality near well-known spas on Usedom is reduced due to Oder river plume impact. Upwelling effects can have negative impact on water quality, too. Intensive blooms of potentially toxic blue-green algae species, are the rule in the lagoon and frequent in the Oder Bight in summer. They are a hazard and limit the acceptance of swimming beaches at the inner coast of Usedom. Practical consequences of variable water quality gradients e.g. on hygienic water sampling are discussed.