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 f or the southwestern Baltic Sea. Adjacent German-Polish coastal waters, the Oder (Szczecin) Lagoon and the Oder (Pomeranian) Bight therefore suffer fro m severe eutrophication and water quality problems. At the same time, summe r (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 s patial 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 wate rs and open Baltic Sea. In the lagoon, it usually takes more than 6 weeks u ntil Oder water enters the large western bay, the Kleines Haff. During tran sport, degradation, transformation and sedimentation processes alter the wa ter quality and prevent the inner coast of Usedom from direct impact of pol luted Oder water. Ongoing nutrient supply promotes intensive algal prolifer ation in all parts of the lagoon and contributes to the low water transpare ncy. Oder water passing the lagoon and entering the Baltic Sea is transport ed over long distances in narrow bands along the shore. Under easterly wind s the water quality near well-known spas on Usedom is reduced due to Oder r iver plume impact. Upwelling effects can have negative impact on water qual ity, too. Intensive blooms of potentially, toxic blue-green algae species, are the ru le in the lagoon and frequent in the Oder Bight in summer. They are a hazar d and limit the acceptance of swimming beaches at the inner coast of Usedom . Practical consequences of variable water quality gradients e.g. on hygien ic water sampling are discussed.