Spatial impact of the Oder river plume on water quality along the south-western Baltic coast Gerald Schernewski a , Thomas Neumann a , Victor Podsetchine b , Herbert Siegel a a Baltic Sea Research Institute Warnemünde, Seestrasse 15, D-118119 Rostock-Warnemünde, Germany b Pirkanmaa Regional Environment Centre, Rautatienkatu 21 B, FIN-33100 Tampere, Finland Received September 13, 2000 ´ Accepted June 6, 2001 Abstract 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. Key words: Szczecin Lagoon ± algal bloom ± eutrophication ± transport modelling ± remote sensing ± coastal zone management Introduction Coastal zones adjacent to large river systems play an outstanding role for trade, transport, agricultural, fisheries and energy production as well as tourism. At the same time they are of extraordinary ecological value and a transformer and sink for terrestric nutrients and pollutants. The drainage area of the Oder (Odra) river covers an area of about 120,000 km 2 with a population above 13 million people, a lot of industry and intensive agricultural use. The Oder indicates the border between Germa- ny and Poland and drains into the Baltic Sea. The coastal zone directly effected by the Oder river can be divided into the inshore Oder (Szczecin) Lagoon with its subdivision into the western Kleines Haff 1438-4639/01/204-143 $ 15.00/0 Corresponding author: Dr. Gerald Schernewski, Baltic Sea Research Institute Warnemünde, Seestraûe 15, D-18119 Rostock-Warnemünde, Germany, Tel.  49 381 5197 207, Fax.  49 381 5197 211, e-mail: Gerald.Schernewski@io-warnemuende.de Int. J. Hyg. Environ. Health 204, 143 ± 155 (2001)  Urban & Fischer Verlag http://www.urbanfischer.de/journals/intjhyg International Journal of Hygiene and Environmental Health