Distribution Patterns of Oder Discharges into the Pomeranian Bight - What Can Be Expected Under 'Normal Conditions'?

The main factors influencing the distribution patterns of material discharged by the river Oder into the Pomeranian Bight are : - the pulsating character of the outflow ('outflow events'); - the physical distribution and mixing of the material by wind controlled currents in the bight; - the modification of the material by biological, physical and chemical processes. Due to the independent action of these factors, the distribution patterns are highly variable. In the TRUMP-project running from 1993 to 1996, the fate of the Oder plume was investigated during seven multi-ship cruises, some of them in co-operation with Polish institutes. A short summary of the results is given here describing 'normal conditions' in the Pomeranian Bight before the flood event in summer 1997. The regional distribution of Oder discharges in the Pomeranian Bight and Baltic Sea is controlled by physical transport and mixing. As the wind is the driving force for the development of currents in this area, two situations are of special interest : the 'west wind situation' and the 'east wind situation'. In both cases, the discontinuous outflow of material can be followed in subsequent distribution patterns. Because biological processes are negligible during winter, this season is characterized by significant correlations between salinity and nutrient salts. However, mixing is not the only mechanism that generates concentration gradients. A variety of additional factors, which are influenced by seasonal variations, affect the distribution of individual species of the discharged material: - variations in the runoff of the river Oder; - (biological, physical and chemical) processes in the Stettin lagoon and in the lagoon of Greifswald; - (biological, physical and chemical) processes in the Pomeranian Bight. Due to the interplay of these effects, the amounts of riverine material in the Pomeranian Bight are highly variable. Concentrations of nutrient salts may change by nearly one order of magnitude within a few days or within a distance of a few miles. For other substances, e.g. DOC, variations are less pronounced, probably due to a higher background concentration in the Baltic Sea. As a consequence of modification processes in the lagoons, concentrations of individual species of the riverine material are not related to each other and thus may strongly influence other processes in the bight, e.g. generating limitating conditions for primary production or affecting transport mechanisms of some trace metals. On the background of the knowledge gained during the TRUMP-project and by the HELCOM monitoring program, it has to be stated that concentrations for most compounds analyzed during the Oder flood in summer 1997 are in a 'normal' range. Exceptionally high concentrations had only been found for organic carbon and organic nitrogen and for silicate. [ABSTRACT FROM AUTHOR]