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Distribution Patterns Of Oder Discharges Into The Pomeranian Bight
What Can Be Expected Under 'Normal Conditions' ?
K. Nagel, V. Mohrholz, & C. Pohl
Institut fur Oslseeforschung
Seestrasse 15
18119 Wamemunde
Abstract
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 cooperation 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.
Regional distribution of Oder discharges in the Pomeranian Bight and the 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 discontinous outflow of material can be followed in subsequent distribution
patterns. Because biological processes are neglectable 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, 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. Exceptional high concentrations had only
been found for organic carbon and organic nitrogen and for silicate.
