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The Fate Of Oder Discharges During An Exceptional High Flood In Summer 
C. Humborg, T. Neumann, F. Pollehne & N. Wasmund 
Institut fur Ostseeforschung 
Seestrafie 15 
18119 Rostock-Wamenuinde 
Abstract 
The distribution of dissolved inorganic nutrients, particulate organic matter and phytoplankton 
pigments in the Oder plume were investigated at peak discharge of the Oder River during an 
exceptional flood event in summer. Two effluent events will be discussed. The first displays a 
situation where water originated most probably from the german part of the Szczecin lagoon 
entered the Pomeranian Bay, the second shows presumably the direct Oder outflow. 
Mixing diagrams of dissolved inorganic nutrients (NO3, PO4, Si0 4 ) reveal a removal of nitrate 
during the first steps of estuarine mixing. In contrast phosphate and silicate showed almost 
conservative mixing patterns. Primary productivity was most certainly nitrogen limited, the 
riverine endmember concentration was one magnitude lower than during spring floods. It 
appears that during a summer flood dissolved inorganic nitrogen is not a s available as in spring 
probably due to the advanced fixation by land vegetation within the drainage area in summer. 
These observations were consistent with the estuarine distribution of particular organic matter 
(Chi a, POC, PON), which showed also conservative mixing patterns during the Oder outflow 
situation. A closer look to the algal pigments (Fucoxanthin, Zeaxanthin) and phytoplankton 
species distributions reveal that diatoms were also mixed conservative, although silicat e in the 
river was readily available. Blue green algae showed a slight positive deviation from the 
conservative mixing line indicating a small net production during mixing. During the first 
observation high concentrations up to 200 pg Chl a mainly of the blue green algae Microcystis 
were found near the Swina mouth. These water masses were most certainly washed out from 
the german part of the Szczecin lagoon, and do not display an active growth in the Pomeranian 
Bay. 
Model results show that the Oder flood event had a more regional character. The entire 
Pomeranian bay was affected by the Oder discharges, whereas adjacent coastal seas were not 
directly influenced. Nevertheless, due to the strong stratification of the water column within 
the Pomeranian Bay, severe anoxia were observed.