The exceptional Oder Flood in summer 1997 nutrient concentrations in the Świna Strait during the years 1996 and 1997 - with emphasis on the flood event

Abstract

Annual variability in nutrient concentrations in the Swina Strait, draining about 75 % of the water from the Szczecin Lagoon, the latter being a recipient of the Oder waters, was found to arise from: the annual/ interannual variability in water/nutrient discharges from the river Oder and the biogeochemical processes taking place in the lower Oder estuary. The other factors influencing these distributions include the pulsating nature of the Swina outflow, and special events such as the flood ’97. A permanent increase in phosphate concentrations over August/September results from intensive phosphate regeneration in the Lagoon and limitation of primary production by low concentrations of nitrogen over the summer period in the region, and from a lower dilution factor during the summer periods. The average seasonal N:P ratios remained in consistent ranges in all seasons but spring, the spring values being, to a great extent, driven by variable Oder discharges. Nutrient limiting role in primary production in the lower Oder estuary switches from phosphorus in spring to nitrogen in the summer time. Great quantities of inorganic nitrogen discharged with the Oder waters are converted in the Lagoon into the organic forms, while organic phosphorus seems to be trapped in the sediments and converted into phosphates, the latter giving a well pronounced annual pattern, not seen in the Oder waters.