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Nutrient loading and retention in lakes of the Jorka river system (Masurian Lakeland, Poland) : seasonal and long-term variation

Hillbricht-Ilkowska A.

Polish Journal of Ecology

2002

Vol. 50, nr 4

459-474

The nutrient (TP, N-NO3, TKN - Kjeldahl nitrogen) loading to lakes (from river, precipitation, direct catchment) and in-lake retention were calculated for five successive lakes of the river-lake system typical of the mosaic, hilly lakeland region (the Jorka system, Masurian Lakeland). The annual values of loading for TP and TN were mostly not higher 1.0 and 15.0 g m^-2 lake area, respectively and rather low compared to other river-lake systems in the lakeland region. However, these values as well as nutrient in-lake retention decreased in 1997-1998 (warm and dry years) compared with 1978 and 19996, together with the decrease in discharge and exchange rate of lake water. In selected cases the retention of TP and TKN became negative which means that the lake functioned as the source of nutrients for downstream fragments of the system. The retention of nitrate-nitrogen was usually positive - lakes remained effective sites for its removal (denitrification). The possible changes in functioning of small catchments and chains of shallow, eutrophic lakes under the conditions of global warming are discussed.