Water samples were collected from lakes: Ostrowo, Piaski, and Przybiernowskie in summer seasons of 1980 do 1997, for 5 or 7 years; additionally, within 1995-1997 samples were collected every three months. The samples were collected from three layers (near-surface, intermediate, and near-bottom) of the water column overlying the deepest site of each lake. The results obtained allowed to classify the lakes as polymictic and eutrophic. Spatial and temporal changes of mineral forms of nitrogen were typical of eutrophic reservoirs showing a clear trend towards deposition of nitrogen in bottom sediment. The eutrophic nature of the lakes studies is confirmed also by the high concentrations of phosphorus compounds. In addition, the lakes are prone to secondary eutrophication.

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