GORZYSŁAW Poleszczuk¹ and  STANISŁAW Sitek^2
1University of Szczecin, Felczaka 3A, 71-412 Szczecin, Poland.
²Sea Fisheries Institute, Pl. Słowiański 11, 72-600 Świnoujście, Poland.

Key words: estuary, water chemistry, Szczecin Lagoon, Lake Wicko Małe.

Abstract.
The results of the research on the quality of the surface water of Lake Wicko Małe (Szczecin Lagoon) conducted in the years 1991-1997 are presented. It is demonstrated that, in 1996, after putting into operation the second (biological) stage of the treatment of sewage discharged into Lake Wicko Małe, the quality of the water in the middle part of the lake improved slightly relative to the quality of the water over the years 1991-1995. This evaluation is based upon the changes in the concentration of mineral nitrogen compounds and dissoloved orthophosphates, as well as indices such as pH, COD-Mn and BOD5. By contrast, NH+4 and Feog concentrations in the water have increased, as it seems, as a result of more intensive processes of enrichment of the water of the lake with ammonium compounds and iron than in the years preceding the launch of the biological sewage treatment plant, at the cost an additional load to the inner reservoir.