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The occurrence of differences in chemical composition between surface and near bottom water in the Great Szczecin Lagoon (Wielki Zalew) was studied between 1986-1994 under calm weather conditions, i.e. with the state of the Lagoon remaining <1ºB for at least three days. In the study 54 measurement series were obtained at 21 measurement stations and the measurements included: water temperature, dissolved oxygen concentration, pH and chlorinity taken at every 0.5-1.0 m on vertical profiles. At selected stations 7 series of day-round measurements were performed with sampling frequency of 2 hours. The selected water quality indicators (temperature, oxygen concentration, pH, Eh, chlorinity, alkalinity and nitrate concentration) were measured in the surface layer (0.5 m below the water surface) and 0.5 m above the bottom. It was found out that under calm conditions water of the Szczecin Lagoon undergoes chemical transformations similar to those in enclosed water bodies.
Key words: aquatic chemistry, estuary, Szczecin Lagoon