The ratio values of the concentrations of the elements which exist in a given water body are important indicators that define the restraining role of nitrogen or phosphorus in the productivity of surface waters. In the years 1986–1996 the relationship between this ratio and thermal conditions, primary production (chlorophyll “a”) and the rate of water mineralisation (conductivity, chloride and sulphate concentrations), and also on their optical properties (visibility of the Secchi disk) were examined in the shallow coastal lakes: Koprowo, Liwia Łuża and Resko Przymorskie. In the waters of the analysed lakes phosphate phosphorus content ranged from 0.028 to 0.365; mineral nitrogen content ranged from 0.289 do 1.897 mg·dm^–3; and the N:P ratio changed from 5.4 to 34.3; the concentrations of chlorides and sulphates amounted from 87.0 to 1079.2 and from 53.0 to 266.4 mg·dm^–3 respectively. The visibility of the Secchi disk equalled to 0.75m, whereas, chlorophyll „a” concentrations changed from33.5 do 73.2mg·m^–3. The values of these indexes prove that the discussed lakes were highly eutrophic. The investigations showed significant changeability of the concentrations of chlorides and sulphates which linked inversely proportionally to the values of the N:P ratio. This relationship was not observed with respect to conductivity. Moreover, the authors stated that the content of chlorophyll “a” in the lake waters of Koprowo, Liwia Łuża and Resko Przymorskie was mostly influenced by phosphorus concentrations (dissolved reactive phosphorus) and light conditions (the visibility of the Secchi disk). Thus, they were the factors restraining the primary production of these water bodies.

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