Abstract
The research in Lake Głębokie was conducted in the years 2008–2010. Sampling from three selected sites was designed in such a way that vertical profiles of the characteristics of interest were obtained. Measurements were performed to determine the content of dissolved oxygen, oxygen saturation expressed as a percentage, water temperature, total phosphorus, chlorophyll a, dry seston mass, and Secchi disk depth. During the course of a year, water temperature varied from 0.58 °C to 24.72 °C. However, a permanent thermal stratification was not observed in the summer season due to complete water mixing. Dissolved oxygen concentrations in the water varied within a range that started at hard-to-measure insignificant values and reached up to 9.63 mg O2 l−1, while oxygen saturation reached a maximum of 116% O2. At the same time, it was observed that oxygen conditions gradually worsened as the depth became greater. The largest oxygen deficits occurred in the winter season of 2009–2010. The oxygen conditions were typical for an eutrophic lake. Its high trophic state was confirmed by an analysis performed using the method proposed by Carlson (1977) as well as by an analysis performed according to the directives of the OECD (1982). The trophic state of this lake is borderline eutrophic and hypertrophic. The oxygen conditions found in this lake ought to be seen as not beneficial to fish.