A Proposal of Protection Techniques in the Catchment of a Lake in the Context of Improving Its Recreational Value

The study was carried out on Lake Rentyńskie (100.8 ha; 9.4 m) situated approximately 20 km to the west of Olsztyn, in the drainage basin of the rivers Giłwa and Pasłęka. The direct catchment area of the lake is 166.2 ha. Forests cover most of the drainage basin area (74%). As revealed in the study, Rentyńskie is a highly eutrophic reservoir. The lake waters were characterized by a high content of nutrients, up to 1.508 mg P dm⁻³ and 11.7 mg N dm⁻³. The high fertility of the lake was also evident in the values of chlorophyll a—75.4 μg dm⁻³, and low water transparency—average 1 m. The total annual phosphorus and nitrogen load to Lake Rentyńskie, calculated according to Giercuszkiewicz-Bajtlik (1990) equals 759.0 kg of phosphorus and 31,869.7 kg of nitrogen, or per unit surface 0.753 g P m⁻² yr⁻¹ and 31.611 N m⁻² yr⁻¹. Allowable and critical load levels to Lake Rentyńskie calculated according to the hydrological model of Vollenweider (1976) equal (respectively) 0.090 g P m⁻² yr⁻¹ and 0.189 g P m⁻² yr⁻¹. From a comparison between the actual phosphorus load and the values calculated according to Vollenweider it can be concluded that the loads not only exceed the allowable values by several times but also the critical values responsible for advanced eutrophication. The study showed that the River Giłwa, which flows through the reservoir, posed a major threat to the analysed lake. In order to improve the water quality in Lake Rentyńskie drastic protective measures should be taken in the basin of the River Giłwa, which is intensively used for agriculture, and also, at the point where the river enters the lake a special system to reduce the level of phosphorus compound concentrations should be set up.