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The study was conducted on Długie Lake in Olsztyn, which for 20 years since the mid-1950s served as a domestic and storm wastewater receiver, which led to its complete degradation. The discontinuation of wastewater inflow in 1976 caused a change in the trophic state from saprotrophic to hypertrophic. Evident improvement of water quality was possible only after the implementation of proper restoration techniques. Długie Lake was subjected to artificial aeration with thermal destratification (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000). After all opportunities to improve water quality in the lake by artificial aeration (low phosphorus sorption capacity of sediment) had been exhausted, it was decided that a phosphorus inactivation method using the coagulant PAX 18 be used (2001)(2002)(2003). Before restoration, the nutrient concentration in the near-bottom water layer of Długie Lake was very high at 22.9 mg TN L −1 and 3.50 mg TP L −1 . The average amount of chlorophyll a was ca. 200 μg L −1 , and the Secchi disc visibility did not exceed 1 m. In 2017, 14 years after termination of the lake restoration process, the total phosphorus concentration at the bottom was 0.21 mg P L −1 on average, and the total nitrogen was 1.5 mg N L −1 . The mean transparency of the water oscillated around approximately 5 m, and the amount of chlorophyll a was 1.86 μg L −1 . Studies have shown that the most important step in reclamation is to prevent pollutants from entering the lake to the maximum extent possible and to use a combination of several reclamation methods as a matter of good practice. Stable environmental conditions have developed in the lake, and the values of chlorophyll a and the Secchi disk visibility indicate that the lake has reached a mesotrophic state.

The paper evaluates the potential impact of internal biogenic pollutants (phosphorus) on the trophic state of the deep, stratified Lake Swiete in Obra, Poland (area 23.3 ha, depth 15.3 m). The amount of phosphorus in interstitial water was analyzed on the basis of spatial studies of 10 cm of surface sediments of bottom sediments. It has been shown that the release of loading accumulated in the water can worsen the trophic state of the lake.

The research was carried out on the flow-through Lake Mielenko (7.8 ha; 1.9 m), which also acts as a stormwater receiver. In 2015, a disposal for road salts was created in the lake’s catchment area. As a result of the inflow of salt-contaminated stormwater, there was a significant increase in the concentration of calcium (57 mg Ca/L), chloride (220 mg Cl/L) and electrolytic conductivity (790 µS/cm). Increased calcium concentrations in lake waters changed their hardness from low to medium-hard. The ecological effect of the change in hydrochemical conditions in Lake Mielenko is the Potamogeton crispus that grows abundantly in this reservoir, which prefer calcium-rich water. The overall aesthetics of the lake have deteriorated significantly, and the availability of water for recreation has also been limited.

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