The research was carried out on a small (52.8 ha) and shallow (4 m) Lake Track in Olsztyn, exposed to a considerably intense anthropogenic pressure. In the middle 1800s this reservoir was dried out and most of the contiguous land was designated for agricultural purposes. The lake was restored in the mid 1900s. At present, the drainage basin of Lake Track is 216 ha. Urban land comprises the largest portion of this area, i.e. 49.3%, barren land 41.4% and forests 6. I%. The lake receives storm waters but for years it had also received sanitary sewage. The results of this research allowed classifying the lake as nutrient-rich, with fairly advanced eutrophication processes. The waters were characteristic of very high nutrients content, up to 0.75 mg P/dm3 and 3.87 mg N/dm3. The high fertility of Lake Track was additionally confirmed by high BOD, values, i.e. up to 9.5 mg 0/dm3, high chlorophyll content, usually from 30 to 40 mg/m3 but reaching 123 mg/m3, and low water transparency, oscillating between 0.6 and 0.9 m. The reason for the lake's high trophic level was no doubt the excessive loading from the drainage basin. The actual nutrient runoff from the drainage basin to the lake exceeded the critical loads, as defined by Vollenweider. Low quality of the lake's waters and the parallel high external loading indicate that preventive measures should be taken, aimed at reducing the external loading.

The research was conducted in a relatively small (26.8 ha) but quite deep (17.3 m) Lake Długie in Olsztyn, Poland. For over 20 years the lake was collecting sewage which eventually caused its complete degradation. In 1987-2000 the lake was restored using the artificial aeration method with destratification of water. The results showed that the artificial aeration effectively limited the internal loading. Application of this restoration method resulted in reduction of phosphorus compounds concentrations in the analyzed water strata. The decrease of TP in bottom sediments (to the level of 3-4 mg P g·' DW) was probably associated with the fact that a new layer of sediments was created, reflecting a change in the aquatic conditions caused by the restoration. The investigations conducted in the reference years showed that the changes were not permanent. A high concentration of phosphorus compounds in bottom sediments, low sorptivc capacity and a tendency to oxygen deficiency, indicate that further possibility to decrease the amount of phosphorus compounds in the lake by this restoration method is limited.