Lake Długie in Olsztyn, Poland, received untreated sewage for over 20 years from a housing estate which probably caused its complete degradation. The lake was regarded in the 1970s as one of the most polluted in Poland; its chemical composition was similar to that of diluted sewage. After the sewage diversion, the amount of organic compounds and nutrients in the water diminished, although the aquatic environment conditions did not improve considerably. Improvement of the trophic state of the lake might have been expected only after applying a relevant restoration technique. Our paper presents the results from a study of the effectiveness of a multi-year restoration of Lake Długie, using the artificial aeration method with destratification. Analysis of the obtained results supports the conclusion that the more than 10-year artificial circulation has resulted in a radical improvement of the environmental conditions in the lake displayed by: increase of oxygen content in the whole water volume, reduction of its consumption rate during both stagnation periods, shortening of the anoxia period in the near-bottom waters, and increase of the redox potential. The artificial aeration has caused a radical decrease of nutrients and organic compounds content in the water, and eventually improved the trophic state. Concentration of phosphorus compounds in the water was reduced mainly due to halted release of the phosphate phosphorus from the bottom sediments. In the case of nitrogen compounds, halted release from the bottom sediments, intensification of the nitrification processes but mainly due to the denitrification process. Despite the considerable reduction of nitrogen and phosphorus compound concentrations in the lake waters, the applied treatment has not resulted in sufficient reduction of the nutrients as to limit the primary production. The lake had remained a heavily eutrophic reservoir which was confirmed by still high values of BOD5, chlorophyll a and low transparency measured by Secchi disk. In the following years of artificial aeration a drop in the reduction rate of the phosphorus compound concentrations was observed which at the parallel lack of iron and manganese in the water indicates that in the case of Lake Długie the capacity for further improvement of its trophic condition in this way had been practically exhausted. Better effects may be expected only after the application of a supportive method, consisting of phosphorus precipitation and fixing in the bottom sediments.
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