# Is It Possible to Restore a Heavily Polluted, Shallow, Urban Lake?
## 4 Discussion  
The results of monitoring have shown that the water and bottom sediment of Karczemne Lake should be classified at the \"below good\" level. The lake does not meet the requirements of the Water Framework Directive of the European Union [41]. With such high nutrient concentrations and excess organic matter, the natural inhibition of the severe degradation of the lake is impossible. In addition to the very poor water quality, the bottom sediments of the analyzed lake also contain extremely high concentrations of pollutants. The concentrations of total phosphorus in the bottom sediments of Karczemne Lake were very high compared to those of other lakes [42; 43; 44]. To date, no other lake in Poland has recorded such high values of phosphorus. The maximum amount of total phosphorus was found at research point number 5: 31.8 mg P g\({}^{-1}\) d.w. in the surface sediments of layer a (0-30 cm). [PERSON] [45] reported that in Karczemne Lake sediments, a maximum of 1.9% TP in dry matter was found, which corresponds to 19 mg P g\({}^{-1}\) d.w., while previous studies carried out by [PERSON] et al. [46] showed a maximum of just over 12 mg P g\({}^{-1}\) d.w. in the surface layer of sediments (10 cm thick) taken from the deepest research point. For comparison, the maximum phosphorus content found by [PERSON] [47] in the bottom sediment of dystrophic Zmarle Lake was slightly above 10 mg P g\({}^{-1}\) d.w. In the sediments of Dlugie Lake in Olsztyn, which was polluted for many years by domestic sewage, the level of phosphorus in the bottom sediments ranged from 4 to 6 mg P g\({}^{-1}\) d.w. [48]. According to [PERSON] et al. [49], the amount of phosphorus in the sediments ranged from approximately 0.5 to 20.2 mg P g\({}^{-1}\) d.w. However, [PERSON] [50] suggested that in the sediments of shallow lakes were not contaminated by sewage, the phosphorus content is usually low and does not exceed 2-3 mg P g\({}^{-1}\) d.w. Karczemne Lake was a domestic sewage receiver for over 60 years. The pollution loads introduced with the sewage inflow deposited on the bottom of the reservoir forming a layer of so-called \"modern sediments\". In oligotrophic lakes, the thickness of the sediment increases by 1 mm each year, and in hypertrophic lakes, it increases by approximately 1 cm [50]. Such high phosphorus content in the bottom sediments, especially in the surface layer, is certainly the result of massive sewage pollution taking place over the last several decades. Station 5, with the highest level of phosphorus, is located in the immediate vicinity of the collector discharging sewage into the lake, which for many years was the main source of pollution. It has been reported that in the deepest layers of the sediment (from 60 cm and in some positions from 90 to 150 cm deep into the sediment), the recorded amounts of total phosphorus are from approximately 0.5 (St. 5) to approximately 3 mg P g\({}^{-1}\) d.w. (St. 18), which gives an overview of the concentration of total phosphorus before the pollution of the lake. Due to the low depth and polymictic nature of the reservoir bottom, sediment certainly undergoes resuspension, which in some sites could cause higher phosphorus contents in the deeper layer of sediment (30-60 cm), e.g., stations 24, 26 and 28. The vertical profiles of the phosphorus and total phosphorus fractions in the sediments at selected research stations show very rich concentrations of phosphorus in more detail as the deposit depth increases in Karczemne Lake. Theoretically, this phosphorus can easily be mobilized, providing a serious load of pollutants to the water of the analyzed reservoir.