## 4 Discussion  
The presented research was carried out to determine whether the applied method of Lake Kortowskie restoration affects the chemistry of the water-sediment interface and the sediment P sorption abilities. Hypolimmetic withdrawal is a method to ensure the removal of excess nutrients and other reduced compounds (e.g., hydrogen sulfide) outside the lake ecosystem, which are released from the lake deposits under anoxia. In [PERSON] et al. (2018), it was shown that an internal loading is responsible for more than 50% of the SRP amount and N-NH\({}_{3}\) in the hypolimnion. In lakes with long pollution history by sewage, the internal loading may significantly exceed external nutrient loading ([PERSON] et al., 2018). Therefore, in theory, using the hypolimmetic withdrawal method should improve the nutrient balance of the lake and reduce the effects of the internal loading phenomenon ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). The results of many years of research into the water of Lake Kortowskie revealed a positive effect of the applied method on the nutrient balance ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). However, using this method also has some limitations. First, the hydrological balance is important, allowing the discharge of hypolimnion water. In the case of dry periods with reduced rainfall, as has been the case in recent years ([PERSON] et al., 2018), draining water with the full capacity of the pipeline could significantly reduce the water level in the reservoir. In addition, the removal of bottom water with maximum efficiency increases the temperature of the hypolimnion but, at the same time, improves oxygen conditions in the bottom zone. This may affect the sedimentation of matter in the lake, as well as the rate of organic matter decomposition in sediment. During the research, there were favorable hydrological conditions allowing the use of the maximum flow in the pipeline (250 l s\({}^{-1}\)). The performed experiments showed that the bottom sediment of Lake Kortowskie was characterized by quite good phosphorus adsorption capacity. Still, the recorded contents of total phosphorus in bottom deposits were not high--the TP amounts were between  
Figure 10: Results of RDA analysis of dependence between P adsorption characteristics (response variables) and chemical composition of bottom sediment (explanatory variables). 1.771 mg P g\({}^{-1}\) DW (St. 2, deposit layer 10-15 cm) and 2.593 mg P g\({}^{-1}\) DW, sediment layer 0-5 cm. The quantitative structure of the phosphorus fraction showed the predominance of fractions that are difficult (HCI-P) and non-bioavailable (res-P). Amounts of labile fractions (NH\({}_{4}\)Cl-P and BD-P) were low at both research stations. P retention by analyzed sediment was especially high (up to 99% earlier adsorbed P) (Tables 3 and 4), which means that the bottom sediment of Lake Kortowskie is very effective at P trapping in oxic conditions, in which the experiments were carried out. This parameter showed statistically significant differences between the control and the experimental station. Higher P retention was noted for sediment taken at St. 1 (control site)--more than 99% of previously adsorbed P, while, at experimental station P, retention was lower (up to 98.99%). It could result from sediment forming in different conditions, affected by the used restoration method. Despite a higher temperature in the hypolimnetic zone induced by the hypolimnetic withdrawal pipeline operating, there were no significant differences in theoretical maximum adsorption capacities between the research stations. P adsorption processes are more effective in higher temperatures [10; 24; 37; 38]. However, tested adsorption characteristics did not show significant differences between sediments taken at the research stations, except for P retention.