## 4 Discussion
Performed RDA analysis showed that the OM, Fe, and Mn contents in deposits are important factors in physical P adsorption processes, influencing P adsorption characteristics. The role of OM, as well as Fe and Mn, in P adsorption by bottom sediment, was well recognized in numerous research, as they formed mobile P pools in sediment, e.g., [26; 38; 39]. A rather unexpected fact of the conducted research was that modeled P adsorption isotherms showed concave isotherm shape, especially in low P concentration. The obvious conclusion implied by modeled isotherm shape was that observed P adsorption was multilayered, not monolayered. According to [PERSON] et al. [39], concave isotherm belongs to the S-class, and it occurs when cooperative adsorption occurs. It means that between the adsorbate molecules, the bond strength is of a similar order of magnitude, or stronger, than the bond strength between adsorbate and adsorbent molecules. Additionally, [PERSON] and [PERSON] [40] mention that concave isotherms occur with strong adsorbate-adsorbate bonding forces. Therefore the P adsorption by the sediment in Lake Kortowskie seems more effective in higher P concentrations in water. According to [PERSON] [31], S-class isotherm can be pictured by the Freundlich model with a 1/n coefficient value exceeding 1, as it was obtained in the present research. The ion exchange isotherms were discussed in the paper by [PERSON] et al. [41]. According to their findings, the adsorption capacity depends on the ion exchange capacity. If exchanged ions have the same valency, the exchange occurs in the adsorption media, and the adsorption curve will be C-type. However, in the case of different valences of exchange ions, the higher valence ions are more adsorbed. Probably that mechanism could influence the concave shape of P adsorption isotherm curves because P adsorption on bottom sediment in lakes occurs in a complex matrix. A similar concave shape of the P adsorption isotherm (with 1/n higher than 1) was obtained by [PERSON] and [PERSON] [26] from the sediment of Lake Klasztorne Male. This lake was restored earlier using Phoslock (benthonite clay modified by lanthanum). This treatment also changed the calcium carbonate content in sediment compared to other lakes in the Kartuzy lakes complex. Because the sediment of Lake Kortowskie is relatively abundant in calcium carbonates, the presence of carbonates is a possible factor influencing P adsorption isotherm shape, though this hypothesis needs further research. The concentrations of phosphorus compounds observed in the water-sediment interface were high, and they confirmed the eutrophic state of Lake Kortowskie. Similar concentrations were observed in the interstitial water of eutrophic lakes [9; 26; 37]. There were some differences in the concentrations of total phosphorus between the control site (St. 1) and the experimental site (St. 2), especially in mineral form amount--this was slightly lower in the experimental station than in the control site (Figures 2 and 3) but was not statistically significant. While no significant differences in analyzed water strata were found between the control and experimental sites, the differences between the research sites were marked in the chemical composition of deposits--total phosphorus content (TP), silicate amount (SiO\({}_{2}\)), fractions of calcium-bound phosphorus (HCI-P), and residual phosphorus (res-P) amounts. The silicate content was significantly higher in deposits of the experimental part, while the amount of TP was significantly lower. This may indicate changes caused by the applied restoration method. An increase in the bottom water temperature accelerates the mineralization processes, which may lead to an increase in the amount of silicate in relation to the amount of organic matter. Typically, a higher silicate content is found in shallow water sediment, which is deposited at higher water temperatures. At the same time, organic matter in warmer waters undergoes rapid mineralization.