## 3 Results  
Characteristic of the Water-Sediment Interface Chemistry as a Background for Sediment P Adsorption Abilities  
The near-bottom and pore water of profundal Lake Kortowskie sediments were rich in nutrients. The concentrations of nitrogen and phosphorus rose together with sediment depth. During sampling, over-bottom water temperature amounted to 8.4 \({}^{\circ}\)C (St. 1) and 8.1 \({}^{\circ}\)C (St. 2). Oxygen concentration was 1.1 mg O\({}_{2}\) dm\({}^{-3}\) (St. 1) and 3.2 mg O\({}_{2}\) dm\({}^{-3}\) (St. 2). The amounts of phosphorus compounds in the overlying waters were similar at both research sites and was lower than 0.5 mg P dm\({}^{-3}\) (Figure 5). At Station 2, the amounts of total phosphorus in the interstitial water were slightly lower, but a smaller share of mineral P form was found in all examined layers of sediments (Figure 2). The range of observed concentrations could be characterized as characteristic of eutrophic lakes [9]. A maximum concentration of total phosphorus was noted in the pore water of the deepest layer (15-20 cm) on St. 2 (9.47 \(\pm\) 0.12 mg P dm\({}^{-3}\)).  
The concentration of iron and manganese (metals influencing P binding in sediment) in the analyzed water strata was not very high (Figure 6). Manganese amounts were below 5 mg Mn dm\({}^{-3}\) at both research stations. Iron concentrations were higher than manganese (the maximum amount was noted in the deepest layer of interstitial water at St. 2--13.4 \(\pm\) 0.4 mg Fe dm\({}^{-3}\)).  
The sediment composition of Lake Kortowskie was similar at both research stations. It can be classified as mixed-type silica-carbonate-organic; none of the major component  
Figure 5: Amounts of P forms (\(\pm\)SD) in the nearbottom and pore water of Lake Kortowskie (total bar length represents the total amount of P).  
Figure 6: Manganese and iron concentration (\(\pm\)SD) in the near-bottom and pore water of Lake Kortowskie.  
amounts did not exceed 50% DW. Other components occurred in small amounts. Among metals responsible for phosphorus binding, Al and Mn occurred in very low concentrations (0.7-0.9%Al\({}_{2}\)O\({}_{3}\) in DW and 0.05-0.10% MnO in DW), Fe level was higher (in the range 2.15-3.95%Fe\({}_{2}\)O\({}_{3}\) in DW), while analyzed sediments were abundant in Ca compounds (28.8-32.5%CaCO\({}_{3}\) in DW) (Figure 7). We did not observe significant differences between particular sediment components at research stations, except silica and phosphorus content (t-value 2.87, n = 8, \(p\) < 0.02 for log%SiO\({}_{2}\) and (t-value \(-\)3.60, n = 8, \(p\) < 0.01 for log% P\({}_{2}\)O\({}_{5}\)).  
The analyzed sediment of Lake Kortowskie was not abundant in phosphorus. The TP amounts ranged between 1.771 mg P g\({}^{-1}\) DW (St. 2, sediment layer 10-15 cm) and 2.593 mg P g\({}^{-1}\) DW. Performed analysis of sediment phosphorus fractions in Lake Kortowskie revealed that the main P fractions in analyzed lake sediment were hardly bioavailable fractions: res-P (residual phosphorus, permanently bound to sediment) and HCI-P (P bound to Ca), which had the highest share of TP (up to 72%TP for res-P at. St. 1 and up to 57%TP for HCI-P at St. 2). Moderately bioavailable fraction NaOH-nrP (P bound to OM) occupied up to 26%TP at St. 1. The rest of fractions occurred in low amounts. One exception was the deepest sediment layer from St. 1. This sediment was abundant in NaOH-rP (P bound to aluminum and iron oxides and hydroxides) and BD-P (redox-sensitive P) fractions (Figure 8). The significant differences in HCI-P, res-P, and TP were observed between research stations (t-value s 4.23, \(-\)4.02 and \(-\)3.63, n = 8, \(p\) < 0.01 for log (HCl-P +1), log (res-P + 1) and log (TP + 1), respectively).  
Figure 7: Amount of components in bottom deposits (\(\pm\)SD) of Lake Kortowskie (in %DW).