## 2 Materials and Methods  
### Description of the Sampling Sites and Sediment Collection  
The study area comprises two natural lakes within the river-lake system, specifically Lake Lekno and Lake Jeziory Wielkie, located in the central-western part of Poland (52\({}^{\circ}\)09\({}^{\prime}\)24\({}^{\prime\prime}\) N 17\({}^{\circ}\)08\({}^{\prime}\)41\({}^{\prime\prime}\) E and 52\({}^{\circ}\)10\({}^{\prime}\)28\({}^{\prime\prime}\) N 17\({}^{\circ}\)07\({}^{\prime}\)48\({}^{\prime\prime}\) E), belonging to the Guszynka river-lake system (Figure 1). Lake Raczynskie is a water source for lakes in this river-lake system, where lakes play a crucial role in retaining orthophosphates (V) in the water, as emphasized by [PERSON] et al. in their work [27]. Their observations highlight the significant impact of anthropogenic factors on the water quality of these lakes. Specifically, a decreasing trend in orthophosphate (V) content was observed in the river-lake system, indicating the reduction potential. Conversely, sporadic peaks in orthophosphate content occurred, such as after the clearing of forests near the shore, underscoring the heightened sensitivity of the analyzed lakes to anthropogenic pressures. Lake Lekno has an average water depth of 2.2 m, with a maximum water depth of 5.0 m, while Lake Jeziory Wielkie has an average water depth of 3.0 m and a maximum water depth of 5.4 m. The study area has a moderate climate, with an average air temperature of 9.1 \({}^{\circ}\)C and an average precipitation of 580.4 mm [27]. In the hydrological year 2020, the sum of precipitation was 510 mm, and in 2022, 520 mm. According to the analysis of precipitation conditions for the 30 th hydrological anniversary, it was discovered that the years 2020 and 2022 were dry.  
The direct catchment area for Lake Lekno spans 1.42 km\({}^{2}\), while that of Lake Jeziory Wielkie covers 7.53 km\({}^{2}\). The research focused on non-stratified lakes, where there is no distinct thermal stratification. The direct catchment area of Lake Lekno has a forested character, with as much as 77.22% of this area covered by forests (with a predominance of mixed forests), while only 3.48% of the direct catchment area is anthropogenic. In contrast, the immediate catchment area of Lake Jeziory Wielkie has a different character (agroforestry). Agricultural areas dominate the catchment, accounting for 45.86% of the area, while forests still maintain a large share of the area, accounting for 42.05% (Figure 2).  
Sampling points and their quantity were determined in proportion to the size of the reservoir, setting a strict grid of squares (FishNet) with a width of 200 m. There were 6 sampling points for Lake Lekno and 15 for Lake Jeziory Wielkie. The latitude and longitude coordinates of these sampling points were determined using a GNSS unit (Figure 3).  
Figure 1: Study area.  
At selected sampling points, as shown in Figure 3, the surface layer of sediments (0-20 cm) was collected from lake bottoms using a van Veen gripper, then sealed in polyethylene bags and immediately transported back to the laboratory in the spring of 2020 and the summer of 2022.  
One portion of the samples was stored in a refrigerator at 4 \({}^{\circ}\)C to analyze the ACP and ALP activity test (within 24 h). The remaining portion of the samples was prepared to analyze the texture. Soil texture was determined using [PERSON]'s areometric method, modified by [PERSON], using the PN-R-040032 standard [28]. The soil texture samples were classified according to the United States Department of Agriculture standards. During  
Figure 3: Sampling points.  
Figure 2: Land use of catchment area, source: CORINE Land Cover (CLC).  
the sampling of bottom sediments, overlying water was also collected from the bottom layer using a Van Dorn sampler. The concentration of org-P was determined using the EN ISO 6878 method [29]. The pH measurements were taken in situ using CP-105 ELMETRON.