## 2 Materials and Methods  
### Lake Lajskie  
Lake Lajskie is a typical example of a reservoir degraded by inappropriate human management. It is a relatively small and shallow lake (Table 1), hydrologically open, and fed by three tributaries. The lake's catchment area has a great influence on this reservoir. It is used mainly for agriculture, which increases the supply of nutrients. The research carried out in the catchment area of this lake in 2008-2020 shows that watercourses draining agricultural catchment areas (about 18 km\({}^{2}\)) provide an annual load of pollutants at the level of 250-650 kg of phosphorus and 2600-12500 kg of nitrogen. Until recently, an additional problem was the discharge of insufficiently treated domestic and economic sewage to one of the lake's tributaries. Historical and contemporary studies [22; 23; 24; 25] have shown strong eutrophication of this reservoir. A characteristic feature of the lake is the fact that, despite its insignificant depth, it suffered from long-term strong oxygen deficits during periods of summer water stagnation and hydrogen sulfide was present in the waters near the bottom.  
Due to the fact that the outflow from Lake Lajskie supplies other, very valuable water ecosystems (nature reserve--Lake K\(\ddot{\text{o}}\)sno, area 550 ha, max depth 45 m), achieving water quality improvement has become a key problem for local authorities and communities.  
Lake Lajskie has been included in the protection and restoration program since 2019. Apart from activities related to the reduction of external pollution loads (arrangement of water and sewage management in the catchment area by the local self-government, filtration zones on tributaries), a multifaceted restoration was also started. To reduce the supply of  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Unit**} & \multirow{2}{*}{**Value**} & \multicolumn{6}{c}{**Bathymetric Surfaces**} \\ \cline{3-8}  & & & **Isobath** & **Area [ha]** & **Between** & **Area [ha]** & **Area [\%]** \\ \hline surface area & ha & 47.84 & 0 m & 47.84 & 0–1 m & 4.39 & 9.18 \\ volume & thou. m\({}^{3}\) & 1592.2 & 1 m & 43.45 & 1–2 m & 7.70 & 16.09 \\ max depth & m & 7.4 & 2 m & 35.75 & 2–3 m & 11.62 & 24.28 \\ mean depth & m & 3.3 & 3 m & 24.13 & 3–4 m & 8.75 & 18.29 \\ relative depth & – & 0.01 & 4 m & 15.37 & 4–5 m & 5.44 & 11.37 \\ depth indicator & - & 0.44 & 5 m & 9.94 & 5–6 m & 3.87 & 8.09 \\ Max length & m & 987.7 & 6 m & 6.07 & 6–7 m & 4.56 & 9.53 \\ Max width & m & 723.3 & 7 m & 1.51 & within 7 m & 1.51 & 3.16 \\ shoreline & m & 3283 & & & & & \\ Shoreline development & m/ha & 68.6 & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic morphometric indicators of Lake Lajskie, based on contemporary measurements [26].  
phosphorus in the water, the method of phosphorus inactivation using an aqueous solution of iron chloride was used (first stage 2019-2020, several applications each year), as well as artificial aeration (oxygenation) of the lake (from 2019). In addition, an intensive program of fishing biomanipulation was launched--restocking predatory fish (pike and zander) and eliminating the excess of small cypriinds. Each of these activities is ultimately aimed at limiting the primary production in the lake and eliminating strong cyanobacterial blooms, which so far, occur practically throughout the summer season. A multi-threaded, holistic set of activities brings results--the quality of water in Lake Lajskie improves year by year. The entity implementing the restoration is the local ecological association.