This research concerns the restoration of the strongly eutrophied Lake Łajskie (Masurian Lake District, Poland) that received pollutants from the agricultural catchment. It is a shallow (max depth 7.4 m) and small (area 48 ha) reservoir with a wide profundal zone characterized by complete deoxidation in summer. Due to its important natural and social role, the lake has undergone a restoration process. Artificial oxygenation is the main method of improving water quality. Due to unfavorable morphometric conditions, the necessity to use multi-point aeration was assumed. An experimental installation based on eight reactors selectively oxygenating only the over-bottom waters was launched in 2019. In 2021, spatial studies of the effectiveness of aerators’ work were carried out at 96 measuring points grouped into 12-test fields around each reactor. The investigations were performed three times during the summer season. It was shown that, in the water of the deepest layers of Lake Łajskie, the oxygen concentration around all reactors increased to an average level of 1–2 mg L−1. The oxygenation efficiency was varied and related to the distance from the aerator and the location in relation to the direction of oxygenated water outflow. The results of the research indicate the legitimacy of designing multi-point oxygenation systems in the restoration of waterbodies degraded as a result of the impact of agriculturally transformed catchments.

Phosphorus is a key factor controlling eutrophication processes. Out of all the parts of the lake ecosystem, the biggest pool of this element (more than 90%) is stored in the surficial layers of bottom sediment. Hence, the bottom sediment’s ability to trap and release P is very important in analyzing a lake ecosystem’s function, particularly when the lake is subjected to restoration. Studies were carried out on Lake Kortowskie (87.2 ha in area, maximum depth 17.2 m), restored in 1956 using the hypolimnetic withdrawal method. The sediment cores were taken at two research stations—experimental (max. depth 17.2 m) and control (max. depth 15.7 m). Experiments were made in laboratory conditions. The results showed that phosphorus adsorption in the bottom deposits of Lake Kortowskie was multilayered. The Freundlich, BET, and Freundlich-Langmuir adsorption models best fit the experimental data. Adsorption isotherms were concave, suggesting that P adsorption effectiveness is higher for higher P concentrations in ambient water.

The research was carried out on the flow-through Lake Mielenko (7.8 ha; 1.9 m), which also acts as a stormwater receiver. In 2015, a disposal for road salts was created in the lake’s catchment area. As a result of the inflow of salt-contaminated stormwater, there was a significant increase in the concentration of calcium (57 mg Ca/L), chloride (220 mg Cl/L) and electrolytic conductivity (790 µS/cm). Increased calcium concentrations in lake waters changed their hardness from low to medium-hard. The ecological effect of the change in hydrochemical conditions in Lake Mielenko is the Potamogeton crispus that grows abundantly in this reservoir, which prefer calcium-rich water. The overall aesthetics of the lake have deteriorated significantly, and the availability of water for recreation has also been limited.

The efficiencies of the restoration of two lakes of varied morphometries and trophic states—meromictic, hypertrophic Lake Klasztorne Małe, and dimictic, eutrophic Lake Klasztorne Duże—with the use of the phosphorus inactivation method with sequential application of iron and aluminum compounds have been compared. The total dose of the agents applied for Lake Klasztorne Małe was 38 tons of PAX 18 (aluminum polychloride) and 14 tons of PIX 111 (iron chloride), and for Lake Klasztorne Duże, it was 74 tons of PAX 18 and 46 tons of PIX 111. After the application of the compounds, better efficiency of phosphate removal from the surface water layers was obtained in the case of the dimictic, eutrophic Lake Klasztorne Duże. The use of two doses of compounds did not lead to complete precipitation of phosphates from the bottom water layers of either lake. It is noteworthy that in the case of both lakes, inhibition of the internal loading process was observed. The obtained results for the Klasztorne lakes showed that the use of two types of compounds makes it possible to reduce the cost of restoration, and moreover, the dosing of iron salts in the coastal areas of the lakes ensures a higher level of ecological safety.