## 3 Results and Discussion
### Multi-Point Aeration--Strengths and Weaknesses of Restoration in Infavorable Morphometry Conditions
Although compressed air is usually the cheapest carrier of oxygen, the restoration of Lake Lajskie, due to the morphometric determinants described above, presupposed the need to obtain the maximum possible efficiency of the devices due to the selection of techniques that do not cause thermal destratification of the lake in summer periods. The most effective gas exchange within a single reactor, with a short retention time in relatively low columns, was expected to ensure the greatest individual impact on a given fragment of the lake. The air only contains 21% of oxygen, and the gas exchange using this medium is dominated by nitrogen, which has been confirmed by previous studies [28; 41; 42]. Its migration to lake waters in gaseous form is not beneficial from the point of view of restoration goals [8]. In fact, the proposed solution is therefore not a technique of artificial aeration, but rather, oxygenation of a degraded lake. The greater environmental efficiency of using pure oxygen compensates for its higher cost compared to compressed air. The estimated cost of the project during the first three years of implementation was approximately EUR 200,000, with annual operating costs (purchase of technical oxygen) amounting to approximately EUR 15-20,000. It should be noted, however, that unlike artificial aeration systems using air compressors, the described installation is independent of electricity sources. This is a definite advantage of a system based on pure oxygen in the absence of power grids near lakes located among agricultural areas that require restoration. The decision to locate oxygenation devices in a larger area of research was the result of collecting empirical experiences during previous experiments. In these experiments, it was assumed that the improvement of the environmental conditions of the tropholytic zone was done by redirecting the oxygenated waters of one of the tributaries to the bottom of the lake [43]. This concept was implemented in 2014-2018. The research carried out during that experiment showed the limited spatial effectiveness of the adopted solution [44]. The water of the watercourse with a higher temperature than that of the metalimnion and the vestigial hypoliminon of Lake Lajskie, flowing just above the bottom, obtained a lower specific gravity compared to the lake water which they fed. As a result, the stream of oxygenated waters was floating, and it was difficult to obtain an even distribution of oxygen in the over-sediment water. The phenomenon of loss of the original temperature of water exposed to aerators often accompanies deep-water aeration systems [40; 45; 46] and is mentioned as one of the most important reasons for reducing the efficiency of oxygenation. In the installation on Lake Lajskie, the problem of heating water drawn from above the bottom in the described reactors was minimized by isolating columns and averaging tanks. Nevertheless, an increase in the heat of the drained water was observed, usually by 0.4-0.7 \({}^{\circ}\)C. This was one of the reasons for the differentiation of the obtained oxygenation results at the depths of 0.5 and 1.0 m.  
Despite the location of individual aerators in a fairly even manner and the orientation of the water outflow so as to support the even distribution of oxygen along with the movement of water, it was not possible to ensure satisfactory oxygen concentration in the entire experimental area. In three sub-reservoirs, apart from reactors five, seven, and eight, the water of the layers directly in contact with the sediments (0.5 m above the bottom) was characterized by only trace amounts (<0.1 mg L\({}^{-1}\)) of this element. The indicated locations are not directly exposed to the outflow of oxygenated water from the reactors. Therefore, in subsequent seasons of the system's operation, it is reasonable to test larger doses of oxygen to central aerators, add additional distribution points, or modify the direction of water outflow from the aerators closest to the blind spots of the oxygenation area.