## 3 Results and Discussion
### Multi-Point Aeration--Strengths and Weaknesses of Restoration in Infavorable Morphometry Conditions  
The contemporary philosophy of lake restoration is based on a thorough diagnosis of the conditions for the functioning of the ecosystem and the selection of techniques and activities to suit the nature of the lake, primarily its hydrological system, morphometry, and water chemistry. Increasingly, rehabilitation programs are based on not one, but several complementary methods [30; 31; 32; 33]. A holistic approach, not only taking into account the change in the water chemistry in the lake, but also the living conditions of biocenoses and strengthening the natural mechanisms of self-purification, gives the best results in counteracting eutrophication [34]. Artificial aeration is crucial for the success of both accompanying methods of restoration of Lake Lajskie, i.e., inactivation of phosphorus with iron coagulants and biomanipulation. The improvement of redox conditions supports the stability and durability of iron-phosphorus connections [35; 36; 37], thus increasing and extending the efficiency of phosphorus inactivation in the deeper zones of iron coagulant application. Increasing the range of oxygenated waters improves the living conditions of the population of settled predatory fish, especially zander, which prefer the penetration of deeper waters [38]. The choice of oxygenation technique is critical to the lake rehabilitation process. It is generally recognized that the method of water mass destratification is more effective [15; 16]. However, it leads to the elimination of thermal stratification, which may be unfavorable for sensitive organisms (e.g., fish of the genus _Coregonidae_). Aeration with thermal stratification is usually more expensive and more difficult to implement. This is due to the need to use systems that isolate the over-bottom water during the oxygenation process, includingcolumn aerators, pulverization tanks and chambers, water supply lines, etc. [16; 29]. Specific morphometric conditions determined the choice of the Lake Lajskie aeration method. The wide and flat area of profundal sediments posed a risk of a negative impact of increased temperature of bottom waters on the metabolism of sediments in the water mass destratification scenario. Hence, it was decided to use a method that preserves thermal stratification, while forecasting the need to use at least several oxygenation reactors. Multi-point aeration is a solution that has already been used for lake renewal; however, scientific reports on this subject are scarce. Most often, one aerator is used in lakes, in the central deep, or only a few located in lake sub-basins. Such a solution was used e.g., in the Trzesiecko lake in Poland (three aerators) and in the Brno reservoir in the Czech Republic (five aerators) [39]. In Lake Tegel, in Berlin (400 ha, maximum depth of 16 m, with an extensive profundal with depth index of 0.4), a system of 15 deep-water aerators improved the oxygenation of the bottom waters on the surface of the central basin with an area of approximately 80 ha. It was assumed that this installation was not supposed to lead to the summer destratification of waters; however, in practice, it contributed to the destabilization of thermal stratification due to the types of limnox aerators used. These devices have an underwater gas exchange reactor and excess air escapes above the body in the form of a stream of bubbles, which favors the ascending movements of the hypoliminon waters and the mixing of thermal layers [40]. Contrary to the limnox deep-water aerators, column aerators were used in the reclamation project of Lake Lajskie. In this project, the reactor chamber is located near the surface and does not require degassing of the underwater installation components. Technical and economic possibilities made it possible to build eight such devices (Figure 1). The choice of pure oxygen as a medium to improve the balance of this element in the lake was not accidental.