## 1 Introduction
The basic installation of the aeration system consists of a compressor located in a room located on the lake shore, a transmission line, and an air discharge system, e.g., a diffuser. Restoration carried out in this way changes the conditions both in the waters of the surface layers (lowering temperature and pH of the water, reducing biomass of phytoplankton, and lowering over-oxygenation) and deeper waters (water oxygenation, reducing the amount of organic matter and nutrients, while significantly increasing water temperature). Avoiding the mixing of water masses (hypolimnion oxygenation) requires the use of special devices and systems preventing the destruction of thermal layers [15; 16; 17]. This is achieved by a mechanical aeration system (a system with a pulverization unit placed on the shore), with partial water lift (Limnox aerator) or with total water lift (e.g., \"ekofox\" and \"stratifox\" tubular aerators, pulverization aerator). Modern constructions enable the simultaneous introduction of a phosphorus-binding coagulant, which can improve the efficiency of the method. The simplest aerators consist of two coaxially mounted pipes--the inner one, to which compressed air is supplied from the compressor located on the shore; and the outer one, reaching above the water table. The water pushed out by the inner tube undergoes partial oxygenation and returns to the hypolimnion by the outer tube. The technique of artificial aeration without destroying thermal layers is less invasive than destratification systems. However, it should be remembered that this technique only affects the bottom zone. Therefore, the improvement of the lake condition is achieved gradually--over the years, the release of nutrients from bottom sediments to water is successively inhibited during periods of water stagnation. Most of the case studies of lakes restoration using artificial aeration described in the literature concern lakes with a clearly conical basin. Typically, aeration is based on the operation of one device located above the deepest part of the lake [15]. Especially in the aeration technique with destratification of thermal layers, the spatial efficiency of aeration significantly improves in lakes with a low depth factor (average depth/maximumdepth). However, the efficiency of oxygenation, regardless of the technique used, is often unsatisfactory, even in lakes with favorable morphometry [18; 19; 20]. This may be caused by the insufficient intensity of the unit processes within the aerator, as well as the difficulty in underwater oxygen distribution beyond the immediate impact zone of the aerator [21]. Even more difficult conditions for effective oxygenation of tropholytic zones are in moderately deep lakes with a relatively flat bottom, with partial stratification or a residual, shallow hypolimnion. They are deep enough to show oxygen deficits at the bottom, but the unfavorable morphometry limits the spatial effectiveness of classic treatments based on single-point aeration. One of such lakes is the hypertrophic Lake Lajskie (area 48 ha, maximum depth 7.4 m). Improving the quality of water in this lake is crucial for maintaining the welfare of one of the most valuable water reserves in the Warmia and Masuria region (Poland), located lower in the catchment area. Hence, a water restoration program has been implemented since 2019, including an innovative multi-point oxygenation system. The purpose of the program is to improve the oxygen conditions over the vide, flat area of the lake bottom. The aim of the work is a preliminary assessment of the effectiveness of oxygenation of the lower layers of the hypolimnion of Lake Lajskie by a network of aerators powered by pure oxygen.