## 2 Materials and Methods
### Methods  
The analysis of changes in the water quality of the Lajskie Lake tropholytic zone subjected to experimental oxygenation was carried out in 2021, during the summer thermal stratification of waters. It was the third season of the system's operation, which was built and launched in 2019.  
Three measurement campaigns were carried out in May, June, and August 2021. The first measurement was made before the oxygenation system was launched, and the second and third were made during the operation of the system in 2021 growing season. The lower layers of the lake's tropholytic zone (0.5 and 1.0 m above the bottom level) were tested.  
Figure 2: Functional diagram of oxygen reactors used during the restoration of Lake Lajskie.  
For the purposes of assessing the operation of the lake restoration system, it was assumed that each of the 8 oxygenation reactors would be the focal point of an individual set of 12 measuring stations (Figure 3). They were located in two transects: latitudinal (points 1-6) and meridian (points 7-12). The span of the transects was 100 m, which means that evenly spaced points on each transect were separated by a distance of 20 m. The distance of the oxygenation reactor column from the central points in a given research area (points 3, 4, 9, and 10) was about 6-8 m. The grid of points, planned in this way, allowed researchers to maintain the assumed accuracy of the measurement methodology and to standardize the method of testing each field object.  
The field work consisted of marking out previously designed points and determining their depth (Garmin echoMAP 50 s echosounder with a built-in GPS receiver, verification of the correctness of indications with a plate depth gauge). Positioning accuracy was 2-5 m on average. Then, at the measuring points, the oxygen content of the above-bottom water was tested (0.5 m and 1.0 m above the bottom level) using the YSI Exo 2 multi-parameter probe. To ensure the best accuracy of readings, the calibration process of the multi-parameter probe was based on own standards prepared each time before field tests. They were prepared  
Figure 3: Bathymetry and layout scheme of measurement stations in Lake Lajskie.  
from lake water using the classic Winkler method [27]. The deoxygenated water standard was obtained by incubating the sample in a sealed glass vessel until oxygen depletion, without the use of reducing reagents.  
The Statistica 13.5 software (StatSoft, Inc., Tulsa, OK, USA) was used to compare the water oxygenation levels around the individual reactors. The multiple regression analysis was performed for the identification of location factors, which are significantly connected to oxygen concentration. That analysis allowed to obtain linear models of type:  
\[\text{Y}=\text{B}_{0}\pm\text{B}_{1}\;\text{X}_{1}\pm\text{B}_{2}\;\text{X}_{2} \pm\dots\;\pm\text{B}_{\text{i}}\;\text{X}_{\text{i}}+\text{E}_{\text{ij}}\]  
where:  
Y--dependent variable (oxygen concentration);  
B\({}_{0}\)--constant (intercept);  
B\({}_{1}\)... B\({}_{\text{i}}\)--regression coefficients;  
X\({}_{1}\)... X\({}_{\text{i}}\)--independent variables;  
E\({}_{\text{ij}}\)--residual component;  
R--multiple correlation coefficient;  
R\({}^{2}\)--multiple determination coefficient.  
Before analysis, data were subjected to log transformation (oxygen values were log (x + 1) transformed) in order to approximate to the normal distribution.  
Two independent variables were tested: distance from reactor (D\({}_{\text{i}}\)) and direction in which reactor released oxygen (D\({}_{\text{r}}\)).