STUDYING THE EUTROPHICATION OF LAKES IN VOLYN POLISSYA (ON THE EXAMPLE OF LAKE ZASVITSKE)
Authors: FESYUK Vasyl , NETROBCHUK Iryna , Dubrovyk Oleksandr 
Published: June 2024

Many diverse lakes are a natural phenomenon of Volyn Polissya. They are of great ecological, recreational and economic importance. As a result of intensive economic activity, pollution and other anthropogenic factors, many lakes are subject to eutrophication. Excessive enrichment of water bodies with nutrients causes the rapid development of algae, deterioration of water quality and disruption of ecological balance. Global climate change has also had a significant impact on this process in recent years. Lake Zasvitske was chosen for the study. It is located in the north-west of Rivne Oblast, in the Varash district, south of the village of Mlyn and north-west of Lake Nobel. It is part of the Nobel National Nature Park. The area of the lake is 22 hectares. Is 620 m long, 350 m wide, has a maximum depth of 16 m, an average depth of 12.5 m, a coastline of 1.75 km and a capacity more than 5 million m3. The lake is fed primarily by atmospheric precipitation and groundwater from. The ecological condition of the watershed is favorable, there are no potentially dangerous objects, the lake is surrounded by pine forests to the north, east and south, and there are equipped small beaches on the western and northwestern sides. The lake water is quite clean. Concentrations of pollutants are exceeded only by the indicator of biological oxygen consumption by 28% and zinc content by 3.1 times. The NDVI and NDAI indices were used to assess the eutrophication of the lake. NDVI (Normalized Difference Vegetation Index) is a quantitative indicator of photosynthetically active biomass most often used to assess eutrophication processes in water bodies. The value of the index varies from -1 to 1. Negative values of the NDVI index indicate the presence of water in the form of clouds and snow; values close to zero (-0.1 to 0.1) correspond to areas with no vegetation cover. Values from 0.2 to 0.9 indicate active vegetation (including in water bodies). NDAI (Normalized Difference Algae Index) is used to diagnose the process of algae overgrowth in a water body and determine the stage of this process. NDVI values increase with the beginning of the transition of the average daily temperature through 0°C, with maximum values reached in late July - early August. Subsequently, the values decrease until the onset of frost. For example, on April 1, 2022. NDVI acquires small positive values up to 0.1. By mid-May, the index values rise to 0.1-0.2. In the eastern part of the lake, where hygrophytes are found, it increases to 0.3. By the end of the month, the NDVI values for most of the reservoir drop again to 0. At the end of June, the index values increase to 0.1 and decrease again. In the first decade of July, the water is cleared of algae. The index acquires negative values (up to -1), which corresponds to clean water. In August-September, the index values vary in the range (-0.1; -0.5). The NDAI index allows us to better trace the seasonality of vegetation processes in the lake. In early April, the development of vegetation processes begins from the belt of semi-flooded hygrophytes along the shores of the reservoir, covering the island rise in the north of the lake. In summer, the intensity of vegetation processes increases, reaching a peak in September. Subsequently, the vegetation dies off and the process repeats in a new vegetation cycle.
Keywords: lake, lake catchment area, limnosystem, geo-ecological state of the catchment area, lake eutrophication, measures to prevent and reduce lake eutrophication.