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Abstract:

Water reservoirs in recent years have been subject to strong anthropogenic pressure leading to their eutrophication and overgrowing processes. The paper analyzes the temporal and spatial changes in overgrowing of the reservoir in order to identify the factors determining the dynamics of this process. The analysis was carried out on the example of Przebędowo reservoir of 12 ha, located in western Poland. 42 Sentinel-2 Level-1C satellite images from the period between the 4^rd July, 2015 and 6^th April, 2018 were used in the study. The analysis of reservoir overgrowth process was based on the Water Adjusted Vegetation Index (WAVI). Overgrowth process was mapped by the simple segmentation algorithm using a threshold value. The analyzes were carried out using the SNAP and ArcGIS software. The results from 42 satellite images were grouped using cluster analysis (CA). Moreover, in order to indicate the factors determining the overgrowth process in terms of time and space, the main component analysis (PCA) was used. The obtained results show that the reservoir is subject to very intensive overgrowth over the entire surface. On the other hand, the dynamics of reservoir overgrowing in a year is related to thermal conditions and to a lower degree to the water level variability. Assessment of inter-annual changes in reservoir overgrowth was impossible because of the differences in temperature and temperature distribution over the analyzed years. The CA analysis allowed a division of the images into 4 groups, which present respectively the phase of plant growth, the phase of maximum coverage with plants, the phase of plants decline and the phase of minimal coverage with plants. The PCA analysis showed that the overgrowth process is initiated from the reservoir shore, the shallowest places are occupied first. Also, along the reservoir there is a gradient of overgrowth from the inlet to the outlet of the reservoir.