Changes in Hydromorphological Conditions in an Endorheic Lake Influenced by Climate and Increasing Water Consumption, and Potential Effects on Water Quality

The study aims to analyse changes in the morphological conditions of the endorheic Lake Borowno (39.06 ha) that occurred in the 20th century and early 21st century. The analysis was based on bathymetric measurements carried out in 2018 and cartographic materials, and performed using QGIS 3.10 and SAGA GIS 6.4 software. Moreover, changes in physical (temperature, transparency), chemical (dissolved oxygen, phosphorus, and nitrogen content) and biological (chlorophyll a, phytoplankton) parameters were analysed based on the results of measurements conducted by the Voivodeship Inspectorate of Environment Protection (VIEP) in Bydgoszcz in the period 1984-2017. It was found that Lake Borowno is subject to a constant reduction in surface area and volume, the rate of which increased in the second decade of the 21st century. The lake's progressive disappearance results from the co-occurrence of hydrological drought over the last several years and an increase in the use of groundwater resources. A decrease in the maximum depth of the lake entails a change in temperature distribution in the vertical profile, which contributes to the tendency toward the lake transforming into an unstratified reservoir. The increase in water temperature affects oxygen content in the bottom water, improves water transparency, and influences the maximum occurrence of chlorophyll a in spring.