Assessment of water quality in a lake with limited tourist pressure due to the COVID-19 lockdown

This study provides a unique opportunity to realistically assess the impact of tourism pressure on the water quality of lakes. A quantitative analysis of the physicochemical and microbiological parameters of water during and after the COVID-19 lockdown period was made. The study covered Sztynorckie Lake located in the Great Masurian Lakes System (GMLS) (northeastern Poland), which is a holiday destination for hundreds of thousands of holidaymakers, and serves as a pillar of the local economy. Significant improvement in the microbiological water quality was observed only during the drastic reduction of human activity due to the pandemic-enforced lockdown. This was confirmed by the results of PCA (X-axis explains 88 % of the variability of the analyzed samples; statistical significance of p<0.05). The rapid increase in the number of coliforms and Escherichia coli (613.1 MPN/100 mL) in the first weekend after the lockdown (June 2020) indicates a point discharge of sewage (including fecal pollution). Illegal discharge of sewage and wastewater is a highly reprehensible offense, and radical legal steps should be taken to prevent it and to protect the water quality. No significant relationship was found between the lockdown and the lake water's physicochemical conditions. In the spring, increased primary production was evidenced by the high concentration of chlorophyll a (52.5 to 70.5 μg/L), suspended particulate matter (20.4 to 21.0 mg/L) and total organic carbon (20.7 to 22.7 mg/L). This is a warning that a nutrient-rich lacustrine ecosystem cannot be restored quickly.