Identifying the water quality of temperate Polish lakes based on major abiotic parameters and indicator phytoplankton assemblages

The investigation conducted in Northeastern Poland aimed to determine the water quality of twenty-five lakes based on leading environmental drivers and indicator phytoplankton species. Water and phytoplankton samples were collected between the 13th and 28th of August, 2023. To evaluate the water quality, the Weighted Arithmetic Water Quality Index (WAWQI), Comprehensive Pollution Index (CPI), and Carlson’s Trophic State Index (TSI) were chosen as appropriate indexes. The minimum values of WAWQI and CPI were observed in ST22 (Lake Wigry) and ST18 (Lake Hańcza) while the highest ones were in ST16, ST1, and ST2, (Rekąty, Suskie, and Piłag) respectively. TSI values were also the lowest in ST18 and the highest in ST10. Pediastrum was the most abundant genus throughout the study. However, on a species level, C. hirundinella and P. boryanum were observed as the most dominant species, reaching the highest biomass in six lakes. According to WAWQI, three lakes were classified as "extremely poor", three as very poor and the rest as poor Based on the results of CPI, four lakes belonged to the "slightly polluted", three to "clean", and the rest to "sub-clean", with non of the lakes classified as medium or heavily polluted. The water quality in the region is declining as a result of tourism and developmental activities. Therefore, this work is expected to contribute to upcoming water management activities. Moreover, based on Carlson's Trophic State Index (TSI) most of the lakes belong to class III (mesotrophic) (10) and IV (eutrophic) (9).