The Use of Phytoplankton in the Assessment of Water Quality in the Lower Section of Poland’s Largest River

Abstract

Phytoplankton studies were carried out in the lower course of Poland’s largest river, the Vistula. This section of the Vistula has changed much since the 19th century due to regulation of the bed and limitation of its flood banks. In the years 1994–2020, hydrobiological studies of physical and chemical parameters and phytoplankton were carried out. On the basis of these studies, this research investigated the water quality of the river. Since 2000, a gradual improvement in water quality in the river has been observed, as manifested by decreasing phytoplankton biomass (from over 15 mg/L in 1994 and 2003 to less than 10 mg/L in the 2000s) and lower chlorophyll a concentrations (from 48 µg/L in 1998 to 12.5 µg/L in 2020). The river’s ecological potential index, calculated on the basis of indicator species of phytoplankton and chlorophyll a concentration, also indicates a progressive improvement. In the 1990s, the ecological potential was poor; at the turn of the century, it was moderate; and in the following years, the phytoplankton studies showed an ecological potential that was good or moderate (slightly below the lower threshold of good potential).