Zooplankton can be used as a reliable bioindicator of the physical and chemical conditions in the aquatic environment. An analysis of the structure of the zooplankton community in Lake Długie was based on the material collected in the years 2001–2003 (after restoration using PAX) and in 2007 (control year). It was found that the number of zooplankton species, their abundance and biomass varied greatly and depended on lake trophy and abiotic environmental factors. The cutoff of sewage inflow and the restoration of the lake resulted in gradual changes in the structure of the zooplankton community, consisting in an increase in species diversity and a marked decrease in the number and abundance of the indicator species of high trophy levels. As shown by the values of zooplankton-based indices, the waters of Lake Długie before restoration were eutrophic with some features of polytrophy, while after restoration they were mesoeutrophic/mesotrophic.

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