Epilimnetic zooplankton communities in soft-water lakes with isoetids in northern Poland

Soft-water lakes are valuable natural habitats and an important aquatic ecosystems. Lobelia lakes, which are classified based on the occurrence of isoetids, are the most characteristic of this group. The aim of our research was to assess the species richness and structure of pelagic zooplankton communities in the epilimnion of 45 Polish soft-water lakes. The lakes analyzed included a wide range of trophic states from oligotrophic to eutrophic. We noted 79 species of zooplankton, including 49 rotifer, 23 cladoceran, and seven copepod species. The species composition of epilimnetic zooplankton communities in these soft-water lakes was poor, and the species richness and zooplankton biomass were low. In these lakes, the species with the highest frequencies accounted for an important part of the total zooplankton biomass of the zooplankton communities, but the species occurring at lower frequencies differed among the lakes. The most common rotifer taxa were: Keratella cochlearis, Keratella cochlearis var. hispida, Polyarthra euryptera, Gastropus stylifer, and Trichocerca similis. The most important crustacean species in the soft-water lakes were: Macrocyclops albidus, Ceriodaphnia quadrangula, and Eudiaptomus sp. The major factors influencing rotifer species richness were total phosphorus, conductivity, and the trophic state of the lakes. Cladoceran species richness was positively related to lake area and pH, while the species richness of copepod communities was negatively related to chlorophyll a concentrations.