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The unicellular flagellates of the genus Phacotus, which form a solid calcite lorica by extracellular precipitation of calcium carbonate are some of the nanoplankters quite frequently occurring in hardwater ecosystems of the temperate zone. The aim of this study was to describe seasonal changes of Phacotus lenticularis populations in several types of water bodies (a glacial lake, small and shallow ponds, and a lowland river) in relation to some environmental parameters. All the water bodies are subject to multiple human pressure: recreation, fishing, agriculture, and urbanization. P. lenticularis individuals preserved in Lugol’s solution were counted under an inverted microscope (after sedimentation in 14 or 9 ml chambers), including both empty and filled loricae. Its populations differed both between seasons and between waters. High P. lenticularis abundance was mainly limited to the summer months. It was observed at temperatures above 19°C and pH values above 7.8, in water characterized by quite high electrolytic conductivity (>500 μS cm-1). Statistical analysis reveals two significant relationships between physicochemical parameters and the abundance of P. lenticularis. This suggests that in the investigated waters the biotic factors and processes are probably more important regulators of changes in the density of its population than nutrients.

The first studies of phytoplankton of the River Warta in Poznań (Poland) were carried out in the 20th century (in 1922–23 and 1950–57). In the growing seasons the dominant groups were diatoms and green algae. Cyanobacteria were noted, but they did not have high abundance. The aim of this work is to present the phytoplankton research conducted on the River Warta in Poznań in the 21st century (in 2003, 2009, 2010 and 2016). In all years the dominance of diatoms and green algae in terms of biomass was noted. However, in late summer cyanobacteria biomass was high and this group became dominant or co-dominant. Spring blooms were created by unicellular centric diatoms, e.g. Stephanodiscus minutulus and colonial green algae: Coelastrum microporum or Micractinium pusillum. In summer, bloom-forming taxa were unicellular centric diatoms, colonial diatoms: Aulacoseira granulata or Fragilaria crotonensis and cyanobacteria: Aphanizomenon flos-aquae and Woronichinia naegeliana. The occurrence of taxa typical of dam reservoirs and lakes suggests the influence of the Jeziorsko Reservoir on the phytoplankton of the River Warta, but it does not exclude the impact of tributaries and oxbow lakes. The research conducted in the 20th and 21st century show important changes in the taxonomical structure and abundance of phytoplankton.