Trophic state assessment based on late summer phytoplankton community structure: a case study for epilimnetic lake water

Phytoplankton species composition, abundance and biomass were studied in samples taken from the epilimnion of Lake Strzeszyńskie in late August of each year from 2000-2005. The number of phytoplankton species was relatively low, with the greatest species diversity observed in the Chlorophyceae. Total phytoplankton abundance was high, with major contributions by picophytoplanktonic cyanobacteria, which formed aggregations. Total phytoplankton biomass was generally low, with the sum of the 2-5 biomass-dominant species always exceeding 50% of the total phytoplankton biomass. The taxonomic composition of the group of biomass dominants was relatively stable inter-annually. On the basis of phytoplankton biomass and chlorophyll a concentration, Lake Strzeszyńskie can be classified as oligo-mesotrophic and mesotrophic, respectively. Trophic index values calculated on the basis of abundance of phytoplankton indicator taxa were higher than the values based on their biomass, and mean values indicate that the lake is mesotrophic or intermediate between mesotrophic and eutrophic.