Multiannual changes of structure and biomass of crustacean zooplankton were studied in the shallow, lowland, polyhumic Siemianówka Reservoir on the upper Narew River in northeast Poland. High ammonium and soluble reactive phosphorus ion availability for phytoplankton, low water transparency, and high dissolved oxygen concentration resulted in humoeutrophication and caused an intensive development of summer cyanobacteria. We compiled and analyzed long-term (1993–2011) crustacean zooplankton community data and chlorophyll a concentrations. Zooplankton biomass and structure in the summer were related to the intensity of the cyanoprokaryota blooms. As chlorophyll a concentration increased, crustacean zooplankton biomass decreased and smaller crustacean species dominated the zooplankton structure. Large species of crustaceans (Daphnia longispina, D. magna, and Eudiaptomus graciloides) disappeared from the reservoir after filamentous cyanobacteria dominated the phytoplankton. Long-term studies suggest that phytoplankton with strong dominance of filamentous cyanobacteria shape the crustacean community but are probably an insignificant food source for the crustacean zooplankton in the Siemianówka Reservoir
