The role of zooplankton in phytoplankton biomass decline and water transparency regulation in a water body subject to high organic and mineral load

Under biogenic load and at high abundance of ichthyofauna (the concentration of total phosphorus P_tot = 0.20 mg P/l, mineral phosphorus P_min = 0.13 mg P/l, and at the ratio N: P = 4: 1 in early summer), zooplankton reduced phytoplankton biomass during summer and autumn and determined high water transparency, reaching >50% of the depth of the water body. It was found that, with a passage of ichthyofauna to artificial food, the trophic press of plankton-eating fish on zooplankton decreases. Under such conditions, with the biomass ratio of plant-eating zooplankton to edible phytoplankton between 0.4 and 7.9, zooplankton can reduce phytoplankton biomass within a day. The major portion (up to 100%) in phytoplankton biomass reduction was due to large-size filterers-phytophages Daphnia longispina and verticators Asplanchna priodonta helvetica.