Ecological Systems and Devices Annotation << Back The Effect of Grazing by Fish on Phytoplankton Development: Experiments in a Flow-Through Ecosystem Gerasimova T.N., Sadchikov A.P. The Patriarshii Pond is a small water body within Moscow boundaries. It shows regular and mass blooming of green algae. Many fi sh species have long, portioned spawning cycles throughout the summer. The pond is constantly inhabited by juvenile fi sh of various sizes, which consume large zooplankton organisms. Because of this, the dimensional structure of the population shifts towards smaller species. Simocephalus vetulus was recorded in the water body only once (July 24). Its abundance was as low as 0.1 spec./L. After that, up to the end of the study, it was never seen in the pond, though it developed very well in an experimental system (into which it penetrated from the water body). In the fl ow-through ecosystem (in the absence of fi sh), the abundance of S. vetulus in August reached 11 880 spec./L. It was not found in the pond because of its grazing by fi sh. Twenty-three zooplankton species were recorded in the pond. Rotifers are the most abundant. They indicate to the presence of large amount of particulate matter (algae, detritus) in the water body. Phytoplankton is represented by 106 species of algae and cyanobacteria and the green algae, by 67 species, among which many form colonies with a size of 100–250 μm, immersed in mucus. Under the effect of zooplankton, the colonies are destroyed and used by it as food. Keywords: eutrophic water body, green algae, pond ichthyofauna, zooplankton, experimental ecosystem. DOI: 10.25791/esip.11.2024.1488 Pp. 58-64.