Nutrients and zooplankton as factors controlling phytoplankton growth in the Cheboksary Reservoir

We present a detailed study of spatial distribution of nutrient elements, phytoplankton and zooplankton in the Cheboksary Reservoir-a eutrophic artificial lake in Central Russia-during summer periods of 2017-2021 and review the relationships between these elements of the food chain to determine what shapes the trophic state of the lake. It is shown that vast differences in nutrient content of the reservoir's tributaries cause its trophic state to change significantly in mouth areas of some larger inflowing rivers. However, in the largest part of the lake the relationship between nutrient content and chlorophyll becomes more subtle, whereas zooplankton community structure steps up as an important factor controlling the phytoplankton growth. It is clear that major efforts in nutrient loading reduction are needed to improve the ecological state of the reservoir, and nitrogen control may be more effective, than phosphorus control. However, described trophic interactions may reduce efficiency of nutrient control and require some biomanipulation measures to improve water quality of the reservoir.