Plankton Primary Production in Volga River Reservoirs: Current Status and Trends in Long-Term Changes

This paper presents data on the primary production of plankton in the Volga River reservoirs during the summers of 2015–2023. Photosynthesis was calculated based on the chlorophyll content at the stations and daily assimilation numbers. The photosynthesis intensity (Amax) varied on average from 2.28 ± 0.08 to 4.30 ± 0.25 mg O2/(L day) in the Upper and Middle Volga reservoirs and from 1.42 ± 0.10 to 1.86 ± 0.12 mg O2/(L day) in the Lower Volga. The integral primary production (ΣA) was mainly from 2.76 ± 0.16 to 3.63 ± 0.22 g O2/(m2 day) and reached 4.40 ± 0.34 g O2/(m2 day) in the Cheboksary Reservoir. Over the 9-year observation period, a tendency towards an increase in Amax in the Ivankovo, Gorky, Kuibyshev, Saratov, and Volgograd reservoirs, as well as increase in ΣA in all reservoirs of the cascade, was observed. The interannual changes in Amax and ΣA do not go beyond the values obtained in 1989–1991.