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

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