Biogenic Elements and Their Significance in the Development of Phytoplankton in Reservoirs of the Upper Volga

Abstract

The content of Chl a and nutrients in the Upper Volga reservoirs in the summer of 2015–2018 has been studied. With Chl a content typical of the summer phytoplankton maximum (from 19.9 ± 3.9 to 48.0 ± 12.0 μg/L), the reservoirs are classified as eutrophic; the Rybinsk Reservoir was classified as mesotrophic only in cool 2017 (10.3 ± 4.7 μg/L). The content of nutrients (from 0.71 ± 0.06 to 1.54 ± 0.07 mg/L Ntot and from 51 ± 12 to 106 ± 3 μg/L Ptot) also corresponds to the eutrophic status of the Upper Volga reservoirs. The average values of the Ntot/Ptot ratio (11–16) indicate the absence of the biogenic limitation of phytoplankton, and a deficiency of nutrients was recorded only in the Rybinsk Reservoir in August 2015 and 2016 and the Ivankovo Reservoir in 2015. In most cases, low values of correlation coefficients (r < 0.7) were noted between the Chl a and nutrients, indicating the complex and multicomponent effect of the latter on phytoplankton. Multiple correlation analysis confirms the high importance of hydrological conditions for the development of autotrophic plankton of the Upper Volga reservoirs.