Abiotic Factors and Their Role in the Development of Phytoplankton in the Lower Volga

Based on field observations carried out during the 2015–2020 summer low water period, the interannual variability of abiotic characteristics and chlorophyll content in the Lower Volga is considered. In years with different thermal conditions and water content, the temperature, transparency, color and electrical conductivity of water are characterized by small variability and demonstrate changes from north to south, according to the zonal features of the Volga cascade. The average nutrient content (0.81–0.99 mg/L N_tot and 101–134 μg/L P_tot) changes insignificantly in the Saratov and Volgograd reservoirs, but decreases in the unregulated lower part of the Volga. The content of N-N\({\text{O}}_{3}^{ - }\) and P-P\({\text{O}}_{4}^{{3 - }}\) in the total nitrogen and phosphorus pool respectively, is 4–9 and 69–74%, the ratio N_tot/P_tot < 10 indicates a possible nitrogen limitation of phytoplankton. The Chl a content corresponds to the mesotrophic category in the Saratov and Volgograd reservoirs (5.3 ± 0.6 and 7.2 ± 0.9 μg/L), and eutrophic in the lower section (13.9 ± 1.5 μg/L). The trophic status of the Lower Volga has not changed in comparison with the last decade of the 20th century. It was found that abiotic factors have a weak effect on the Chl a content in the Saratov reservoir, moderate in the Volgograd reservoir and almost completely control the development of phytoplankton in the unregulated lower part of the Volga (R² = 0.21, 0.59, and 0.91). The data obtained supplement the observations of previous years and form the basis for long-term monitoring of ecosystems of large artificial reservoirs.