Long-Term Changes in the Main Indicators of the Trophic State of the Large Plain Reservoir Under the Influence of Climatic Transformation and Successional Processes
According to field observations in 2001–2019 in the Volgograd Reservoir, significant negative trends in allochthonous and total organic matter and mineral nitrogen were noted. Winter warming led to an increase in the volume of waters rich in organic matter of a humic nature, which led to an increase in color and iron content. An increase in the intensity of bioproduction processes against the background of higher water temperature contributed to a growth in the proportion of easily oxidized fractions of organic matter in comparison with the initial period of the reservoir’s existence. During the first two decades of the new century, there was a tendency towards simplification of the phytoplankton community, along with a negative trend in the total biomass and the main dominant groups - diatoms, cyanoprokaryotes and green algae. The ratio of the functional groups of phytoplankton changed: the proportion of diatoms decreased with a rise in Cyanoprokaryota and mixotrophic phytoflagellates. The dimensional characteristics declined and the course of the seasonal dynamics of biomass changed from the spring peak dominance of diatoms to the summer maximum of cyanoprokaryotes dominance. The restructuring of the phytoplankton community is a response to a change in the regime of biogenic elements and organic matter as a result of climatic transformation and successional processes.