Long-Term Dynamics of Chlorophyll in Plankton of Different Sites in a Large Plain Reservoir

On the basis of field observations in 2009-2019, the seasonal and long-term dynamics of chlorophyll at six standard stations of the Rybinsk Reservoir is considered. The fluorescence method is used to determine chlorophyll. The average chlorophyll content at stations varies from the minimum 3.5-10.8 to the maximum 16.9-40.5 mu g/L in different years and from 12.0 at station 5 (Sredny Dvor) up to 21.6 mu g/L at station 2 (Mologa) on average for the entire period. The seasonal dynamics of chlorophyll is characterized by spring; summer; and, in some years, autumn maxima. With the same type of seasonal dynamics, the change in chlorophyll at the stations is to varying degrees coupled in the years of observation and is most often correlated in neighboring areas. Significant differences in the amount of chlorophyll at the stations are observed during surveys carried out within one day and are determined by the complex hydrological structure of the reservoir; active dynamic processes; and, in the spring, by the thermal regime. The presence of stable large circulation zones smoothes out the spatial differences of chlorophyll, the average seasonal concentrations of which do not differ significantly at all six stations in years with an average water level, and at four stations of the Main Reach in extremely high-water years. A close correlation in long-term dynamics is revealed for the average chlorophyll concentrations for the growing season at six stations, as well as at each station and the reservoir as a whole. The results of the work confirm the reliability of the data obtained for assessing the ecological state of the Rybinsk Reservoir.