On the possible connection between phytoplankton dynamics in a large lowland reservoir and long-term variations in solar activity

Our work is based on the assumption of a connection between the development of phytoplankton as chlorophyll a (CHL) and the long-term dynamics of solar activity expressed as sunspot numbers (Wolf numbers, W) by the example of the Rybinsk Reservoir (Upper Volga, Russia). We analyzed the possible relationship between mean for the growing season CHL and W over the period of solar cycles 20–24 (1969–2019), as well as the relationship between CHL in water and bottom sediments with W during the entire cycle 24 (2009–2019). A moderate tendency towards a decrease in CHL with increasing W was revealed for cycles 20 and 22, but for cycle 21 there was no correlation between them at all. A linear increase in CHL with increasing W was detected for cycle 23 and the most distinct for the cycle 24. A close relationship with high positive correlation coefficients equal to 0.74 and 0.70 have been established between W and CHL in water and bottom sediments during cycle 24. For sedimentary pigments, the degree of correlation with W depends on the type of bottom sediments. A reliable connection is typical for deep areas with a high rate of sludge accumulation; in areas with unfavorable conditions for sedimentation, such a relationship has not been identified. It is assumed that the increased correlation between CHL and solar activity in the cycle 24 compared to the previous cycles is due to the anomaly of this cycle, characterized by unprecedentedly low W values.