Relations between variations in the lake bacterioplankton abundance and the lake trophic state: Evidence from the 20-year monitoring

Trophic state is a key biological characteristic of water body that integrates the main hydroecological factors. However, the character of the interrelation between bacterioplankton dynamics and the lake trophic state remained obscure. The long-term (1995-2015) monitoring data were carried out to examine the relations between bacterial parameters (abundance, biomass, and cell biovolume) and main hydroecological factors (chlorophyll, phosphorus, nitrogen, organic matter) in three water bodies: Lake Batorino, Lake Myastro, and Lake Naroch, which differ by trophic state. The results of the analysis of the data show that the growth of the chlorophyll-based trophic state index (TSIChl) as well as dissolved organic carbon (DOC), total phosphorus (TP) and 5-day biochemical oxygen demand (BOD5) in the chain Lake Naroch - Lake Myastro - Lake Batorino is accompanied by a linear increase in the bacterioplankton abundance, while fluctuations of bacterioplankton abundances and trophic state parameters in each of the Naroch Lakes are not correlated with each other. It is shown that temperature is the factor, which impacts seasonal bacterioplankton dynamics. The ratio of the time-averaged abundance of bacteria in July to the time-averaged abundance of bacteria in May remains virtually unchanged compared to the ratio between July and October. However, similar ratios for chlorophyll-a undergo significant changes. This result leaves room for further investigation of the factors, which can influence trophic state of the lakes, on the assumption that bacterial abundance is considered as an invariant measure.