The community structure of Lake Kinneret (Israel) microorganism populations: Size distribution and succession

The abundance and succession of bacterial populations, and their biovolume has been studied in the deepest station of Lake Kinneret, Israel during spring-summer. These parameters were then related to the chemical, biological and physical/hydromechanical conditions in the lake. Medium sized cells (2-3 μm) dominated the bacterial population in the spring, then in mid-summer the smaller cell fraction (1 μm) took over. Relatively low numbers of large cells (10 μm) were counted, but the biovolume of this fraction was significant, especially in mid-summer. Both the abundance and biovolume of medium sized cells were positively correlated with NH₄/⁺ and O₂ concentrations, but not with NO₃/^-, inorganic orthophosphate (P(i)) and Total-P. At water temperatures above 28°C, the biovolume of medium sized cells decreased, probably due to faster grazing of zooplankton. When water circulation in the vertical plane was dominant, large cells disappeared from the water column, due to sinking to the bottom. We conclude that chemical parameters, such as NH₄/⁺ and O₂ concentrations, had a long-term effect on bacterial succession and size distribution, whereas hydromechanical mechanisms, such as turbulent mixing in the horizontal or vertical planes, had a short-term effect.