Determination of Eubacterial and Cyanobacterial Size and Number in Lake Baikal Using Epifluorescence

Chroococcoid cyanobacteria, (mean size = 0.79 μm, likely Synchetocystis limnetica Popovsk) and total eubacteria (mean size = 0.33 μm), from Lake Baikal, USSR, were enumerated using epifluorescence microscopy and sized with image analysis. Bacterial densities ranged from 0.44 · 10⁶ cells ml^?1 at 250 m to 2.3 · 10⁶ cells ml^?1 at the surface. Mean eubacterial abundance was 1.3 · 10⁶ cells ml^?1. Cyanobacterial densities were more variable, ranging from 0.42 · 10⁴ cells ml^?1 at 250 m to 9.8 · 10⁴ cells ml^?1 at the surface, with a mean abundance of 2.7 · 10⁴ cells ml^?1. The cyanobacteria, in particular, occurred in clusters resembling “marine snow”. Our results indicate that Lake Baikal picoplankton size and density are similar to other large lakes but may have a more diverse community structure than in other large oligotrophic lakes.