Cyanobacterial mats of solar lake were followed up during the four seasons of a year 1997 through daylight and night.  Also, the free water microorganisms groups related to soil fertility were followed included: Total colony counts, asymbiotic-N-fixers, nitrifying bacteria, sulphur-oxidizing and reducing bacteria. The ratio between total phytoplanktons and bacteria in water was calculated as indicator for biological activity in the Solar Lake The obtained results showed that there are a positive correlation between the counts of free water bacteria and phytoplanktonic mass in the lake. The highest bacterial counts were recorded during day time of spring season. The ratio of algosphere to free water bacteria was generally more than one, reached highest value beginning of stratification period (autumn season). On the other hand at the highest figure of total aerobic bacterial counts and the lowest figure was recorded by sulphur reducing bacteria.