Measurement results on spatial distribution (vertical and horizontal) of phytoplankton chlorophyll and its photosynthetic activity are discussed. The fluorescence parameters were used in the measurements. A fraction of micro-algae biomass, being in the photosynthesis region, in the total photosynthetics biomass distributed throughout the water column, was estimated. The phytoplankton assessment above the thermocline shows the algae fraction to be between 30 and 50% of the whole amount. Consequently, less than a half of photosynthesizing organisms of Lake Baikal can participate in "water-atmosphere" gas exchange in the period of spring homothermy formation.