Depth distribution of freshwater microorganisms and physicochemical characteristics in Iskar reservoir

The depth distribution of different physiological and taxonomical groups of freshwater bacteria in Iskar reservoir - the main drinking water source for Sofia city, was studied. Samples were taken from several depth layers in the area of intake water tower of the reservoir and studied for presence of aerobic, anaerobic heterotrophs, oligotrophs, ammonifiers, nitrifiers and denitrifiers, genera Aeromonas and Pseudomonas and coliform bacteria with cultivation methods. We found close distribution of observed microbial groups according to the correlation coefficients of their numbers in different water layers. Two zones with similar distributions were presented - upper zone in the upper layers from the surface down to the fifth meter and another zone under 10 meters depth. Exception was the observed distribution of microbial groups in the water layer at 25 meters, which showed closer correlation with the upper zones than with the contiguous layers. We determined also some physicochemical, chlorophyll a and phaeophytin concentration, chemical oxygen demand (COD), nitrogen forms and phosphate concentration. Correlation was presented also between the bacterial numbers, chlorophyll a and organic matter (COD). The dominating groups are aerobic heterotrophs and oligotrophs through the whole water column representing up to 31.34% respectively 48.58% from all culturable bacteria.