VERTICAL DISTRIBUTION AND SEASONAL CHANGES OF MICROBIAL COMMUNITY IN THE LAKE OHRID PELAGIC REGION

The aim of the study was to determine vertical distribution, seasonal changes in the number of bacterioplankton and its qualitative composition in the waters of Lake Ohrid pelagic region. Samples were collected in a vertical profile (surface and depths of 10m, 20m, 30m, 40m, 50m, 75m, 100m, 150m, 200m, 250m and 275m) from the pelagic zone of Lake Ohrid during 2007-2008. In the Lake Ohrid pelagic region the maximal counts of bacteria were found in the trophogenic zone, in summer, and in the zone near the bottom, probably reflecting the accumulation of particulate organic matter, and concentration of soluble organic substrates excreted by phytoplankton. Generally, the lake is in the category of clean waters with a domination of oligotrophic bacteria. According to the received results for saprophytic bacteria, the pelagic water of Lake Ohrid is still of I class (oligotrophic).As for representation of physiological groups of bacteria, the general conclusion is that all groups have very similar seasonal dynamics and relatively low abundance. It is a common characteristic of all investigated physiological groups of bacteria: proteolytic, amilolytic, lipolytic, phosphor-mineralizing, phosphor-mobilizing, nitrogen-fixing and cellulolytic bacteria, that they exhibited two development maxima in the summer to fall (when decomposition of dead plankton occurs) and a minimum in the winter-spring period. Organic phosphorus-mineralizing bacteria are much more abundant than phosphor-mobilizing bacteria. However their number in the pelagial region is small and insignificant, as the lake is oligotrophic and relatively low quantities of the phosphorus containing matter. Anyhow, obtained results indicate that Lake Ohrid is a biologically controlled ecosystem of oligotrophic character.