[Structure and function of pelagic zooplankton in various types of lakes (using the example of small lakes in northwestern Russia].

Changes of structure and functioning of pelagic zooplankton under lake eutrophication were studied in a group of small (without outlet) lakes in southern Karelia and Leningrad region. The lakes were morphologically similar and located within the same climatic zone. Differences in their trophic status were connected with anthropogenic eutrophication. Correlation between species number and feeding resources of a lake is determined as: Y = (8.01 +/- 3.85) + (0.29 +/- 0.07)X1 + (6.75 +/- 1.52)X2, r2 = 0.95, (1) Y--number of zooplankton species, X1--average chlorophyll concentration for the season, mg/1; X2--average value of biochemical oxygen demand (expressed in mgC/1). Average biomass of zooplankton for season also depends on food: logY = (-0.054 +/- 0.224) + (0.242 +/- 0.094)logX1 + (0.170 +/- 0.179)logX2, n = 13, r2 = 0.87, (2) Y--average zooplankton biomass for the season, kcal/m3, X1 and X2 the same as (1). Increase in species number and zooplankton bimass determined primarily by Rotatoria occur in parallel to chlorophyll concentration and activity of bacteria. In acid lakes rotifers are not important in energy balance. In mezotrophic lakes two main energy paths are formed--through rotifers and through Crustacea. The role of rotifers is extremely important in eutrophic lakes where about 80% of energy paths through them. The ratio community production to energy consumption for the growing season is also depended on the community structure (species number, equitability, connectence): Y = (2.257 +/- 0.026) - (0.368 +/- 0.031)X1 + (5.160 +/- 0.442)X2, r2 = 0.99, (3) Y--average seasonal production of zooplankton, kcal/m2; X1--maximal meaning of Shannon index (bites), calculated on biomass value; X2--connectence of the community, calculated according Briand (1983). Eutrophication changes the stability of lakes relative to external influences, while low productive lakes are very sensitive to the increase in nutrient load, high productive lakes are more influenced by changes in fish predation.