Samples were taken by 6 L bathometers at standard depths and at the thermocline layer. The standard sedimentation technique for samples preparation (Usachev, 1926) was used. Samples were preserved by buffered formalin 40% and were settled within 30 days. Then a volume of water was siphoned off and samples were concentrated to 10-20 ml. After thorough mixing the subsamples (0.05 or 0.1 ml) were transferred to the counting chamber. Organisms were counted in two subsamples at ×300-400 magnification.