An in situ method for the investigation of vertical distributions of zooplankton in lakes: test of a two-compartment enclosure

Two-section enclosures were designed for the investigation of the effect of various physicochemical and biological factors on vertical distribution of zooplankton in situ. The framework of the enclosure was a cylindrical polyethylene column without any partitions inside, in which the isolation of animals in different sections after in situ exposure was achieved by pinching the flexible central part of the column. Enclosures were tested at the brackish stratified meromictic Lake Shira (Russia, Khakasia). The absence of fish and carnivorous zooplankton in the lake suggests that the vertical distribution of zooplankton is mainly determined by physicochemical gradients in the water column. Experiments and field observations demonstrated that all age and size groups of Arctodiaptomus salinus and Brachionus plicatilis strongly avoided surface layers during the daylight. The escape of zooplankton from the anoxic hypolimnion was less active. Statistically significant avoidance was observed only for copepodites C4–C5 and females of A. salinus. The relatively simple construction of the columns and easy handling during the experiment were the factors that favoured the use of this device to perform in situ basic tests of the effect of different factors on the vertical distribution of zooplankton.