IMIS

The dataset for 2010 was obtained within the framework of the Ukrainian national monitoring program. Mesozooplankton sampling followed the same standardized protocol, employing a vertical Juday net with a 0.1 m² opening area and 150 μm mesh size. Samples were collected by vertical tows extending from 2 m above the bottom, or from the lower limit of the oxic layer, to the surface at predefined depth intervals. The effective wire length was calculated considering both the cable angle and the target sampling depth. Collected material was fixed in 4% borax-buffered formaldehyde (pH 8.0–8.2; one part 40% formaldehyde to nine parts water/sample) and stored in plastic containers until processing. In the laboratory, each sample was concentrated to 100–150 cm³ before being split into sub-samples. Taxonomic composition, abundance, and biomass (estimated from species-specific individual weights) were determined using a Bogorov chamber and a stereoscopic zoom microscope. For each sub-sample, no fewer than 100 specimens of each of the three dominant species were enumerated. This counting approach yields an estimated abundance precision of about 20% for the dominant taxa (Alexandrov et al., 2014). Species identification was carried out according to Mordukhay-Boltovskoy (1968, 1969, 1972).