Comparison of methods for nocturnal sampling of predatory zooplankters in shallow waters

The aim of the study was to assess the suitability of a plankton net (diameter of 60 cm, mesh size of 500 μm) and a column sampler (length of 200 cm, diameter of 5 cm) for estimating the density of zooplankton predatory species (Neomysis integer, Leptodora kindtii, Cercopagis pengoi). Nocturnal sampling was performed once a month (May—November 2018) in the Vistula Lagoon (southern Baltic) in the range depth of 1.3—3.6 m. Statistical analysis indicated no significant differences between the N. integer and C. pengoi density estimated by the two sampling gears. In the case of L. kindtii, the mean density obtained by the column sampler was higher when analyzing all samples together and/or deep-water samples only (p < 0.02). However, no such differences were found at shallow stations i.e. up to ca. 2 m in depth. It was assumed that the more suitable sampling equipment for estimating zooplankton abundance in a shallow, well-mixed transitional (brackish) basin is the column sampler. This type of gear, so far used mainly for sampling of micro and mesozooplankton, allows the simultaneous nocturnal collection of the entire zooplankton size spectrum, including representatives of large predatory species. The suitability of light traps for qualitative studies of zooplankton species responding positively to light under the high turbidity of the Vistula Lagoon was also investigated. The traps proved to be most useful for N. integer (100% frequency), and much less for L. kindtii (46.2%) and C. pengoi (27.3%).