Role of the small-size fraction of mesozooplankton in the pelagic community of the northeastern part of the Black Sea in the autumn

The role of the small-size (SF; 0.1-0.5 mm) and large-size (LF; 0.5-20.0 mm) fractions in the biomass and abundance of mesozooplankton (0.1-20.0 mm) was assessed using the database of samples obtained during the cruises of RV Akvanavt in the northeastern Black Sea in November 2000 and October 2006. The mesozooplankton was collected by means of Juday nets (37/50, filtering gauze 160 mu m) and Niskin bottles in two areas: (1) the shelf and continental slope (30-1480 m depth) and (2) the deep sea (depths of more than 1500 m). The plankton net was considerably less effective in collecting the SF of the mesozooplankton (by a factor of 30-36) than the Niskin bottles. When comparing the SF and LF, we estimated the abundance and biomass of the SF in the samples obtained with the Niskin bottles. The abundance of the SF in the deep-sea area was 2.5 times lower compared to the shelf and continental slope, and the LF abundance was 5.0 times lower in the same way. The abundance of the SF constituted 88% of the total mesozooplankton on the shelf and continental slope, and 78% in the deep-sea area. The biomass of the SF was higher as well on the shelf and continental slope. Meroplankton played a significant role in the SF zooplankton abundance (0.5 x 10(3) + 0.16 ind. m(-3)) in this area. The SF grazing impact was 10% of the total mesozooplankton grazing on the shelf and continental slope, and 17% in the deepsea area. Appendicularia and nauplii of copepods had the greatest contribution to the mesozooplankton grazing among the SF group.