Structural-functional characteristics оf zooplankton in the ob estuary and adjacent Kara sea shelf in summer

The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well as on feeding of zooplankton were obtained during the period of intensive river runoff. The biomass of zooplankton in terms of wet weight (without jelly animals and chaetognaths) varied over a wide range from 40 to 1880 mg/m3. The biomass was dominared by the Limnocalanus macrurus population, represented by all age stages, and Pseudocalanus spp . (an average of 60% and 17% of biomass, respectively. Maxima of zooplankton biomass were associated with the boundary of the distribution of river waters and with the southern periphery of the estuary frontal zone (EFZ). The short-term temporal variation of the abundance of zooplankton was observed when the transect was repeated after 2 days: the biomass at the southern periphery of the EFZ decreased from 1900 to 250 mg/m3). The observed changes is supposed to be caused by the shift of the southern boundary of the EFZ under the change in the wind regime. At high phytoplankton biomass in the estuary (>7 μg Chl-a/l), the daily ingestion of autotrophic phytoplankton of different copepodite stages of Limnocalanus macrurus was 5.77, 4.46, 2.59 and 1.4 μg C/ind/day in CVI, CV, CIV and CIII, respectively. Energy intake by younger copepodite stages not only covered the metabolic requirements, but also enabled the copepods to growth. At a relatively low Chl-a concentration (<2 μg/l) on the shelf, the mean values for older stages were lower (1.56 μg C/ind/day and 1.17 μg C/ind/day in CVI and CV, respectively) and hardly met the metabolic demands. The zooplankton grazing impact estimated with the gut fluorescence method varied from 0.2 to 7.3% of phytoplankton standing stock and from 2.2 to 30% of primary production. The results are discussed in relation to the specific hydrophysical conditions in the summer period and in comparison with the data obtained earlier for the autumn season.