Feeding and distribution of zooplankton in the desalinated “lens” in the Kara Sea: Impact of the vertical salinity gradient

The feeding rates and distribution of dominant zooplankton in the area of the Kara Sea “lens” (upper desalinated layer) were studied in August 2014. Zooplankton abundance was low along the transect across the center of the lens but noticeably increased at the margin of the lens. Calanus glacialis, C. finmarchicus, Oithona similis, and Pseudocalanus spp. dominated among zooplankton, constituting 60–80% of the total biomass. Within the lens area, chlorophyll peaked at a depth of about 10 m, whereas zooplankton was distributed deeper than 30 m, and no diel vertical migrations were recorded. Outside the lens, zooplankton was distributed in the upper mixed layer. Specific daily rations on phytoplankton within the lens area were considerably higher than outside (5–50 and 0.2–6%, respectively). We hypothesized that accumulation of the suspended matter over a sharp density (salinity) gradient contributes to an increase in daily rations. A significant positive correlation between the values of daily ration and salinity gradients was obtained. The grazing impact of zooplankton on the primary production and the phytoplankton biomass was maximum (94 and 8%) at the lens margin, which was due to the combination of high values of zooplankton abundance and daily rations.