The research was done at transect (11 stations) from inner part of the Khatanga Bay in the south to continental slope area in the north from 17 to 20 September 2017. Four biotops with different parameters of pelagic environment, composition, quantitative characteristics and vertical distribution of phytoplankton were allocated: inner part of the Khatanga Bay, estuarine frontal zone, western shelf of the Laptev Sea and continental slope area. Inner part of the Khatanga Bay and continental slope area were characterized by the highest values of phytoplankton numbers and biomass, which reached 1106 cell/l и 160 mg/m3, respectively. Formation of maximum at the depth of 45 meters was typical for phytoplankton vertical distribution in continental slope area. Algae biomass in the maximum reached 400 mg/m3 which was the highest value for the transect. Well pronounced latitudinal zoning in phytoplankton communities structure was revealed in the western part of the Laptev Sea which was similar to that in another areas the Arctic seas under strong impact of Siberian rivers discharge.

Role of plankton in the vertical flux in the East Siberian Sea was studied in the 69 cruise of the RV Akademik Mstislav Keldysh in September 2017. Vertical fluxes were measured in sediment traps samples collected in the area of Indigirka river plume and in the marine shelf area. Mass vertical flux and particulate organic carbon flux varied from 80 to 530 mg/м2/d and from 16 to 49 mgС/м2/d, accordingly. Phytoplankton in sediment traps was dominated by cysts and spores of diatoms and dinoflagellates. Phytoplankton flux increased with depths from 0.220.33 to 1.21.3 мgС/м2/d. Fecal pellet fluxes (712 mgС/м2/d) was almost similar at two studied stations and did not change with depth. Zooplankton in the traps was dominated by houses of larvacean and carcasses of copepods Jashnovia tolli and Calanus glacialis Flux of zooplankton varied from 3 to 17 mgС/m2/d. The influence of the continental runoff reflected in a decrease of the proportion of planktonogenic components in the vertical flux of organic carbon. In the river plume area their total contribution to organic carbon flux did not exceed 30%; on the marine shelf it reached 80%.