Species composition, density, biomass and distribution of zooplankton on shelf and slope of northwestern part of the Laptev Sea and shallow-water shelf of the East Siberian Sea were studied in 2015 summer period. Using analysis of stable isotopes of carbon (13 С) and nitrogen (15N), trophic status of abundant zooplankton and fish species, and their positions in food webs of these two seas, were compared. Gut content analysis, as well stable isotope data, showed mutual changes in the trophic status of dominant fish species arctic cod, Boreogadus saida, with age.