Demersal–Pelagic Relationships of Marine Organisms in the Kara Sea as Inferred from δ¹⁵N and δ¹³C Ratios and the Diet of Fish

To compare trophic characteristics in pelagic and demersal communities in the Kara Sea, carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) in fish tissues, as well as in zooplankton and zoobenthos, were analyzed. Relatively low levels of δ¹³C were observed in marine organisms in the study area. Based on the persistent differences in carbon isotope ratios between zooplankton and zoobenthos, the boundary between the pelagic and demersal communities corresponds to an average δ¹³C of –21.5‰. The δ¹⁵N levels of the analyzed marine organisms in pelagic and demersal communities ranged from 6.4 (in the bivalve Serripes groenlandicus) to 16.3‰ (in the longear eelpout Lycodes seminudus) and occupied three trophic levels, reflecting the component of predation. Trophic relationships in the Kara Sea demonstrate a high degree of interdependence of benthic and pelagic communities, since pelagic and demersal fish are consumers of both benthic and pelagic food to one degree or another. The contents of the ¹³C carbon isotope in tissues and the studies of fish diet have shown that about 40% of the examined species of demersal nekton are mainly linked to the pelagic food web, bypassing the traditional deep-sea web. In pelagic fish, pelagic organisms form the basis of the diet, although in Arctic cod, the dominant nekton species, the proportion of zoobenthos increases with increasing body size, while in long rough dab, it decreases.