Trophic structure of the abyssal benthic community in the Sea of Japan inferred from stable isotope and fatty acid analyses

The abyss of the Sea of Japan represents an example of an isolated deep-sea environment that contains mostly endemic fauna and has a complicated Quaternary history. To determine the trophic structure and sources supporting this abyssal benthic community, the carbon and nitrogen stable isotope ratios and fatty acid (FA) compositions of key invertebrate species and sedimentary organic matter (SOM) were analysed. Samples were collected at a range of depths from 2481 to 3666 m in the deep-water basin of the Sea of Japan in August 2010. Species of the most abundant invertebrates-including polychaetes, sea anemones, peracarid crustaceans, bivalves and brittle stars-showed similar delta N-15 values, corresponding to relatively high and similar trophic positions. Analysis of FA trophic markers showed that all of these equally N-15-enriched omnivores, carnivores and scavenger species in the Sea of Japan abyssal environment fed mostly on sinking zooplankton animals. The resulting FA profiles of these species showed a high 18:1 omega 9/18:1 omega 7 ratio, and the 22:6 omega 3 and 20:5 omega 3 polyunsaturated FAs were the most abundant FAs. Only one macrobenthic species, the filter-feeding Thyasira (Parathyasira) sp., had low 22:6 omega 3 and 20:5 omega 3 FA proportions, but it exhibited significant levels of 18:2 omega 6 and 16:1 omega 10 FAs characteristic of the SOM of the deep waters of the Sea of Japan. These data reveal the dominant role of descending zooplankton as a food resource for mega- and macrobenthos in this marginal deep-water environment. Despite the proximity of the productive shelf area, which exports plant residues to the deep-water basin of the Sea of Japan, we found no isotopic or FA indications of feeding on allochtonous detritus of seagrasses or macroalgae among abyssal consumers. Our data did not support cannibalism as a feeding mode of the abundant abyssal carnivorous polychaetes, as had previously been suggested. Different key invertebrate species of the Sea of Japan abyssal food web occupied similar trophic positions and fed predominantly on descended zooplankton. We suggest that the simple structure of the Sea of Japan abyssal food web, lacking abundant deposit-feeder food chains, is the result of the young evolutionary age of this community rather than the low availability of bottom detritus or the specific structure of the pelagic community that provides abundant downward flow of zooplankton.