Influences of depth and pelagic subsidies on the size-based trophic structure of Beaufort Sea fish communities

摘要

The quality and quantity of food reaching the seafloor is negatively related to water column depth. Lower food availability at depth may in turn affect trophic structure and result in depth-related patterns in biomass and body size (size spectra) in demersal fish communities. Here, we used demersal fishes collected from 4 habitat depths in the Beaufort Sea, spanning 18 to 1000 m, to investigate depth-related changes in the slopes of biomass size-spectra relationships and whether they are related to benthic resource use. Significant positive relationships between biomass-weighted trophic level (estimated from delta N-15) and log(2) body mass classes demonstrated strongly size-structured fish food webs at all habitat depths, with larger individuals occupying higher trophic levels. Total fish community biomass peaked between 350 and 500 m. The decline in biomass production beyond 500 m was greater for smaller size classes, resulting in significantly shallower (less negative) size-spectra slopes with increasing habitat depth. Large size classes in deep habitats were dominated by species known to obtain pelagic subsidies. Lower isotopic distinction between benthic and benthopelagic fish species, based on C-13 measurements, suggested lower dietary differentiation between feeding guilds in deeper habitats. We suggest pelagic subsidies allow high relative biomass production in larger size classes of fishes in deeper habitats, whereas decreasing availability of benthic resources limit biomass production in smaller size classes. Size spectra of demersal fish communities may provide an important indicator of structure in both pelagic and benthic Arctic environments, and be a useful parameter to include in longterm monitoring programs.