Benthic food-web structure under differing water mass properties in the southern Chukchi Sea

We used carbon and nitrogen stable isotope ratios as markers of food source connections and trophic position to evaluate whether benthic food-web structure varied among water masses with different productivity regimes in the southern Chukchi Sea. Benthic communities and suspended particulate organic matter (POM) were sampled at nine stations located in four water masses during the 2004 Russian-American Long-term Census of the Arctic (RUSALCA) cruise. POM δ13C values were depleted in the relatively unproductive Alaska Coastal Water (ACW, −24.2‰) and at the Russian Coast (RC, −24.5) compared to the enriched signatures of highly productive Anadyr Water (AW, −21.1‰) and the intermediate value (−23.6‰) of Bering Shelf Water (BSW). Corresponding differences in POM C/N reflected higher nutritive content of AW (6.19) compared to ACW (8.45). Carbon isotopic values of sediments were also most depleted in the nearshore waters of the ACW (−24.8‰) and RC (−23.4‰), versus BSW (−22.8‰) and AW (−22.1‰). In addition, the low δ15N values and high C/N ratios associated with sediments under the ACW (2.9‰ and 10.0, respectively) compared to the other three water masses (range 4.5–4.9‰ and 6.8–7.5, respectively) are likely explained by a terrestrial signal associated with the higher freshwater input into the ACW. Consequently, the δ13C value of POM in the ACW is likely driven by a large fraction of refractory material of terrestrial origin, and POM may not always be a reliable baseline for trophic level calculations in the ACW. Excluding POM, δ15N isotope spread among the same 42 taxa of invertebrates and fishes was 8.5‰ in ACW and 7.5‰ in AW, compared to 12.0‰ and 9.6‰ with the inclusion of δ15N POM values. Almost without exception, consumers in the ACW had higher δ15N values than their AW counterparts (average difference 2.5‰). However, food webs in ACW and AW (as well as in the BSW and RC) did not differ substantially in length (four trophic levels) when based on primary consumers as the baseline. The relatively high proportion of consumers within the first trophic level in AW suggests that there is a more direct coupling of benthic consumers to the very high pelagic primary production in these waters, which is also reflected in the high benthic infaunal biomass at low trophic levels (TL2) reported in the literature for this area. We conclude that differences in regional water column productivity in the southern Chukchi Sea may be manifested primarily in the quantitative representation of various trophic levels and less in qualitative characteristics such as food-web length or relative distribution of trophic levels.