Spatial and temporal dependence on sea ice algae in the Chukchi Sea, Arctic Ocean, inferred from bivalve stable isotopic composition

The Chukchi Sea is one of the most productive Arctic seas in the world. Around 10% of its net primary production originates from sea ice algae, much of which falls ungrazed to a relatively shallow (40-50m) shelf. The chlorophyll a derived from sinking ice algae is thought to supports a robust macrobenthic faunal community, dominated by bivalves, which in turn supports higher trophic organisms such as Pacific walrus (Odibenus rosmarus divergens), and bearded seal (Erignathus barbatus). However, recent reductions in ice extent and thickness could shift primary production from under-ice to open-water environment, thus reducing ice algal production and delivery to benthic biota. We used stable isotope analyses on benthic bivalve samples, collected in summer between 2002 and 2015, to identify contributions of ice algal production to benthic organisms and track their spatial and temporal variations. Ice algae contributions were indicated by δ13C values in bivalves, as ice algae are isotopically heavy compared to phytoplankton and would be reflected in consumers. This 14-yr period was marked by an 8%, decrease in Arctic ice extent, which was especially pronounced and spatially variable in the Chukchi Sea. We examined variability in the δ13C values in the common bivalves Astarte spp., Ennucula tenuis and Macoma spp. over space and time using one-way ANOVAs with Bonferroni correction to consider the potential for variation in ice algae contributions. All bivalve δ13C values were within a range (-21.84‰ to -17.62‰) that suggests some ice algal contribution. Among stations, E. tenuis and Astarte spp. did not significantly differ in their individual δ13C values. In contrast, Macoma spp. had significantly enriched δ13C values at one station south of Point Hope (δ13C = -17.75‰, F5,8 = 1.211, P < 0.05) in 2015. There were no significant (P > 0.05) differences in δ13C values from year to year for samples pooled across stations within a taxon. As the only taxon collected at the station south of Point Hope, Macoma spp. δ13C values indicate that this station may have assimilated a higher proportion of ice algae in comparison to other stations. Nevertheless, the overall lack of variation in δ13C values across space and time suggests that variation in ice conditions might not strongly influence the relative carbon contributions.