The significance of sea ice algae in the Pacific Arctic determined by highly branched isoprenoid biomarkers

Abstract

Our current understanding of ice algae as a carbon source at the base of the Arctic food web is limited because of difficulties unequivocally distinguishing sympagic (sea ice) from pelagic primary production once assimilated by consumers. For this study, I tested the utility of highly branched isoprenoids (HBI), which are unusual lipids produced by diatoms. This includes a biomarker found exclusively in Arctic sea ice termed the ice proxy with 25-carbon atoms (IP25) and two other HBIs with sea ice and pelagic sources. HBI measurements in the Pacific Arctic (the northern Bering and Chukchi seas) were sparse compared to the rest of the Arctic prior to this investigation. Analysis of surface sediments and cores collected across the continental shelf revealed a latitudinal gradient of increasing sympagic HBIs. Some of the highest concentrations of IP25 recorded in the Arctic were found in the Chukchi Sea. Fluxes of IP25 indicated year-round export of ice algal lipids in this region. Persistent diatom fluxes and rapid burial of sympagic carbon are likely a sustaining resource for infaunal communities throughout the year. As such, HBIs were measured in benthic primary consumers and indicated an elevated utilization of ice algae by surface and subsurface deposit feeders, while suspension feeders by contrast showed greater pelagic organic carbon utilization. Sympagic organic carbon signatures were largely influenced by the HBI content in local sediments. This led to the identification of two species with possible dependencies on ice algae. This method was extended to transient, higher trophic organisms by measurement of HBIs in Pacific walrus livers harvested during subsistence hunting activities. Relative HBI proportions were shown to relate to foraging location and revealed a higher reliance on sympagic organic carbon by female and juvenile Pacific walruses relative to males. This is likely due to a greater requirement for sea ice habitat by females and calves in the Bering and Chukchi seas. This study showed that HBI biomarkers can robustly track sea ice organic carbon contributions through the Pacific Arctic food web and should be considered alongside other trophic markers in future monitoring efforts in response to climate change.