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Sea ice is a dominant feature of marine ecosystems in the Arctic. Its presence directly or indirectly impacts Arctic marine ecosystems, especially on the shelves where benthic and pelagic systems are extensively coupled. If the extent and thickness of sea ice continue to decline, we predict a shift in the type of algal material reaching the benthos (from ice algae to phytoplankton), which will potentially impact the food requirements of the benthos. We have several pieces of evidence showing that both types of ice algae (below-ice ice algae dominated by Melosira arctica and within-ice ice algae dominated by Nitzchia frigida) presently reach the benthos in significant quantities. What we don’t know, and what we propose to address is: “What is the digestibility of ice algae and phytoplankton-derived organic matter by the Arctic macrobenthos?” From the perspective of a macrofaunal organism, digestibility includes three separate components: 1) selection (is encountered organic material ingested or rejected?); 2) absorption (is ingested organic material absorbed during passage through the gut, or does it get egested in the feces?); and 3) assimilation (is absorbed organic material assimilated into biomass?). We propose a series of hypotheses to guide our assessment of digestibility: H1: There is no difference in the quality of ice algae and phytoplankton as food for benthic organisms. H1i: There is no difference in the long-term assimilation of ice algae and phytoplankton by benthic organisms of different trophic groups (suspension feeders, deposit feeders, omnivores). H1ii: There is no difference in the short-term absorption efficiency among different trophic groups feeding on phytoplankton and ice algae. H2: The response of benthic organisms to ice algae and phytoplankton as food sources is the same when assessed on a Pan-Arctic scale. Assessment of long-term assimilation of the various types of algae (within-ice ice algae; below-ice ice algae; and phytoplankton) will be conducted by determining lipid biomarkers and their isotopic ratios, and by determining CHN and protein signatures of organisms collected during all aspects of the work (summer ’02; spring ’03; fall ’03; and summer ’04 in both Norway and Kotzebue, Alaska). Assessment of short-term absorption will first use the ash-ratio method in a whole core delivery experiment. Following the whole-core experiments, dominant taxa from each trophic group will be identified and used in a comparison of 1) absorption efficiencies as calculated by the ash-ratio method, and 2) carbon retention efficiencies as calculated using a pulse-chase radiotracer approach. Finally, we will repeat the dominant taxa absorption efficiency experiments in both Svalbard, Norway at the Ny Ålesund lab and in Kotzebue Sound, Alaska.