Identifying variable sea ice carbon contributions to the Arctic ecosystem: A case study using highly branched isoprenoid lipid biomarkers in Cumberland Sound ringed seals

We analyzed liver samples from 322 ringed seals (Pusa hispida) collected from Cumberland Sound (southeast Baffin Island) to test our ability to differentiate between carbon sources in near apex predators. Highly branched isoprenoids (HBIs) were present in all samples, and their distributions were consistent with recognized seal habitat use. HBI distributions in mature seals (>= 5 yr) confirmed a less variable carbon source during winter, consistent with geographically restricted sexually mediated territorialism. In contrast, HBI distributions were more variable for immature seals (< 5 yr old), consistent with increased movements and body growth-mediated habitat selection. The ubiquitous presence of sea ice-derived HBIs (e.g., the 'Ice Proxy with 25 carbons') in every seal collected throughout January-December indicates that springtime sea ice primary production remains important for ringed seals throughout the year. HBI distributions remain largely unaltered by trophic transfer, enabling them to document short-term (< 4 weeks) and seasonal changes in carbon. This important characteristic of HBIs facilitated interpretation of sea ice-derived carbon use by seals over annual and interannual timeframes and identified strong associations between sea ice carbon use and insolation as well as sea ice extent. Analysis of HBI distributions could be used to monitor and predict the response of Arctic organisms to reducing sea ice extent and the associated decline in future sea ice primary production over a range of temporal scales.