Biomarkers of change: tracking diets along an environmental gradient in an Antarctic benthic ecosystem.

Along the West Antarctic Peninsula (WAP), steep environmental gradients can be observed in response to climate change, resulting in longer ice-free period, warmer temperatures, and changes in primary production. Through the TANGO project, 5 sites, with macroalgal forests and rocky reefs, were studied from the southern to the northern WAP. This large spatial gradient encompassed sites with persistent sea-ice in summer, along with slightly more temperate areas, with ice-free waters all year long. The sampling of basal food sources and consumers (i.e., 8 phyla, 21 taxa and 5 trophic guilds), aimed to understand whether trophic interactions in benthic consumers are influenced by environmental differences and food sources availability. Fatty Acids (FA) content and Highly Branched Isoprenoids (HBI) were used as trophic biomarkers. Macroalgae had high 18:1n-9, 20:4n-6 or 20:5n-3 content, while other food sources (e.g. sediment organic matter) had higher C16 or C18 content, known as diatoms or bacterial biomarkers. For benthic consumers, a clear distinction was observed between filter feeders and grazers. Grazing snails Margarella antarctica contained higher proportions of diatom biomarkers, and therefore a FA content more similar to filter feeders (e.g. Laternula elliptica), than the limpet Nacella concinna, another grazing gastropod. Also, HBI ratios suggested that sea-ice algae consumption varied depending on the site and species. Filter feeders such as the lantern clam L. elliptica showed no HBI signals related to sea-ice algae. Conversely, grazers such as N. concinna and the urchin Sterechinus neumayeri, and deposit feeders such as Terebellidae worms, showed locally important HBI incorporation into their tissues, related to longer-lasting sea-ice cover. Biomarker analysis highlighted species-specific dietary dependencies on different food sources. These contrasts in trophic ecology are likely to modulate consumer response to current and future changes.