What do grazers really graze on?

Sea ice dynamics strongly influences primary production and its availability for local communities in polar ecosystems. In spring, sea ice melting promotes phytoplanktonic and benthic (i.e. macroalgae and microphytobenthos) primary production, and causes a massive export of sea ice algae. All of these represent potential food sources for benthic organisms. Along the West Antarctic Peninsula, two gastropods (Nacella concinna and Margarella antarctica) and a sea urchin (Sterechinus neumayeri) represent some of the most abundant benthic grazing species. Understanding their trophic plasticity when faced with such a diversity of potential food sources is necessary to understand the flow of energy within these communities. To address this objective, analysis of fatty acid composition was carried out on primary food sources and consumers. A clear distinction was observed in the fatty acid composition of the three benthic species. The limpet N. concinna showed higher proportions of fatty acids described as macroalgal biomarkers (i.e.,18:1n-9, 20:4n-6, 20:5n-3) than the other two species. M. antarctica had higher proportions of fatty acids described as a biomarker of microorganisms (i.e., 16:1n-7, 16:4n-1, ante or iso, C16 or C18 PUFA). S. neumayeri, meanwhile, contained a higher proportion of fatty acids described as copepod biomarkers (i.e., 20:1n-9, 20:1n-11, 22:1n-9). Given these results, it would appear that these three grazers feed on different food sources. N. concinna may feed more on macroalgae, M. antarctica on microorganisms and S. neumayeri may have a more mixed diet ranging from sources to primary consumers, such as filter feeders, which might explain the proportion of zooplankton biomarkers. These differences could be driven by both morphological differences in feeding structures and different prey selection, interactively modulating the role of grazers in Southern Ocean food webs.