Lipids as trophic biomarkers: use and inputs in a food web study of Antarctic benthic ecosystems.

The benthic compartment of the Southern Ocean is highly constrained by sea ice spatio-temporal dynamics. Along the West Antarctic Peninsula (WAP), sea ice dynamic is changing in response to climate change. This results in a longer ice-free period, an increase of light availability, and changes in primary producers supporting benthic organisms. Overall, whether these changes could affect the feeding habits and trophic interactions of these Antarctic benthic organisms remains yet to be fully understood. In this study, we tackled the following question: can environmental changes altering food source availability affect the diet of key consumer species? Through the TANGO project sampling campaigns in 2023, we have collected and analyzed the fatty acid compositions of primary producers and benthic consumers in 5 different stations from southern WAP (67°33'06''S, 67°12'53''W). Here, we will focus on one grazer and one suspensivorous consumer. Macroalgae species had higher content of 20:4n-6, while other sources (sediment organic matter and microphytobenthos) were characterized by more 16:1n-7, a fatty acid characteristic of diatoms. Significant spatial differences were found for the circum-polar benthic limpet Nacella concinna. We observed that a bacterial biomarker (18:1n-7) was significantly more abundant in sedimentary habitats. These stations were also those likely to experience greater ice-related disturbances (e.g. glacier melting, iceberg scouring, sea ice cover change). On the other hand, in other rocky-dominated stations, higher content of 20:4n-6 was observed which is consistent with a diet based on macroalgae. We might assume that N. concinna tends to turn to other sources when one is missing, such as microbial film when macroalgae abundances are low. Looking at a suspensivorous tunicate, Cnemidocarpa verrucose, fatty acid composition showed more consumption of primary production derived from microalgae. The main fatty acid discriminating this ascidian from the limpet was 22:6n-3, a dinoflagellate biomarker. In addition, 16:1n-7 (diatoms biomarker) concentrations were higher in C. verrucosa. Among the stations, this filtering species also showed differences in fatty acids composition. In a station near a glacier front, less content of diatom (20:5n-3) and dinoflagellate (22:6n-3) biomarker were found. In contrast, more abundant cryptophyta biomarker (18:4n-3) were found for some individuals. This might reflect changes in phytoplankton communities. Given these findings, environmental conditions and ice disturbance appear to affect benthic consumers' diets. Such effects differ depending of the species and food habits and therefore highlight the multifaceted impact of ecosystem alterations.