Decoupling Between Benthic Trophic and Functional Structure in Sub-Antarctic Patagonian Fjords Under Contrasting Environmental Stressors

Benthic communities are a fundamental component of coastal marine ecosystems, regulating energy flow, mediating trophic transfer, and supporting a wide range of ecological functions closely linked to habitat characteristics. In Marine Protected Areas (MPAs), integrative approaches that simultaneously address trophic and functional dimensions are essential to assess ecosystem functioning and resilience to environmental disturbances. This study aimed to characterize and compare the trophic structure and functional diversity of benthic communities in sub-Antarctic fjords and channels of the Magellan region (southern Chile), all of which are located within MPAs and represent contrasting environmental settings characterized by either proglacial influence or a documented history of recurrent harmful algal bloom (HAB) occurrence and paralytic shellfish toxin (PST) exposure. We hypothesized that benthic communities under these contrasting environmental stressors would differ in carbon assimilation pathways, community isotopic niche breadth, and functional organization. Trophic structure was evaluated using stable isotope analysis (δ13C and δ15N), Bayesian mixing models to estimate basal carbon source contributions, and community-level isotopic metrics. Functional diversity was quantified using biological and ecological traits to calculate community- and species-level functional indices. Our results revealed marked differences in trophic organization among localities, whereas global functional diversity indices were consistently low and similar across environmental settings. Communities from localities characterized by recurrent HAB/PST exposure showed limited trophic reliance on pelagic-derived organic matter, maintaining benthic-based energy pathways, whereas proglacial localities exhibited higher allochthonous subsidies and broader isotopic niches. Functional uniqueness and specialization metrics indicated strong functional redundancy and a high dependence on a small set of functionally distinctive species. These findings establish a baseline for remote MPAs and demonstrate the value of integrating trophic and functional metrics in long-term monitoring and adaptive management frameworks.