Benthic biodiversity in Collins Bay (King George Island), Antarctica: evidence for small-scale community structuring

The Antarctic Peninsula's coastal benthic ecosystems are shaped by highly dynamic environmental drivers, hence high-resolution biodiversity studies across depth gradients are key for improve our understanding current impacts of drivers on benthic communities and future projections. Benthic megafauna and seaweeds were surveyed in Collins Bay, King George Island, using ROV video transects (10-80 m depth) and analysed for diversity and community composition in relation to environmental factors. Soft substrates dominated the area (82.44% of frames), and 157 taxa from 15 phyla were recorded, with Asteroidea occurring in 62.14% of sampled depths and Odontaster validus being the most frequently observed species. Benthic community composition differed significantly with depth, distance from the glacier, and substrate (PERMANOVA; depth: F = 16.10, R² = 11.91%, p < 0.001; distance: F = 20.38, R² = 5.03%, p < 0.05; substrate: F = 7.20, R² = 2.66%, p < 0.001), with depth explaining the largest proportion of variation. Species richness showed distance-dependent depth responses: no significant depth-related changes occurred close to the glacier, richness peaked at intermediate depths (50-60 m) at intermediate distances (negative binomial GLMM, p < 0.001), and increased strongly with depth at sites furthest from the glacier (p < 0.001). Functional composition also varied along these gradients. Sessile filter feeders dominated deeper glacial-proximal habitats, while mobile taxa increased in relative abundance with distance from the glacier, although larval dispersal strategies remained invariant across depths and distances. Macroalgal assemblages were primarily structured by depth, with species richness declining significantly below 40-50 m across all distance categories. Overall, our findings demonstrate that glacial retreat-related processes modulate how bathymetry structures benthic biodiversity, generating strong small-scale heterogeneity within a single fjord. This study provides a fine-scale baseline for detecting future changes in Antarctic benthic ecosystems under ongoing climate-driven glacial change.