Ecological responses of Antarctic Chaetoceros spp. to simulated melting and salinity shifts

Climate-driven freshening and shifting sea-ice dynamics are altering surface salinity regimes in coastal Antarctic waters, with profound ecological consequences. Phytoplankton, as the foundation of polar marine food webs and a key driver of biogeochemical cycles, are particularly sensitive to salinity variability. In this study, we exposed a mixed <i>Chaetoceros</i> culture from the Terra Nova Bay (the Ross Sea) to a range of salinity conditions representative of both recent microhabitats and projected future scenarios. By tracking short-term and acclimated responses across multiple functional traits-including photosynthetic efficiency, cell size and morphology, pigment composition, and nutrient uptake-we identified distinct acclimation strategies shaped by the severity and direction of salinity stress. These findings reveal how salinity fluctuations can restructure phytoplankton physiology in ways that influence trophic transfer efficiency, carbon export potential, and community resilience. For instance, shifts toward smaller, less-pigmented cells under moderate hyposalinity reduce food quality for grazers and alter energy flow through the food web, while extreme salinity events favor microbial recycling over carbon export. Moreover, salinity-driven changes in pigment:chlorophyll a ratios have implications for interpreting remote sensing data and chemotaxonomic reconstructions. By linking physiological plasticity to ecosystem-level processes, this study underscores the central role of salinity as an ecological filter in polar systems and highlights the need to incorporate salinity variability into models of phytoplankton dynamics and Southern Ocean biogeochemistry under climate change.