Stable isotope insights into the habitat use of two benthic invertebrates along Antarctic glacier-influenced fjords

Retreating glaciers are rapidly transforming fjord ecosystems along the Western Antarctic Peninsula (WAP), yet the trophic responses of benthic fauna remain poorly understood. We analysed δ13C, δ15N, δ34S, and C:N ratios in two soft-bottom species collected along distance gradients from retreating glaciers in two WAP fjords. The study included the deposit-feeding bivalve Nuculana inaequisculpta from Marian Cove (King George Island) and the opportunistic-feeding sea anemone Edwardsia sp. from Sheldon Cove (Adelaide Island). Near the land-terminating glacier at Marian Cove, bivalves were depleted in 13C, consistent with reduced phytoplankton growth under surface meltwater influence. Near the marine-terminating glacier at Sheldon Cove, anemones were 13C-enriched, in line with enhanced phytoplankton blooms driven by deep meltwater inflow. δ15N, δ34S, and C:N ratios showed no fjord-scale trends and appeared governed by local factors. Low δ34S values reflected suboxic sediment inputs in both fjords, possibly linked to denitrification and elevated δ15N baselines. When associated with blooms, anemones appeared to rely less on sedimentary sources while bivalves showed signs of sediment hypoxia. In both species, reliance on suboxic sediments was potentially related to poor nutritional condition. Environmental factors linked to submarine sills may have improved bivalve nutritional condition and increased anemone trophic level. Overall, benthic δ34S emerged as an excellent indicator in Antarctic marine habitats, and δ13C could be a tracer of productivity associated with glacier configuration in Antarctic fjords. Local meltwater conditions may limit organic matter flux to the benthos, nuancing the role of newly ice-free areas as carbon sinks within a negative climate feedback.