Winter disturbances drive community change in subarctic eelgrass meadows

The dynamics of winter sea ice and icefoot play a fundamental role in structuring intertidal ecosystems in subarctic environments. However, ongoing climate warming is altering ice regimes across northern shorelines. In the St. Lawrence marine estuary (Canada), reduced formation of icefoot increasingly exposes benthic habitats to winter-related disturbances. We investigated how such disturbance influences eelgrass ( Zostera marina L.) meadows and associated benthic communities using a before-and-after comparison spanning the 2024-2025 winter season. We used a sampling design that compared areas with no change (Stable substratum conditions), strong cover reduction (Abraded), and bare sediment (Reference), as identified from aerial survey imagery. Substantial losses in eelgrass cover (∼20%) were observed, with Abraded areas showing reduced taxa richness but comparable total wet biomass of sampled assemblages. Our results revealed that persistent small mussel aggregates, attached to remnant eelgrass rhizomes in Abraded areas, played a key role in maintaining associated biomass and community structure. Mussels acted as secondary foundation species, formed biogenic habitat and partially compensated for eelgrass loss by maintaining refuges and colonization surfaces for associated fauna. These findings suggest that resistance to winter disturbance depends not only on the severity of habitat loss but also on the persistence of interacting foundation species and their capacity for functional replacement. Reduced coastal protection by icefoot increases intertidal exposure to physical and physiological stressors, progressively altering subarctic benthic assemblages and leading to simplified community structure. Such changes warrant attention regarding their consequences for ecosystem functioning and the services they provide.