The effect of ice regimes on intertidal benthic communities in the St. Lawrence Lower Estuary

Supervisor : Mathieu Cusson (UQAC)

Co-supervisor : Christian Nozais (UQAR)

With ongoing climate warming, coastal ecosystems are increasingly exposed to winter disturbances. Storms, ice abrasion, and extreme cold events strongly structure mid-intertidal benthic communities in the St. Lawrence marine estuary. Under typical conditions, these benthic assemblages are thermally and mechanically protected by a stable ice foot, which provides resistance to harsh winter conditions. However, this protection is progressively weakening as the ice foot becomes altered, unstable, and mobile. This shift increases the exposure of benthic communities to drifting sea ice and intensifies the physical disturbances they experience. 

The aim of this study is to compare biological cover and to characterize the structure of mid-intertidal benthic communities across three key habitats (eelgrass meadows, macroalgal beds, and mussel beds) following a winter season, under contrasting ice regimes. 

Using fine-scale (1 m) and broad-scale (60 m) photographic monitoring of habitat-forming communities, the areal cover of structuring species was quantified in autumn 2024 and spring 2025. For each habitat, three zones were identified: stable zones (presence of communities before and after winter), abrasion zones (loss of communities after winter), and reference zones (absence of communities before and after winter), within which biological sampling was conducted. Results indicate that exposed sites (lacking protection from an ice foot) experienced greater post-winter denudation than protected sites. For instance, cover losses of up to 30% were observed in eelgrass meadows, and approximately 70% in mussel beds. This pronounced loss of habitat-forming species resulted in decreased richness, diversity, and evenness of associated species. Locally, sites abraded by ice during winter tended to converge toward reference zones (complete absence of structure) and diverge from control zones. Overall, these findings highlight the critical role of coastal ice in structuring benthic communities and allow anticipation of the impacts of increasingly unpredictable winters characterized by episodically colder conditions and reduced ice cover on the resilience and composition of mid-intertidal habitats in the subarctic St. Lawrence marine system.