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

Supervisor : Mathieu Cusson (UQAC)

Co-supervisor : Christian Nozais (UQAR)

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

The objective of this study is to compare biological cover and to determine the structure of mid-intertidal benthic communities within three key habitats (eelgrass, macroalgae, and mussel beds) following a winter season under different ice regimes.

Using photographic monitoring at both small (1 m) and large (60 m) spatial scales above habitat-forming communities, the surface cover of foundation species was quantified for fall 2024 and spring 2025. For each habitat, three types of zones were defined: 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 (not protected by an ice foot) exhibit more pronounced denudation than protected sites after the winter season. For example, cover losses of up to 30% were observed in eelgrass meadows and approximately 70% in mussel beds. This increased loss of foundation species leads to declines in species richness, diversity, and evenness of associated assemblages. Locally, sites abraded by ice during winter tend to converge toward reference conditions (complete absence of structure) and diverge from control zones.

These findings highlight the critical role of coastal ice in structuring benthic communities and provide insight into the potential impacts of increasingly unpredictable winters, sometimes colder yet less ice-covered, on the resilience and composition of mid-intertidal habitats in the subarctic St. Lawrence marine system.