Biodiversity and community dynamics of subtidal benthic rocky habitats in the St. Lawrence

Supervisor : Mathieu Cusson (UQAC)

Co-supervisor : Fanny Noisette (ISMER-UQAR), Kathleen Macgregor (IML-DFO)

Subtidal rocky substrates are important habitats in the coastal ecosystems. Their large macroalgae can support diverse macrobenthic communities and provide  structural complexity for colonization, which promotes high species diversity. However, these habitats remain significantly understudied compared to intertidal rocky reefs, largely due to challenges such as inaccessibility and the need for specialized equipment for sampling. Their ecological stability makes these communities valuable indicators of environmental change and disturbance. By integrating taxonomic and functional trait-based approaches, it is possible to gain a comprehensive understanding of species roles, community structure, and pattern of biodiversity according to specific features of rocky habitats. While local-scale studies have been conducted in this region, large-scale investigations are conspicuously lacking. Such research is critical to advancing our understanding of the processes that shape community structure, particularly in dynamic systems such as the St. Lawrence marine system, where sharp environmental gradients exert a significant influence on ecological patterns.

The objective of this large-scale study is to understand the key drivers shaping community structure and composition in subtidal macrobenthic rocky communities of the marine St. Lawrence. It will examine how environmental gradients, such as temperature, salinity, and substrate type, influence patterns of biodiversity and functional roles within these ecosystems.

Data collection will be collected each autumn through SCUBA diving along the coast from Tadoussac to Sept-Îles. A standardized sampling protocol will be used at up to 25 sites along 350 km of coastline, allowing systematic monitoring of subtidal communities. Sampling will be carried out with 30 m transects at depths of 2, 6, and 14 m to assess macrobenthic community structure and record invasive species. Underwater photography will assess substrate complexity and size structure of key species, particularly the green sea urchins, with biomass measurements. Environmental parameters (temperature, turbidity, pH, oxygen) will be recorded to analyze their impact on community structure and biodiversity patterns.

Through this research, we aim to identify the key drivers of community variability, thereby contributing to the expanding knowledge base required for effective ecosystem conservation and management. Understanding the complex dynamics of subtidal rocky communities is crucial for advancing ecological research and enhancing our understanding of the resilience and vulnerability of marine ecosystems in the context of rapid environmental change. By examining these understudied habitats, this research not only advances the science of marine biodiversity but also emphasizes the significance of protecting these vital ecosystems in the face of ongoing environmental change.

This research seeks to identify the drivers of community variability and improve our understanding of the resilience and vulnerability of subtidal rocky ecosystems in the face of environmental change. By focusing on these understudied habitats, it advances marine biodiversity science and underscores the importance of protecting these critical ecosystems for effective conservation and management.