Snapshots of ice-free season dynamics in the near-shore water column of the northern Gulf of St. Lawrence, Canada

Abstract. Coastal ecosystems are highly dynamic and vulnerable to both climate changes and anthropogenic pressures. The Sept-Îles region, located in the northwestern Gulf of St. Lawrence, is a high-use subarctic coastal system with diverse urban, industrial and maritime activities. This study presents analyses of monthly water column profiles at 35 sites focusing on temperature, salinity and chlorophyll fluorescence, used as a proxy of phytoplankton biomass, during the ice-free season. Using a conductivity, temperature and depth (CTD) sensor, water column profiles were collected from May to October 2022 along the coastline, at sites between 2- and 52- meters depth. Results revealed a thermocline developing in spring, intensifying in summer and disappearing in autumn. Chlorophyll a (Chl a) concentrations peaked below the thermocline in July, while secondary increases were recorded at the surface in September, consistent with observations of an autumn bloom in similar environments. These findings highlight the complex dynamic of physical and biological parameters in the coastal water column, and the importance of the timing of sampling to fully capture seasonal variability. To improve future research in the area, measuring nutrient concentrations would be essential for detecting potential upwelling events and better explaining phytoplankton variation during summer. This study provides a valuable baseline for future investigations and justifies the continuation of measurements of water column variability in the region, in the context of rapid climate change. The complete dataset is available via https://doi.org/10.5683/SP3/ALRWON (Arseneault & Saulnier-Talbot, 2025a).