Exploring the submerged vertical walls of the Saguenay Fjord, Québec, Canada: biodiversity and distribution of benthic epifauna

IntroductionThe Saguenay Fjord, located in Quebec, Canada, is the southernmost fjord in the Northern Hemisphere. Despite its accessibility via the St. Lawrence system, economic significance, and unique ecosystems, few studies have explored its benthic biodiversity. The latest studies, dating back to 1968 & 1996, sampled only a few seafloor stations and have identified just over 300 benthic species. The vertical walls of the fjord, which also provide benthic habitats, had never been studied. Our objectives were to (1) characterize the epibenthic communities of these walls, (2) identify the environmental drivers influencing them, and (3) test whether spatial distribution patterns occur along upstream-downstream and depth gradients.MethodsWe sampled the walls using a remotely operated underwater vehicle (ROV), conducting video transects down to 100 m at 33 stations. At each station, a CTD multiprobe measured water column parameters (turbidity, depth, temperature, salinity, dissolved oxygen, fluorescence, and pH). The videos were analyzed to identify the species and quantify their abundance, richness, and coverage across stations and depths.ResultsThe dominant species were Tentorium semisuberites, Didemnum albidum, and organisms from the Ophiuroidae family.DiscussionResults show a bay effect characterized by higher abundance and biodiversity at the mouth of the fjord, possibly due to the saline inflow from the St. Lawrence Estuary. Biodiversity and abundance were also greater below the thermohalocline, which was observed at 20 m.ConclusionIn conclusion, the study of fjords requires a three-dimensional approach that integrates not only the fjord floor but also its walls, as well as longitudinal and vertical environmental gradients that influence species distribution.