Eve Rioux, Jory Cabrol, Véronique Lesage

The Estuary and Gulf of St. Lawrence experienced major changes in its trophodynamics over the past decades as a result of overfishing, acoustic and chemical pollution, climate change, and the increase in abundance of other top predators. These stressors and changes in prey-predator relationships are suspected to had deleterious effects on the endangered St. Lawrence beluga (Delphinapterus leucas) population. Thus, the aim of our study was to examine the evolution of the trophic structure of the St. Lawrence ecosystem between two periods of contrasting anthropogenic and climate stressors (1995—2003 vs. 2019—2021), and thereby provide a context for observed changes in beluga isotopic signatures and estimated diet. To do so, we analyzed stable isotope ratios of carbon and nitrogen of 1,286 samples of marine mammals, fish and invertebrate species to explore changes in trophic niche over time. Our results highlighted significant changes between the two periods in the isotopic signatures and Bayesian standard ellipse areas of several marine mammal species and most of the fish and invertebrate species, confirming substantial modifications in the community structure of the St. Lawrence ecosystem between the two periods. We also observed little to no niche overlap (0 to 29.4%) between the two periods for all species, along with a change in trophic niche partitioning, especially for rorqual species but also, for the majority of the pelagic and demersal fishes, suggesting new prey-predator relationships. Our study highlighted some degree of dietary plasticity in several of the species examined, and suggest a long-term change in the ecosystem structure of the Estuary and Gulf of St. Lawrence, with likely effects on diet composition, isotopic signature and energetics of St. Lawrence Estuary beluga.