Habitat coupling dynamics of mobile consumers along a freshwater and marine resource gradient in a sub-Arctic estuarine system

Food webs consist of numerous connections between consumers and resources that can couple adjacent ecosystems together by the movement of nutrients, prey, and consumers leading to habitat coupling. Habitat coupling and isotopic niche dynamics among mobile consumers at the individual and population-level has been examined extensively in freshwater systems but has received little attention in sub-Arctic estuaries, which act as transition zones between freshwater and marine habitats and resources. The objective of this study was to quantify the diet composition between freshwater-and marine-derived resources and the isotopic niche size of mobile consumers (13 fishes and 2 seal species) within the lower Churchill River, Manitoba, Canada using stable isotopes (δ13C, δ15N and δ34S) in a Bayesian framework. Cisco, lake whitefish, and northern pike represented the habitat couplers in this system and also exhibited the greatest amount of individual variability since most (75%, 56%, 65% of individuals for cisco, lake whitefish, and pike respectively) consumed a mix of both freshwater-and marine-derived resources. The largest isotopic niche sizes were found for lake whitefish and northern pike, whereas the smallest isotopic niche sizes were found for both harbour and ringed seals. The isotopic niche of lake whitefish overlapped with the most species (5 in total), which supports their broader use of both freshwater- and marine-derived resources. Habitat couplers in this system exhibited more variability in their foraging strategy than the other consumers, which aligns with their known life histories and migratory or resident movement strategies. Future changes in the relative availability of freshwater and marine-derived resources due to climate and anthropogenic stressors could pose consequences to these habitat coupling species and overall trophic dynamics of sub-Arctic estuarine systems. •Habitat coupling occurs when consumers integrate resources from different habitats.•Cisco, lake whitefish, and northern pike were the habitat coupling species.•Lake whitefish had the highest niche overlap with many species.•Habitat coupling behaviour aligned with species life histories and movement strategies.•Habitat coupling species are important for the ecosystem structure and function.