Long-term convergence of salmonid isotopic niche in Lake Ontario

Abstract

Investigating niches through time to reveal species’ responses to ecological stressors is critical for successful resource management. Stable isotope analysis of archived fish scales provides a unique opportunity to retrospectively study species’ isotopic niches. We analysed the carbon and nitrogen stable isotopes of lake trout (Salvelinus namaycush), rainbow trout (Oncorhynchus mykiss), and Chinook salmon (O. tshawytscha) from Lake Ontario in 1975, 1991, 2001, 2013, and 2019 to investigate how salmonids’ isotopic niche position and similarity have changed over time. These years capture major stressors, including phosphorus control, dreissenid mussel establishment, and round goby establishment. We found long-term changes in the trophic ecology of Lake Ontario salmonids. The δ¹³C values were less negative in 2001 than 1991, indicating higher littoral benthic carbon reliance after dreissenid mussel establishment. Additionally, the δ¹⁵N values were lower in 2019 than earlier sampling years (1975 and/or 1991). Isotopic niche overlap between some salmonids was high (≥ 60 %) in 2013 and 2019, concurrent with a decline in the salmonid community carbon and nitrogen range. Lake Ontario salmonid isotopic niches have converged through time, suggesting energy sources for salmonids have become more similar. These changes may increase interspecific interactions between salmonids and reduce food web adaptive capacity. Our analyses affirm pervasive negative impacts of ecological stressors (invasive species and nutrient changes) that impact lakes worldwide.