Trophic Structure and the Isotopic Niche Dynamics of the Tasiujaq (Eclipse Sound, Nunavut, Canada) Marine Food Web

ABSTRACT Rapid warming in the Arctic is causing cumulative impacts on ecosystems, including widespread reduction in sea ice extent, resulting in shifts in primary production, which can have cascading bottom‐up trophic influences on marine food web structure and function. Understanding trophic interactions and the structure of local food webs across the Canadian Arctic remains fragmented, yet it is essential to inform conservation efforts and provide a comparative reference to assess future food web shifts driven by climate change. We used stable isotope analysis (δ 13 C and δ 15 N) to determine the isotopic niche dynamics (e.g., size and overlap) and trophic positions among invertebrates, fishes and marine mammals sampled from 2015 to 2019 to quantify the trophic interactions and trophic structure of the Eclipse Sound food web. Mean trophic positions ranged from 2.0 for copepods to 4.4 for sea tadpoles, with some individuals, such as narwhal and ringed seals, occupying a trophic position of 5.3, indicating a marine food web that spans at least four trophic levels. Across taxonomic groups, isotopic niche size was largest in the invertebrate taxonomic group (7.87‰ 2 ), followed by fishes (3.26‰ 2 ) and then marine mammals (2.03‰ 2 ). The core isotopic niche overlap between taxonomic groups ranged from no overlap between invertebrates and fishes, or between marine mammals and invertebrates and 44% overlap between fishes and marine mammals. Overall, we present the marine food web structure of over 50 species of invertebrates, fish and mammals during the open‐water period in Eclipse Sound, Nunavut, and provide a reference point for future assessments of food web dynamics and conservation goals.