Environmental gradients shape community composition, energy pathways, and trophic dynamics in a coastal Arctic food web

Natalie Vachon, Lucassie Arragutainaq, Joel Heath, Paloma C Carvalho, Steven H Ferguson, Kimberly L Howland, Kelsey F Johnson, Tatiana Meta, CJ Mundy, Sarah Rauf, Abby De Los Rios, Bruno Rosenberg, Tyson Scott, Kallie S Strong, Cortney A Watt, Colin Garroway, David J Yurkowski

A central question in ecology is how environmental heterogeneity structures community composition and trophic organization, and whether changes in physical conditions alter energy pathways without changing overall network connectivity. Arctic food webs have generally been quantified at broader spatial scales which must average spatial heterogeneity, limiting the ability to quantify asymmetric ecosystem responses to climate- and anthropogenic-driven change at local scales. Resolving entire food webs at finer spatial scales is therefore essential to identify mechanisms linking environmental gradients to community, composition, energy flow and trophic structure. Here, we use Qikiqtait, Nunavut, Canada to characterize a coastal Arctic marine food web by integrating biotracers, DNA metabarcoding, community metrics, and network analysis across >110 species and >1200 samples, from invertebrates to marine mammals. We identified consistent north–south gradients in temperature, salinity, and primary production, with warmer, fresher, phytoplankton-dominated waters in the south and colder, more saline conditions in the north. These gradients were reflected consistently across community composition and trophic dynamics. Northern food webs exhibited higher fish species richness and stronger reliance on ice-derived carbon. Southern food webs were dominated by brittle stars and Arctic cod (Boreogadus saida), with greater pelagic contributions to energy pathways. Spatial differences in trophic position were most pronounced among sessile invertebrates, echinoderms, and decapods, reflecting shifts in trophic roles. Although overall food web connectance was similar across regions, species mediating energy flow north of the islands were benthic, whereas both benthic and pelagic species were central in the south. These results demonstrate that fine-scale environmental heterogeneity can reorganize energy pathways and trophic roles without altering overall network structure. In the rapidly warming Arctic, where sea ice loss and altered hydrology are increasing spatial heterogeneity, such localized responses are likely to generate asymmetric ecosystem change. By resolving food web structure in detail at one location, this study provides mechanistic insight into how climate- and anthropogenic-driven change may propagate through Arctic marine ecosystems and informs broader predictions of ecosystem reorganization.