Patterns of trophic niche overlap of diadromous and marine Arctic fishes in Beaufort Sea coastal lagoons

Ongoing climatic changes in the coastal Arctic can influence the food webs that support a variety of fish species and subsistence fisheries in coastal lagoons. Along the Beaufort Sea coast, lagoons provide productive summer feeding habitats for both diadromous fishes migrating from freshwater and marine fishes migrating from the shelf. We compared trophic niche size and niche overlap between diadromous species, Arctic Cisco ( Coregonus autumnalis ), Least Cisco ( Coregonus sardinella ), and Dolly Varden ( Salvelinus malma ), and marine species, Polar Cod ( Boreogadus saida ), Fourhorn Sculpin ( Myoxocephalus quadricornis ), and Saffron Cod ( Eleginus gracilis ), across lagoon systems differing in freshwater input and oceanic exchange. Using complementary biomarkers (stomach contents, bulk δ 13 C and δ 15 N, compound-specific amino acid δ 13 C, and fatty acid profiles), we found that diadromous fishes consistently exhibited broader trophic niches than marine fishes, reflecting their ability to exploit both freshwater and offshore pelagic resources. Trophic overlap occurred across the two life histories in both lagoon types, but was greater in high-exchange lagoons, primarily due to shared reliance on amphipods, mysids, and marine carbon sources. Considering the different biomarker turnover times, this overlap likely extends from late winter ice cover into open water periods. These results suggest that competition for prey is probable among species with contrasting life histories, particularly during periods of low prey abundance. By resolving seasonal and spatial patterns of trophic overlap, our findings provide important baseline knowledge for modeling future scenarios of lagoon connectivity and for informing subsistence fisheries management under ongoing changes in the Arctic. • Diadromous fishes showed broader trophic niches than marine fishes. • Diadromous niches reflect broader diets spanning freshwater and offshore pelagic sources. • Trophic overlap was greater in high exchange lagoons than in low exchange lagoons. • Various biomarker reflected an approximate seasonal progression of trophic niche overlap.