Environmental change in the Arctic is reflected in the decadal trend of stable nitrogen isotopes in harp seal teeth over the past 60 years.

Unprecedented rates of environmental change in the Arctic has resulted in multiple concurrent stressors, which impact the entire food web from primary producers to predators. Monitoring changes in Arctic food web structure is urgently needed and necessitates accurate tools. Harp seals (Pagophilus groenlandicus), are ice-dependent predators with a wide Arctic distribution and are an excellent indicator of food web structure. δ¹⁵N values of bulk tissues (δ¹⁵N_bulk) are powerful tracers in estimating trophic position but are influenced by δ¹⁵N values at the base of the food web, or baseline. Compound-specific δ¹⁵N values of amino acids (δ¹⁵N_AA) simultaneously fingerprints both the δ¹⁵N values at the baseline, which is conservatively traced by δ¹⁵N values of source AA (δ¹⁵N_source), and trophic position in predators, which is isolated by δ¹⁵N values of trophic AA (δ¹⁵N_trophic). We analysed δ¹⁵N_bulk and δ¹⁵N_AA values in harp seal teeth from the Barents Sea, Greenland Sea and Labrador Seato assess decadal trends in harp seal trophic position in the Arctic over the past 60 years. The significant decline in δ¹⁵N_source valuesrevealed that the decline in δ¹⁵N_bulk values was driven by environmental change influencing the δ¹⁵N values at the base of the food web, e.g. via Atlantification. Contrary to what could be concluded from δ¹⁵N_bulk alone, the trophic position of seals estimated with δ¹⁵N_source and δ¹⁵N_trophic remained constant overtime despite changes in the underlying food web, e.g. the northward shift of boreal species due to warming temperature and removal of prey due to commercial fishing. Our results highlight the power of δ¹⁵N_AA analyses in accounting for changes in δ¹⁵N baseline to accurately interpret temporal changes in seal trophic position. Our robust estimation of harp seal trophic position over the last six decades can be used in population modelling to improve seal management strategies.