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

Camille De La Vega1, Rachel Jeffreys2, Joanna Kershaw3, David J Yurkowski4, Anne Kirstine Frie5, Steve Ferguson4, Garry B Stenson6, Martin Biuw7, Sophie Smout8 and Claire Mahaffey1, (1)University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom, (2)University of Liverpool, Earth, Ocean and Ecological Sciences, United Kingdom, (3)University of St Andrews, St Andrews, Fife, KY16 8LB, UK, United Kingdom, (4)Freshwater Institute, Fisheries and Oceans Canada, Winnipeg, MB R3T 2N6, Canada, MB, Canada, (5)Institute of marine research, Nordnes NO-5817 Bergen, Norway, Norway, (6)Northwest Atlantic Fisheries Centre, Fisheries and Oceans Canada, St. John's, NF, Canada, (7)Norwegian Polar Institute, Polar Environmental Centre, Tromsø, Norway, Norway, (8)University of St Andrews, United Kingdom
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 (δ15Nbulk) 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 (δ15NAA) simultaneously fingerprints both the δ15N values at the baseline, which is conservatively traced by δ15N values of source AA (δ15Nsource), and trophic position in predators, which is isolated by δ15N values of trophic AA (δ15Ntrophic). We analysed δ15Nbulk and δ15NAA 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 δ15Nsource valuesrevealed that the decline in δ15Nbulk 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 δ15Nbulk alone, the trophic position of seals estimated with δ15Nsource and δ15Ntrophic 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 δ15NAA 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.