Arctic seals as tracers of environmental and ecological change

Erling S. Nordøy,Tore Haug,Claire Mahaffey,Alessandro Tagliabue,Jo Hopkins,David J. Yurkowski,Steven H. Ferguson,Robyn E. Tuerena,Garry B. Stenson,Martin Biuw,Rachel M. Jeffreys,Sophie Smout,Camille de la Vega

doi:10.1002/lol2.10176

Abstract

AbstractKnowledge of species trophic position (TP) is an essential component of ecosystem management. Determining TP from stable nitrogen isotopes (δ15N) in predators requires understanding how these tracers vary across environments and how they relate to predator isotope composition. We used two seal species as a model for determining TP across large spatial scales in the Arctic. δ15N in seawater nitrate (δ15NNO3) and seal muscle amino acids (δ15NAA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ15NNO3 and δ15NAA. Our results show that the spatial variation in δ15NAA in seals reflects the δ15NNO3 end members in Pacific vs. Atlantic waters. This study provides a reference for best practice on accurate comparison of TP in predators and as such, provides a framework to assess the impact of environmental and human‐induced changes on ecosystems at pan‐Arctic scales.

Highlights

Spatial variation in the baseline δ15NNO3 of seawater was enriched in 15N by ~ 2‰ in the Pacific influenced Canadian Archipelago water (6.8‰), compared to the Barents Sea (5.1 Æ 0.2‰) and Labrador Shelf (5.0 Æ 0.3‰; Table 1). δ15N of phenylalanine (δ15NPhe), representing the δ15N of the baseline in seal tissues ranged from 6.2 Æ 0.9‰ (Greenland Sea) to 9.8 Æ 0.7‰ (Baffin Island) in harp seals, and from 11.2 Æ 0.2‰ (Baffin Island) to 12.1 Æ 0.6‰ (Canadian Archipelago) in ringed seals (Fig. 4b,c)
When using the δ15NAA approach and correcting for variations in the baseline using δ15NPhe, our results show that: (1) within each seal species, the TPrel does not vary across the Arctic, confirming our prediction and (2) ringed seals are at a lower trophic position (TP) than harp seals, contradicting our prediction
These findings highlight the power of using δ15NAA when examining spatial variation in TP of predators and demonstrate the need to account for variation in the δ15N of the baseline to avoid misinterpretation of δ15Nbulk in consumers

Summary

Introduction

Seal sampling A total of 210 muscle samples were obtained from the longissimus dorsi of adult (older than 6 year old) harp and ringed seals at six sites across the Arctic and sub-Arctic (Southern Barents Sea, Northern Barents Sea, Greenland Sea, Labrador shelf, Baffin Island, and Canadian Archipelago; Fig. 2, Table 1). For harp seals that were sampled at more than two sites, we applied ANOVAs followed by Tukey pairwise comparison tests on δ15Nbulk, δ15NPhe, and TPrel to test the effect of the most accurate explanatory factors derived from the model selection (Supporting Information S4).

Results

Conclusion