A stable isotope (δ13C, δ15N) model for the North Water food web: Implications for evaluating trophodynamics and the flow of energy and contaminants

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Deep-Sea Research Part II: Topical Studies in Oceanography





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The North Water Polynya is an area of high biological activity that supports large numbers of higher trophic-level organisms such as seabirds and marine mammals. An overall objective of the Upper Trophic-Level Group of the International North Water Polynya Study (NOW) was to evaluate carbon and contaminant flux through these high trophic-level (TL) consumers. Crucial to an evaluation of the role of such consumers, however, was the establishment of primary trophic linkages within the North Water food web. We used δ15N values of food web components from particulate organic matter (POM) through polar bears (Ursus maritimus) to create a trophic-level model based on the assumptions that Calanus hyperboreus occupies TL 2.0 and there is a 2.4‰ trophic enrichment in 15N between birds and their diets, and a 3.8‰ trophic enrichment for all other components. This model placed the planktivorous dovekie (Alle alle) at TL 3.3, ringed seal (Phoca hispida) at TL 4.5, and polar bear at TL 5.5. The copepods C. hyperboreus, Chiridius glacialis and Euchaeta glacialis formed atrophic continuum (TL 2.0-3.0) from primary herbivore through omnivore to primary carnivore. Invertebrates were generally sorted according to planktonic, benthic and epibenthic feeding groups. Seabirds formed three trophic groups, with dovekie occupying the lowest, black-legged kittiwake (Rissa tridactyla), northern fulmar (Fulmarus glacialis), thick-billed murre (Uria aalge), and ivory gull (Pagophilia eburnea) intermediate (TL 3.9-4.0), and glaucous gull (Larus hyperboreus) the highest (TL 4.6) trophic positions. Among marine mammals, walrus (Odobenus rosmarus) occupied the lowest (TL 3.2) and bearded seal (Erignathus barbatus), ringed seal, beluga whale (Delphinapterus leucas), and narwhal (Monodon monoceros) intermediate positions (TL 4.1-4.6). In addition to arctic cod (Boreogadus saida), we suggest that lower trophic-level prey, in particular the amphipod Themisto libellula, contribute fundamentally in transferring energy and carbon flux to higher trophic-level seabirds and marine mammals. We measured PCB 153 among selected organisms to investigate the behavior of bioaccumulating contaminants within the food web. Our isotopic model confirmed the trophic magnification of PCB 153 in this high-Arctic food web due to a strong correlation between contaminant concentration and organism δ15N values, demonstrating the utility of combining isotopic and contaminant approaches to food-web studies. Stable-carbon isotope analysis confirmed an enrichment in 13C between POM and ice algae (-22.3 vs. -17.7‰). Benthic organisms were generally enriched in 13C compared to pelagic species. We discuss individual species isotopic data and the general utility of our stable isotope model for defining carbon flux and contaminant flow through the North Water food web. © 2002 Elsevier Science Ltd. All rights reserved.