Evidence for the Incorporation of Terrestrial Carbon in Arctic Coastal Food Webs in the Western Beaufort Sea

The nearshore shelf of the Beaufort Sea is defined by extreme physical and biological gradients that have a distinctive influence on its productivity and trophic structure. Massive freshwater discharge from the Mackenzie River, along with numerous smaller rivers and streams elsewhere along the coast, produce an environment that is decidedly estuarine in character, especially in late spring and summer. Because of its low in situ productivity, allochthonous inputs of organic carbon, identifiable on the basis of isotopic values, are important to the functioning of this arctic estuarine system. Coastal erosion and river discharge are largely responsible for introducing high concentrations of suspended sediment from upland regions into the nearshore zone. The depletion in the 13C content of invertebrate and vertebrate consumers, which drops about 4-5 ppt eastward along the eastern Alaskan Beaufort Sea coast, may reflect the assimilation of this terrestrial organic matter into local food webs. Isotopic tracer studies of amphidromous fishes in the Simpson Island barrier island lagoon ecosystem revealed that terrestrial (peat) carbon may contribute as much as 30 to 50% of their total dietary requirements. On the eastern Alaska Beaufort Sea coast, carbon isotopic values of arctic cod collected in semi-enclosed lagoons were more depleted by 3-4 ppt compared to fish collected in the coastal Beaufort Sea. Calculations from isotopic mixing equations indicate cod from lagoons may derive 70% of their carbon from terrestrial sources. Nitrogen isotopic values of lagoon fish were also 4 ppt lower than coastal specimens, reflective of the lower del 15N values of terrestrial derived nitrogen (0-1.5 ppt compared to 5-7 ppt for phytoplankton). The possible role of terrestrially derived carbon to arctic estuarine food webs is especially important in view of the current warming trend in the arctic environment and the role of advective processes that transport carbon along the nearshore shelf.