Stable isotopes of sulfur, carbon, and nitrogen were used to investigate the incorporation of nutrients into food chains (fish, invertebrates, and plants) and the influence of migration on the nutritional origins of fish in two freshwater systems in the Mackenzie Delta region. Sulfur isotope analyses of fish muscle showed that broad whitefish (Coregonus nasus) in Travaillant Lake and lake whitefish (Coregonus clupeaformis) in the Kukjuktuk Creek system were migrant populations which had grown mostly on sources outside the local food base. A marine food source was indicated for the lake whitefish, while no specific source was determined for the broad whitefish. The delta-N-15 clearly defined the trophic levels in the local food chains in both systems. Fish were in three levels in Travaillant Lake. Amphipods were in a level below the lowest fish and sediments and macrophytes were in the lowest level. The Kukjuktuk Creek fish were in two levels above a level including macrophytes, amphipods, and zooplankton. The delta-C-13 varied widely even within single fish species local to Travaillant Lake. The delta-C-13 did not differentiate migrants from local fish. Plants of both the C3 and C4 photosynthetic pathway were indicated by carbon isotopes.
