Environmental contaminants in colonial waterbirds from Great Slave Lake, NWT: spatial, temporal and food-chain considerations

Great Slave Lake in the Northwest Territories, Canada, differs regionally i n trophic status and local and regional inputs of contaminants. Spatial and temporal trends in contaminant levels in bioindicator species such as colo nial waterbirds could offer insights into the potential for contaminant bio accumulation in Great Slave Lake. Persistent chlorinated hydrocarbon contam inants, mercury (Hg), and selenium (Se) were examined in herring gull (Laur us argentatus) eggs and livers collected from various locations on Great Sl ave Lake in 1995. Eggs were collected in May and June, and livers in May an d August. Also, the relationship between contaminants and trophic level, as inferred from stable-nitrogen isotope analysis (delta(15)N), was examined in four colonial waterbird species: herring gull, mew gull (L. canus), Casp ian tern (Sterna caspia), and black tern (Clilidonias niger). Finally, the co-accumulation of mercury and selenium was examined in eggs of these birds . There were no differences in chlorinated hydrocarbon concentrations among four sampling sites (colonies). Concentrations did not differ between herr ing gull adults collected in early May and those collected in early August. Chlorinated hydrocarbon concentrations in eggs of herring gull, mew gull, Caspian tern, and black tern were related to their trophic positions as inf erred from their delta(15)N values in their lipid-free egg yolks. Concentra tions in these colonial waterbirds were much higher than those in fish from Great Slave Lake, but lower than those in their conspecifics from the Grea t Lakes. It is probable that a relatively large proportion of the chlorinat ed hydrocarbon contaminant load in colonial waterbird eggs on Great Slave L ake results from exposure to and storage of such contaminants at more heavi ly contaminated wintering and staging areas. This possibility limits the us efulness of colonial waterbirds as indicators of chlorinated hydrocarbon bi oaccumulation in Great Slave Lake. Selenium and mercury concentrations in h erring gull eggs differed significantly among the four breeding colonies, a nd concentrations in adults declined between May and August. Selenium and m ercury were positively correlated in eggs of all species.