INFLUENCE OF TROPHIC POSITION AND FEEDING LOCATION ON CONTAMINANT LEVELS IN THE GULF OF THE FARALLONES FOOD-WEB REVEALED BY STABLE-ISOTOPE ANALYSIS

In this study, we present the levels of organochlorine (Sigma DDT, Sig ma HCH, Sigma chlordane, HCB, and Sigma PCBs) and metal (Pb, Hg, and S e) contaminants and their relationship to stable carbon and nitrogen i sotope values in the Gulf of the Farallones marine food web. This food web consisted of two species of euphausiids (Euphausia pacifica and T hysanoessa spinifera), two fish species [short-bellied rockfish (Sebas tes jordani) and anchovy (Engraulis mordax)], four bird species [commo n murre (Uria aalge), Brandt's cormorant (Phalacrocorax penicillatus), rhinoceros auklet (Cerorhinca monocerata), and pigeon guillemot (Cepp hus columba)], and the northern sea lion (Eumetopias jubatus). We used a novel method of using egg albumen to determine stable isotope value s. The values of delta(13)C ranged from -20.1 parts per thousand in th e euphausiids to -15.0 parts per thousand in the northern sea lion and were consistant with a pelagic/offshore vs benthic/inshore results fo und in other studies. Values of delta(15)N in the Gulf of the Farallon es food web ranged from 11.2 parts per thousand in the euphasiids to 1 9.8 parts per thousand in the northern sea lion and generally demonstr ate an equivalence with trophic level. The levels of organochlorine co mpounds were lowest in the euphausiids [Sigma DDT 11, and Sigma PCB 4. 5 mu g/kg dry weight geometric mean (GM)] and highest in the northern sea lion blubber (Sigma DDT 9500 and Sigma PCB 3500 mu g/kg dry weight GM). The highest levels of organochlorine compounds in the birds were in the common murre (Sigma DDT 8200 and Sigma PCB 5900 mu g/kg dry we ight GM). Levels of Pb, Hg, and Se ranged from 80 to 1000, from 100 to 19000, and from 1900 to 4100 mu ug/kg dry weight GM, respectively. Al l of the organochlorine compounds and Hg were significantly correlated with delta(15)N values in the food web. Lower values of delta(15)N in egg albumen than in the muscle tissue from common murres reflect a sw itch in diet to a lower trophic position during the egg formation peri od. The high contaminant levels in the murre suggest a mobilization of stored lipids into the eggs.