GEOGRAPHIC TREND IN MERCURY MEASURED IN COMMON LOON FEATHERS AND BLOOD

The common loon (Gavia immer) is a high-trophic-level, long-lived, obl igate piscivore at risk from elevated levels of Hg through biomagnific ation and bioaccumulation. From 1991 to 1996 feather (n = 455) and blo od (n = 381) samples from adult loons were collected between June and September in five regions of North America: Alaska, northwestern Unite d States, Upper Great Lakes, New England, and the Canadian Maritimes. Concentrations of Hg in adults ranged from 2.8 to 36.7 mu g/g (fresh w eight) in feathers and from 0.12 to 7.80 mu g/g (wet weight) in whole blood. Blood Kg concentrations in 3 to 6-week-old juveniles ranged fro m 0.03 to 0.78 mu g/g (wet weight) (n = 183). To better interpret expo sure data, relationships between blood and feather Hg concentrations w ere examined among age and sex classes. Blood and feather Hg concentra tions from the same individuals were significantly correlated and vari ed geographically (r(2) ranged from 0.03 to 0.48). Blood and feather H g correlated strongest in areas with the highest blood Kg levels, indi cating a possible carryover of breeding season Hg that is depurated du ring winter remigial molt. Mean blood and feather Hg concentrations in males were significantly higher than concentrations in females for ea ch region. The mean blood Hg concentration in adults was 10 times high er than that in juveniles, and feather Hg concentrations significantly increased over 1 to 4-year periods in recaptured individuals. Geograp hic stratification indicates a significant increasing regional trend i n adult and juvenile blood Hg concentrations from west to east. This g radient resembles U.S. Environmental Protection Agency-modeled predict ions of total anthropogenic Hg deposition across the United States. Th is gradient is dearest across regions. Within-region blood Hg concentr ations in adults and juveniles across nine sites of one region, the Up per Great Lakes, were less influenced by variations in geographic Hg d eposition than by hydrology and lake chemistry. Loons breeding on low- pH lakes in the Upper Great Lakes and in all lake types of northeaster n North America are most at risk from Hg.