Decadal time trends of mercury (Hg) concentrations in Arctic biota suggest that anthropogenic Hg is not the single dominant factor modulating Hg exposure to Arctic wildlife. Here, we present Hg speciation (monomethyl-Hg) and stable isotopic composition (C, N, Hg) of 53 Alaskan ringed seal liver samples covering a period of 14 years (1988-2002). In vivo metabolic effects and foraging ecology explain most of the observed 1.6 ‰ variation in liver d202Hg, but not ?199Hg. Ringed seal habitat use and migration were the most likely factors explaining ?199Hg variations. Average ?199Hg in ringed seal liver samples from Barrow increased significantly from +0.38±0.08‰ (±SE, n=5) in 1988 to +0.59±0.07‰ (±SE, n=7) in 2002 (4.1±1.2% per year, p