The watershed, by way of four tributaries, provides the principal input of total Hg (Hg(T)) to Onondaga Lake, an urban hypereutrophic lake (Henry et al., 1995). In general, atmospheric deposition is the primary source of Hg to the watershed in the absence of point source discharges or mineral contributions. The majority of the Hg deposited from the atmosphere to the watershed is retained by the soils. The flux of Hg from the atmosphere to the watershed of Onondaga Lake is estimated at 20-30 micrograms.m-2.yr-1, based on measurements of Hg deposition in urbanized regions of the United States and Europe. Watershed specific yield from three of the four tributaries that feed Onondaga Lake ranges from 5.3 to 7.5 micrograms.m-2.yr-1. These watershed Hg export rates are higher than those observed for forested areas and represent approximately 25 percent of the estimated deposition to this urban location. The Hg(T) specific yield from the fourth tributary, Ninemile Creek, was substantially higher. This creek had the highest suspended solids loading and receives runoff from an inactive chloralkali plant site. A summertime buildup of Hg species was observed in the hypolimnion of this thermally stratified lake (Jacobs et al., 1995). Sediment trap collections indicated that particle settling facilitates the accumulation of Hg in the hypolimnion. The sources of CH3Hg to the hypolimnion include tributary input and in situ bacterially mediated methylation. The impact of the entrainment of Hg-rich hypolimnetic water following fall destratification on the bioaccumulation of CH3Hg was not clear, because plankton CH3Hg levels were not generally higher following fall overturn.