Decreasing mercury in northern Wisconsin: Temporal patterns in bulk precipitation and a precipitation-dominated lake

Long-term monitoring of a precipitation-dominated lake in northern Wisconsin indicates decreasing mercury concentrations over the past decade. In the surface waters of Little Rock Lake, aqueous Hg-T has decreased at an average rate of 0.04 ng/L/-or roughly 40% from 1988 to 1999, The concentration of aqueous meHg(T) in Little Rock Lake has decreased too-at an average rate of 0.004 ng/L/y (roughly 50% since 1988). Atmospheric monitoring also indicates decreased mercury in bulk precipitation. For those years when we have a continuous data record (1994 through 1999), the annual Hg cycle in bulk precipitation has been damped and the average rate of bulk atmospheric Hg deposition decreased by roughly 50% (from 11.2 ug/m(2)/y to 5.5 ug/m(2)/y). These trends suggest that the lake waters have responded to decreased atmospheric Hg deposition-perhaps driven by reduced anthropogenic emissions in the region. Ancillary data and mass balance modeling are used to more fully investigate recent changes in the aquatic mercury cycle.