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 HgT has decreased at an average rate of 0.04 ng/L/yor roughly 40% from 1988 to 1999. The concentration of aqueous meHgT 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/m2/y to 5.5 ug/m2/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.