Seasonal enrichment and depletion of Hg and SO4 in Little Rock Lake: relationship to seasonal changes in atmospheric deposition

Hg and SO4 are priority pollutants in fresh waters throughout the northern hemisphere; both have atmospheric sources associated with anthropogenic emissions to the lower troposphere. Although depositional change has had a demonstrable impact on lakes over decadal time scales, effects over shorter time scales remain uncertain. To better understand the responsiveness of lakes to changing pollutant inputs, we examined the coupling of water chemistry to atmospheric deposition over time scales of weeks to years in Little Rock Lake. The results indicate that the lake is tightly coupled to its air shed with respect to Hg. The annual cycle of Hg in surface waters closely tracks the annual cycle of Hg in precipitation. For SO4, the annual cycle in lake water is damped and not in phase with the cycle of atmospheric deposition because of more intense internal recycling. These observations are in keeping with the residence times of Hg and S in lake water (Hg in months; S in years). Nevertheless, because atmospheric deposition is the dominant source of both pollutants, trends in lake water track trends in precipitation over a decadal time scale. Thus, remote lakes may be highly responsive to short term changes in Hg deposition, whereas responses to SO4 deposition may be more gradual.