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.

Sulfur burial in and loss from the sediments of Little Rock Lake, Wisconsin

Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of atmospheric deposition of substances to the lake, or other processes occurring within lakes. In this study, the sedimentary record of sulfur was examined in Little Rock Lake, Wisconsin. This lake received experimental sulfate additions from 1985 through 1990, and a mass balance showed that 35% (425 kg) of experimental sulfur additions were buried in the sediments. Sediment cores collected before and after sulfur additions confirmed that burial of 180–360 kg of sulfur occurred during the 6 years of experimental sulfur additions. However, cores collected in 1992 and 1996 contained less sulfur than cores taken in 1990 and suggested that sulfur incorporation in sediments is not permanent. To examine seasonal sulfur loss from sediments, sulfur inventories in the top 4 cm of sediments were measured in multiple cores before and after fall overturn. At water depths greater than 5 m, surface sediments retrieved after fall overturn had significantly less sulfur than did those collected before overturn. The small seasonal release from the sediments (25 kg S in 1996) might be caused by organic matter decomposition or sulfide oxidation.