MERCURY CYCLING IN A NORTHERN WISCONSIN SEEPAGE LAKE - THE ROLE OF PARTICULATE MATTER IN VERTICAL TRANSPORT

During summer stratification, total mercury (Hg(T)) reached maximum concentrations in the O2-depleted, hypolimnion of Little Rock Lake, WI. Initially, the hypolimnetic increase was attributed solely to redox-controlled release of Hg from bottom sediments. However, subsequent depth profiles of Hg indicated that hypolimnetic Hg enrichment could also result from the downward transport and recycling of particulate Hg prior to incorporation in the sediments. Contrasts between Fe and Hg cycles in this lake reinforce this notion. Increases in hypolimnetic Fe were observed during both summer and winter O2 decreases. In contrast, hypolimnetic Hg concentrations declined during winter. In the ice-free season, the distribution of particulate mercury (Hg(p)) correlated with the distribution of chlorophyllous particulates in this lake, re-emphasizing the importance of biotic processes in controlling Hg cycling in the hypolimnion.