Research on HgT and MeHg cycling is presented from Spring Lake, a small seepage lake in northern Minnesota, USA within the Marcell Experimental Forest. The research is a part of a larger project at the Marcell Experimental Forest designed to measure how atmospheric inputs affect terrestrial and aquatic cycling of mercury. Our objectives at Spring Lake were to use mass balance approaches to quantify source strength of mercury in the watershed, study redox transformations, determine internal and external loading for both HgT and MeHg, and model Hg cycling in the lake. Methods included field sampling of environmental compartments (air, water, sediment, land, zooplankton, fish, seston), laboratory studies on the photo-transformations of mercury, and aquatic modeling using Stella as the simulation platform. Many aqueous redox pathways exist for mercury including photoreduction, photo-oxidation, biotic redox processes, dark oxidation, and enhanced oxidation in the presence of chloride. Quantification and modeling of evasion rates of elemental mercury were made during the daytime, yielding an average loss of 1.1 ng m-2 h-1 for 2001 to 2002. Losses of Hg0(aq) spikes in the dark over ten days in the laboratory were observed in lake water (0.02 h-1), high resistivity deionized water, and HPLC grade water (0.002 h-1). Analysis of five sediment cores from Spring Lake showed the accumulation rate of HgT was 21.4 ug m-2 y-1 (averaged from 1990 to 2000), two orders of magnitude greater than accumulation of MeHg in lake sediment. Because atmospheric deposition does not account for the observed accumulation rates (HgT wet deposition = 8.0 ug m-2 y-1, MeHg = 0.078 ug m-2 y-1), measurements of HgT and MeHg in near-shore wetlands and in lakewater were made to elucidate the contributions of watershed and in-lake processes to the mass budgets. Photodegradation rates of MeHg also were quantified and yielded loss rates of 0.4 y-1 in mid-July and 0.04 y-1 in mid-October in lakewater 3 cm from the surface. Profiles of MeHg in sediment porewater suggest that early in the year there may be a small diffusive flux of MeHg from sediment to water (0.06 ug m-2 y-1) but springtime porewater concentrations of MeHg are relatively low (~0.5 ng L-1). In late summer to early fall, porewater concentrations of MeHg were higher (1.5 to 2.2 ng L-1) and showed distinct peaks that correlated with maxima in sulfate reducing activity at 5 and 15 cm. The percent of HgT present as MeHg was highest in the lakewater (9.4%) and decreased with sediment depth in both the solid phase and porewater phase. With respect to mass loading, direct wet and dry deposition together accounted for about half of the HgT accumulation in lake sediment. In comparison to the mass (burden) of HgT and MeHg in the lake, annual atmospheric inputs are 3 times the HgT mass and about equal to the MeHg mass.