Whole-lake burdens and spatial distribution of mercury in surficial sediments in Wisconsin seepage lakes
We quantified total mercury in surficial sediments (uppermost 5 cm) of six small seepage lakes. Fifty cores were taken from each lake, based on a random sampling design stratified by water depth. Volumetric concentrations (mass per volume of wet sediment) more accurately portrayed the depth distribution of mercury in the lakes than did dry-weight concentrations, which underrepresented the significance of the shallow-water sediments as a reservoir of potentially available mercury. Estimates of whole-lake burdens (masses) of mercury in the surficial sediment, which represent the maximum amount of sedimentary mercury available for methylation, ranged from 45 to 149 g. These sedimentary pools of mercury greatly exceeded the inventories of the metal in water, seston, and fish, and the release of mercury from the sediments could significantly increase its bioaccumulation. Areal burdens of mercury (mass per unit of lake area) varied fivefold among lakes (1.65–7.84 g/ha) and were strongly correlated with lake pH, but were not correlated with dissolved organic carbon, conductance, or chlorophyll a of lakes. We hypothesize that the observed variation in areal burdens of mercury was partly due to variation among lakes in the pH-related efflux of gaseous mercury (Hg0) from the lakes to the atmosphere.