Assessment of the spatial distributions of total- and methyl-mercury and their relationship to sediment geochemistry from a whole-lake perspective

Abstract

The aim of this study was to determine the spatial variability for total- and methylmercury in surface sediments (0–2 cm) across a single whole-lake basin, and to relate this variability to the sediment’s geochemical composition. 83 surface sediment samples from Stor-Strömsjön – a lake with multiple sub-basins located in northern Sweden – were analyzed for geochemical composition as well as total-mercury (total-Hg) and methylmercury (methyl-Hg; 35 samples) concentrations. Our results indicate that variations in fine-grained mineral matter (36%) and organic matter (34%) explain an equal amount of the total-Hg variation, but that their relative importance varies between different parts of the lake. Total-Hg concentrations were similar in locations controlled by organic matter or fine-grained mineral matter (average 109 ng g␣1); however, total-Hg inventories (mass per unit area) were significantly higher in the latter (35 and 53 mg m␣2, respectively). Methyl-Hg concentrations are largely (55% of variance) controlled by water depth and sulfur concentration, which supports the importance of within lake methylation reported from other studies. Both for concentrations and inventories the spatial distribution for methyl-Hg in surface sediments is patchy, and interestingly the highest methyl-Hg inventory (1.4 mg m␣2) was found in a shallow location with coarse-grained minerogenic sediment (very low organic matter). A large spatial variability, even within a single lake, is something that needs to be recognized, e.g., when studying processes affecting mercury cycling, mercury loadings and when using lake sediments to reconstruct historic mercury deposition.