The Vertical Distribution of Mercury Species in Wisconsin Lakes: Accumulation in Plankton Layers (Chapter 1.11)
The aquatic cycling of highly bioreactive trace elements such as Hg is potentially influenced by layers of physiologically active plankton which develop in the thermocline and hypolimnia of lakes. Here we examine the distribution of waterborne mercury species (Hg and methyl-Hg in dissolved and particulate forms) with respect to the vertical distribution of plankton layers in three quite different Wisconsin lakes. We also examine corresponding distributions of waterborne Fe, Mn, dissolved oxygen, organic carbon, sulfate, and sulfide. To resolve fine-scale spatial features of the plankton layers, the watercolumn was mapped using computer-based, bio-optical techniques with an absolute resolving power of about 2 em. Mercury accumulated in all the plankton layers we observed, reaching maximum concentrations of 45 ng Hg/L (compared to 1 ng/L in the epilimnion), but there were distinct differences in the distributions of Hg and methyl-Hg. Methyl-Hg accumulated primarily in plankton layers below the oxic/anoxic (0/A) boundary, reaching 12 ng/L in anoxic layers (\textgreater 100-fold higher than [methyl-Hg] epilimnion). Spatially and seasonally, [Hg] tended to track the concentration of chlorophyll-a (eukaryotic phytoplankton and cyanobacteria); [methyl-Hg]tracked the distribution of bacteriochlorophylls (phototrophic sulfur bacteria). Maxima of bacteriochlorophyll and methyl-Hg both occurred near the sulfide/sulfate transition zone below the oxic/anoxic boundary. Unlike Fe and Mn, the concentration of Hg and methyl-Hg tended to decrease in the region between the lowest plankton layers and the sediment surface. While the orthograde distributions of Fe and Mn indicate redox-driven diffusion from sediments into the overlying watercolumn, more complex processes appear to determine the observed vertical distribution of mercury species. Given the strong affinity of Hg for biotic particles (log Kd = 5 to 6), either Hg scavenging/settling from above or Hg diffusion/sorption to plankton layers could account for the elevated [Hg] observed in all plankton layers. Both mechanisms could be involved if Hg was released from settling detritus and then resorbed by cells within the layers. Since [methyl-Hg] maxima were confined to anoxic plankton layers, either diffusion/sorption from below (i.e., sediments) or de novo production of methyl-Hg within these layers is implied. Our data tend to support the hypothesis that methyl-Hg is formed within anoxic layers via conversion of Hg derived from the upper watercolumn. We explore this hypothesis and its implications.