Is Clear Lake methylmercury distribution decoupled from bulk mercury loading?

Clear Lake is the site of the abandoned Sulphur Bank Mercury Mine, active periodically from 1873 to 1957, resulting in approximately 100 Mg of mercury (Hg) being deposited into the lake's ecosystem. Concentrations of total (primarily inorganic) Hg (TotHg) in Clear Lake are some of the highest reported worldwide for sediments (up to 4.4 x 10(5) ng/g [ppb dry mass]) and water (up to 4 x 10(-1) microg/L [= ppb]). However, the ratio of methylmercury (MeHg) to TotHg at Clear Lake indicates that the methylation process is mostly decoupled from bulk inorganic Hg loading, with Hg in lower trophic level biota significantly less than anticipated compared with other Hg-contaminated sites worldwide. This may be due to several factors, including: (1) reduced bioavailability of Hg derived from the mine (i.e., cinnabar, metacinnabar, and corderoite), (2) the alkaline nature of the lake water, (3) the shallow depth of the lake, which prevents stratification and subsequent methylation in a stratified hypolimnion, and (4) possible dilution of MeHg by a highly productive system. However, while bulk inorganic Hg loading to the lake may not contribute significantly to the bioaccumulation of Hg, acid mine drainage (AMD) from the mine likely promotes Hg methylation by sulfate-reducing and iron-reducing bacteria, making AMD a vehicle for the production of highly bioavailable Hg. If Clear Lake were deeper, less productive, or less alkaline, biota would likely contain much more MeHg than they do presently. Comparisons of MeHg:TotHg ratios in sediments, water, and biota from sites worldwide suggest that the highest production of MeHg may be found at sites influenced by chloralkali plants, followed by sites influenced by gold and silver mines, with the lowest production of MeHg observed at cinnabar and metacinnabar Hg mines. These data also suggest that the total maximum daily load (TMDL) process for Hg at Clear Lake, as currently implemented to reduce contamination in fishes for the protection of wildlife and humans, may be flawed because the metric used to implement Hg load reduction (i.e., TotHg) is not directly proportional to the critical form of Hg that is being bioaccumulated (i.e., MeHg).