Mercury uptake patterns of biota in a seasonally anoxic northern California Reservoir

Biotic uptake of mercury (Hg) in Davis Creek Reservoir, California increased dramatically in conjunction with the entrainment of anoxic hypolimnetic water into the mixed layer. This indicated a seasonal pulse increase of bioavailable Hg associated with thermal destratification. The effect was more pronounced in juvenile bass (70–200% seasonal increases in muscle Hg concentration), as compared to adults (15–25% increases), and was most distinct in zooplankton, which spiked to concentrations of 3–6 mg/kg, dry weight, immediately following fall destratification (130–270% seasonal increases over pre-fall levels). In addition to the general buildup of methyl Hg in the hypolimnion under anoxic conditions, a dense layer of photosynthetic anaerobic bacteria just beneath the thermocline is implicated as a potentially important seasonal source of methyl Hg to reservoir fish. Hg increases in juvenile and adult fish correspond to late summer and fall entrainment of upper hypolimnetic water, while zooplankton spike increases may be partially related to ingestion or adsorption of Hg-scavenging manganese oxides, which precipitate following full turnover. A simple and effective, syringe-based cold vapor atomic absorption method for total Hg is also described.