An assessment of bioavailability and bioaccumulation of mercury species in freshwater food chains

Mercury (Hg), in the form of methylmercury (MeHg), bioaccumulates in aquatic food webs often resulting in fish consumption advisories. Bioaccumulation, or the uptake in excess of elimination by biota, can be a complex process. Mercury enters the base of the aquatic food chain through uptake from water by algae. Bioavailable Hg is Hg which is incorporated by the algae. This thesis tests the hypothesis that strong ligands control bioavailability of aqueous Hg to algae by binding Hg in solution and preventing uptake.; Mercury bioaccumulation was studied both in field and laboratory. Within Isle Royale National Park, Hg concentrations in Sargent Lake, a lake with a fish consumption advisory for Hg, were compared to those in Lake Richie---a non-advisory lake. Differences included possible higher bioavailability of filterable Hg in Sargent Lake due to lower DOC levels and higher bioaccumulation by zooplankton and adult perch in Sargent Lake. Other factors such as food web structure may be important. For example, the food web was shown to be more pelagic in Sargent Lake.; A laboratory bioassay was developed to study factors affecting bioavailability of Hg to algae. Kinetics of Hg accumulation by a representative freshwater alga under well-defined conditions were determined. Some weakly-bound inorganic complexes of MeHg appeared to be bioavailable. The conditional equilibrium stability constant for binding of MeHg to algae was also estimated (∼10 16). This estimate was used to model binding of MeHg to algae in the presence of natural ligands, i.e., dissolved organic carbon (DOC).; Using the bioassay with natural waters, inhibition of bioavailability was observed with increasing DOC for both inorganic Hg(II) and MeHg. Above 10 mg/L of DOC, bioconcentration factors (BCFs) for both Hg species decreased markedly. The largest variability in BCF for both Hg(II) and MeHg was at DOC concentrations of <10 mg/L. At DOC concentrations <10 mg/L, potential bioavailability of Hg to algae is greatest and may be determined by DOC quality, strong inorganic ligands, pH, conductivity or other factors. Size-fractionation of particles and colloids in water did not have a significant effect on uptake of Hg by algae.