MeHg

The methylmercury(I)-imidazole system in solution contains [MeHg(Him)], [MeHg(im)] and [(McHgl Im] species (77JA8106). In the latter the imidazo-late ligand fulfils a bridging function and is coordinated via both nitrogen heteroatoms. 

Watersheds are sinks for atmospheric Hg deposition (Grigal 2002). However, they are highly variable in their ability to retain inputs of total Hg (THg), convert ionic Hg (Hg(ll)) to bioavailable methylmercury (MeHg), and snpply Hg(II) and MeHg to downstream aqnatic ecosystems, ultimately influencing exposure to sensitive biota and hnmans. 

FIGURE 2.8 Inputs and losses of a) THg and b) MeHg for watersheds in Europe and North America. 

Mercury in soil is not only likely to have a different potential for evasion and methylation than Hg in runoff, but soil Hg may be perturbed by land disturbance. Land disturbances that are particularly relevant to Hg cycling include the formation of wetlands and flooding of reservoirs (Rudd 1995 see Chapter 3). Disturbances such as clear-cutting can also result in marked increases in the release of THg and MeHg from soils (Munthe and Hultbeig 2003 Porvari et al. 2003). Fire can result in large Hg losses by volatilization (Grigal 2002). 

Summary of collections and measurements of THg and MeHg that should be made at intensive watershed Hg monitoring sites... 

Hg(0), PHg, RGHg), wet deposition, throughfall, and litterfall, as discussed in the program to determine total ecosystem deposition (see Section 2.2.8). A summary of the measurements of Hg species that should be made in an intensive watershed Hg monitoring program is provided in Table 2.4. We envision that stream water measurements of total and dissolved THg and total and dissolved MeHg would also be made. 

Given the limitations associated with several of the possible Hg species in water and sediment, it was concluded that the most likely applicable Hg species were HgT and MeHg. However, 7 indicators were identified that ate based on HgT and MeHg measmements on sediment and water samples (see Table 3.1) and discussed next in the context of the evalnation criteria. 

Criterion Importance of criterion HgT in sediment (top 1-2 cm) MeHg in sediment (top 1-2 cm) Percent MeHg in sediment Instantaneous methylation rate Sedimentary accumulation rate of Hg HgT in surface water MeHg in surface water... 

Availability of existing databases To select hiotic indicators with a significant role in the trophic transfer of MeHg in aquatic food wehs Medium Low Low Low Medium to Low Medium Medium... 

FIGURE 3.3 Pearson correlation coefficients between fish (Gambusia) Hg concentration and MeHg concentrations in various environmental media sediment, porewater, surface water, and suspended particulate matter (SPM) from the Florida Everglades (1995-1998). 

FIGURE 3.5 a) Relationship between HgT and MeHg in surface sediments across 49 ecosystems (from Benoit et al. 2003) and b) relationship between HgT and MeHg in surface sediments from 122 streams across the United States. (Source From Krabbenhoft et al. 1999.)... 

As with HgT, concentration profiles for MeHg in sediment often show dramatic changes with depth and considerable spatial variability. Typically, maximum concentrations ate observed at or near the oxic/anoxic interface, which is generally near... 

FIGURE 3.6 Measured concentrations of Hg and MeHg in the top 2 cm of sediments through time in 2001, at 4 discrete sediment sampling sites within Lake 658, the study lake of the METAALICUS project. 

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