Dissecting the spatial scales of mercury accumulation in Ontario lake sediment.

Total mercury concentration was analyzed in 171 lakes from pre-industrial (>30 cm depth; Hg(pre-industrial)) and present-day sediments (0.5-1 cm; Hg(present-day)). Numerous hot or cold spots of sediment mercury enrichment (Hg EF; Hg(pre-industrial)/Hg(present-day)) were evident as determined by local tests of autocorrelation, although in most cases, the maximum correlation among sites was not the nearest neighbor, indicating a strong influence of watershed characteristics. Hg EF was correlated with the area of open water (ha) (r = 0.91, p = 0.035), mine tailings (r = 0.94, p = 0.019), and organic deposits in surficial geology of the watershed (r = -0.91, p = 0.034). Through use of local rather than global regression coefficients, R(2) increased from 0.20 (p = 0.005) to 0.60 (p = 0.013). A broad spatial pattern (>500 km) observed only in Hg(pre-industrial) was best explained by mean annual precipitation (shared variance = 3.5%), while finer spatial patterns only observed in Hg(present-day) and Hg EF were best explained by pH (average shared variance = 10.8%).

Temporal analysis of net fluvial methylmercury loading in a dystrophic and a clear water lake.

The concentration of methylmercury (MeHg) in aquatic ecosystems is the net result of the highly dynamic abiotic and biotic processes of mercury methylation and demethylation. In this study, we conduct an examination of the net fluvial loading of methylmercury (MeHg(Net)=MeHg(Watershed)-MeHg(Lake outflow)) across a 3 year time frame in both a dystrophic lake and an oligotrophic lake. A significant portion of MeHg(Net) variance in both lakes could be attributed to a seasonal pattern (11.4%, p=0.009; oligotrophic, and 27.0%, p<0.0001; dystrophic) which in both cases, was most correlated with air temperature. The dystrophic lake appeared to be a net source of methylmercury (MeHg(Net)=-1.9+/-0.3 mg MeHg d(-1)) while the oligotrophic lake appeared to be a net sink (MeHg(Net)=0.4+/-0.2 mg MeHg d(-1)), indicating that there was net methylation in the dystrophic lake and net demethylation in the oligotrophic lake. Higher MeHg loading to the lakes occurred during the summer and between seasons there was a difference in MeHg(Net) of 1.1+/-0.3 mg MeHg d(-1) and 3.1+/-0.6 mg MeHg d(-1). Seasonal patterns of MeHg(Net) in the oligotrophic lake lagged behind the dystrophic lake by 39 days. The short term variation in MeHg(Net) was dominated by precipitation (t=2.73, p=0.008; dystrophic, t=2.53, p=0.017; oligotrophic).