The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons

To understand the geochemical cycle of Hg in hypereutrophic freshwater lake, two sampling campaigns were conducted in Lake Taihu in China during May and September of 2009. The concentrations of unfiltered total Hg (unfTHg) were in the range of 6.8–83 ng L−1 (28 ± 18 ng L−1) in the lake water and total Hg in the sediment was 12–470 ng g−1, both of which are higher than in other background lakes. The concentration of unfTHg in ∼11% of the lake water samples exceeded the second class of the Chinese environmental standards for surface water of 50 ng L−1 (GB 3838-2002), indicating that a high ecological risk is posed by the Hg in Lake Taihu. However, the concentrations of unfiltered total MeHg (unfMeHg) were relatively low in the lake water (0.14 ± 0.05 ng L−1, excluding two samples with 0.81 and 1.0 ng L−1). Lake sediment MeHg varied from 0.2–0.96 ng g−1, with generally low ratios of MeHg/THg of <1%. The low concentrations of TMeHg in the lake water may have resulted from a strong uptake by the high primary productivity and the demethylation of MeHg in oxic conditions. In addition, contrary to the results of previous research conducted in deep-water lakes and reservoirs, the low concentrations of MeHg and low ratio of MeHg/THg in the lake sediment indicates that the net methylation of Hg was not accelerated by the elevated organic matter load created by the eutrophication of Lake Taihu. The results also showed that sediments were a source of THg and MeHg in the water. Higher diffusion fluxes of THg and MeHg may be partly responsible for the higher concentrations of THg in the lake water in May, 2009.