Uptake of mercury by fish in an experimental boreal reservoir.

We studied the uptake of mercury (Hg) by finescale dace (Phoxinus neogaeus) in an experimental boreal reservoir for 2 years prior to flooding and 3 years after flooding, and in a natural wetland pond over the same 5-year period. Hg uptake was much higher after flooding as compared to uptake before flooding and in the natural pond. After flooding, Hg concentrations in late summer were usually 2-3x higher than concentrations observed prior to flooding. Net uptake of Hg by fish over the summer in the experimental reservoir was 0.25 and -0.07 microg per fish in the 2 years before flooding as compared to 0.63, 0.64, and 0.42 microg per fish in the 3 years after flooding. Thus, Hg uptake by fish responded quickly to flooding and was highest in the first 2 years following impoundment. Uptake in the reference pond ranged from 0.10 to 0.28 microg of Hg per fish over the same 5-year period. Calculated fluxes of Hg on an areal basis ranged from 0.04-0.09 microg m-2 year-1 in the reference pond, were 0.08 and -0.02 microg m-2 year-1 in the experimental reservoir prior to flooding, and ranged from 0.14-0.22 microg m-2 year-1 in the experimental reservoir after flooding. These fluxes were much smaller than fluxes of methyl mercury (MeHg) through the zooplankton and emerging insect communities. Most (71-89%) of the mercury measured in the muscle of finescale dace was MeHg, and the proportion that was MeHg decreased over the summer period prior to flooding, but increased over the summer after flooding. Growth of fish was not significantly affected by flooding. Fish ate predominantly benthic invertebrates (64-84% of food items found in stomachs), with lesser proportions of crustacean zooplankton (16-31% of items found in stomachs) and feeding was similar after as compared to before flooding. Therefore, differences in Hg uptake did not appear to be the result of changes in diet.