Methylmercury production in the water column of an ultraoligotrophic lake of Northern Patagonia, Argentina

Methyl-mercury (CH 3Hg +) production was studied in freshwaters from lake Moreno, an ultraoligotrophic system belonging to Northern Patagonia. Hg 2+ labelled with high specific activity 197Hg was spiked to water samples in concentrations of 10 ng l −1, and incubated in laboratory for 3 d time trends under different conditions. Experimental water was sampled daily to evaluate CH 3 197Hg + production. Lake water used in the experiments was sampled just below the upper limit of the metalimnion (∼30 m depth), where maximum values of chlorophyll a have been measured previously. Sampling was performed in late autumn, when the plankton fraction