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

Methyl-mercury (CH3Hg+) production was studied in freshwaters from lake Moreno, an ultraoligotrophic system belonging to Northern Patagonia. Hg2+ 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 CH3197Hg+ 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 <50 μm exhibited mercury concentrations up to 260 μg g−1 dry weight. The experiments analysed lake water filtered through 50, 20, and 0.2 μm (filter-sterilized) mesh nets. ASTM grade 1 water was also incubated for control. All the experiments were run in an environmental chamber under controlled temperature and light regime.High Hg2+ conversion to CH3Hg+, up to 50%, was measured in lake water, in a process stimulated by light. CH3Hg+ production was two-fold higher after 3 d of incubation with illumination compared to total darkness. Sterile lake water showed conversions up to 30%, while the planktonic components seem to enhance the CH3Hg+ production. Overall, our results provide evidence that lake Moreno waters favour CH3Hg+ production in processes stimulated by light. Although biotic components certainly contribute to enhance mercury methylation, water chemistry plays a key role in this process. We hypothesize that dissolved organic matter, particularly its quality, could be decisive.