Mercury in an ultraoligotrophic North Patagonian Andean lake (Argentina): concentration patterns in different components of the water column

ARRIBÉRE, M., QUEIMALIÑOS, C.P., RIBEIRO GUEVARA, S., DIÉGUEZ, M.C., FAJON, V. Y HORVAT

Ghiyang, China

Congreso; 9th International Conference on Mercury as a Global Pollutant; 2009

Piedmont deep ultraoligotrophic lakes of Northern Patagonia (Argentina) are located in pristine areas along a wide latitudinal range. The area is characterized by high levels of UV radiation and strong westerly winds, which have an enormous impact on lake dynamics by determining the underwater light climate and the thermal structure of the water column. Recently, several studies have reported moderate to high total mercury concentrations in different trophic compartments in deep Andean lakes. External sources of Hg for eastern Patagonian terrestrial and aquatic ecosystems are still not clear, however, wet atmospheric deposition could be recalled as one potential source. Total mercury contents were measured seasonally at four sites of Lake Moreno, in different planktonic size fractions (10-53, 53-200 and >200 mm). The lowest size fraction presented the highest levels of total Hg, with values between 0.3 to 30 mg g-1 DW  in spring and summer, and raising from 90 to above 250 mg g-1 DW  in fall, while winter samples showed lower Hg. Mercury contents in the fraction 53-200 mm were lower, ranging between 0.1 and 10 mg g-1 DW  in spring and summer, and reaching up to 50 to 90 mg g-1 DW  in fall and winter. Hg contents in the fraction >200 mm did not exceed 2 mg g-1 DW, except for one spring sample which reached 8 mg g-1 DW. Empirical studies in Lake Moreno showed that the water column, the first target of Hg deposition, converted up to 50% Hg2+ into CH3Hg+, with sterile lake water showing up to 30% and planktonic components enhancing up to 20% the CH3Hg+ production. An evaluation of dissolved and particulate natural CH3Hg+ in Lake Moreno revealed moderate to high values. Methylmercury in the fraction <50 mm collected at 5, 30 and 80 m depth ranged between 0.2 and 0.7 ng L-1, and fluctuated from 0.2 to 0.4 in filtered-sterile water. Overall, our results show that there is not evident bioaccumulation of total Hg in Lake Moreno since the highest total Hg contents are in the smallest planktonic fraction, which is consistent with the pattern presented in the dilution hypothesis. Moderate to high CH3Hg+ concentrations are found in sterile lake water and the highest methylation potential as well. Nevertheless, further studies are needed to elucidate the responsible mechanism of these results.