Seasonal dynamics of mercury in plankton in an ultraoligotrophic lake: a ä13 C approach

ARCAGNI, M.; CAMPBELL, L.; RIZZO, A.; ARRIBERE, M.A.; KYSER, K.; KLASSEN, K.; RIBEIRO GUEVARA, S.

Halifax, Nova Scotia

Conferencia; 10th International Conference on Mercury as a Global Pollutant; 2011

Total mercury concentration (THg) and stable carbon isotope ratios (D13C) were seasonally measured in three different plankton fractions (F1: 10-53 µm, F2: 53-200 µm, F3: >200 µm) to assess seasonal association between THg content and C sources to plankton in Lake Moreno, an ultraoligotrophic lake in Northeastern Patagonia (Argentina), which is divided in two basins, East (ME) and West (MW) that present differing hydrogeomorphic and catchment features. THg contents were significantly higher in F1 (up to 209.4 µg g-1 dry weight (DW) and 257.9 µg g-1 DW in ME and MW, respectively) than in the other two fractions. A seasonal trend was observed in the THg contents in F1, with the highest concentrations in fall, and the lowest in summer. However, a different trend was observed in the other two fractions which presented lower THg in fall and winter (except F2 in ME) and higher concentrations in spring and summer. D13C values were in general higher in ME than in MW and a seasonal trend was also observed in both basins. The highest and lowest values were observed in fall and spring respectively, in both basins for all fractions. A positive correlation was observed between THg and D13C in both basins, indicating that the high THg concentrations in plankton were probably associated with allochthonous C sources. This findings are also supported by another allochthony indicator evaluated in a previous work, the color:chlorophyla ratio which showed a seasonal pattern with a higher ratio in fall and lower in spring. Precipitations in the area are more abundant during fall and winter, being fall the season where it rains the most, with the consequent input of allochthonous material to the lake due to runoff and stream discharges. The higher THg contents recorded in spring could be related to the deposition of mercury, via atmospheric transport, in the snowpack that accumulates in the high altitudes during winter, and flushing afterwards to the aquatic systems during the spring snowmelt. This water flowing to Lake Moreno in spring carries out much less organic materials when compared with the fall runoff, because it flows mostly through rocky surfaces.