Mercury in pristine lakes of Andean Patagonia (Argentina): the potential of basal organisms of pelagic food webs in the incorporation of Hg2+ from the dissolved phase

DIEGUEZ, MARÍA DEL CARMEN; RIBEIRO GUEVARA, SERGIO; SOTO CÁRDENAS, CAROLINA; GEREA, MARINA; GARCÍA, PATRICIA E.; MARVIN-DIPASQUALE, MARK; QUEIMALIÑOS, CLAUDIA P.

Edimburgo

Conferencia; 11th International Conference on Mercury as a Global Pollutant; 2013

Patagonia is a global reservoir of pristine freshwater ecosystems. Most of these glacial lakes are located in mountain valleys of the Andes (800 m a.s.l), bear low nutrient and DOM concentrations, attributable to ultra to oligotrophic conditions. Further, moderate to high levels of total mercury (THg) have been measured in water, sediments and biota resident in benthic and pelagic communities. Forest fires and volcanic activity along with atmospheric transport and precipitation are plausible sources of Hg to terrestrial and aquatic ecosystems in this region. Recent local measurements of THg in the atmosphere indicate concentrations ranging from 0.5 to 1.6 ng m-3. Natural water samples from these lakes show that less than 3% of the THg pool is MeHg and the remainder is presumably dominated by inorganic Hg2+. These results suggest that, compared to MeHg, inorganic Hg2+ in the dissolved phase is at greater concentration for incorporation at the base of the pelagic food web. In this investigation we analyzed experimentally: a- the uptake of Hg2+ by picoplankton (0.2-2.7 µm), pico and nanoplankton (0.2-20 µm) and microplankton (20-50 µm) in natural water from four Patagonian lakes and, b- the effect of natural lake DOM concentration on the incorporation of Hg2+ by the picocyanobacteria Synechococcus. Our main hypotheses were that the Hg2+ incorporation of the different planktonic size fractions is related to a certain surface index that ultimately determines the potential for Hg2+ incorporation. Hg2+ incorporation in basal organisms is mediated by natural lake DOM. The experimental set up consisted inreplicated batch incubations of fractionated natural plankton of each of the four lakes inoculated with the radioisotope 197Hg2+ to a final concentration ranging from 10 to 12 ng L-1; and ii-replicated incubations of Synechococcus (106 cell mL-1) in water from the four lakes and dilutions from surface and bottom water from one lake inoculated with 197Hg2+. For each experimental set up, the incorporation of Hg2+ concentrations in water, organisms and filtrates were measured through the activity of 197Hg2+ in the samples, comparing them to the standards measured in the same geometry with a HPGe detector. Results showed that heterotrophic and autotrophic bacteria, nanoflagellates and small algae (0.2 and 20 µm), constitute the main entry points of Hg2+ in the pelagic compartment due to their high surface indexes which were a key factor mediating the extent of Hg2+ adsorption. However, the uptake of Hg2+ by these organisms was ultimately controlled by DOM quantity and quality. In this line, the incorporation of Hg2+ by Synechococcus increased exponentially with the dilution of water, thus supporting the idea that DOM concentration dampens the availability of Hg2+ for basal organisms.