Environmental drivers of Mercury patterns in two connected oligotrophic lakes of Andean Patagonia (Argentina).

DIEGUEZ, MARÍA DEL CARMEN; FERNANDEZ, ZAIDA; ARCAGNI, MARINA; RIZZO, ANDREA; SOTO CÁRDENAS, CAROLINA

Simposio; Mercury in the Biosphere: Ecological and Geochemical Aspects; 2025

Pristine ecosystems devoid of direct anthropogenic mercury (Hg) sources provide unique models to study the influence of environmental and physicochemical factors on Hg dynamics. In aquatic systems, dissolved organic matter (DOM) has a key role in the biogeochemical cycle of Hg, intervening in its transport, speciation, availability and trophic transfer. Nahuel Huapi National Park (NHNP, Patagonia) is frequently disturbed by volcanic eruptions, and its climate and mountainous forested landscape promote wet and dry Hg deposition. Lower in the landscape, oligotrophic lakes collect the terrestrial runoff enriched with nutrients, C, and Hg. This study focuses on seasonal and spatial dynamics of total Hg concentrations [THg] in two connected piedmont lakes, the shallow flowing lake Pire (ca. 20 m) and the deep branch Brazo Rincón (BR; ca.100 m) of Lake Nahuel Huapi, adjacent to the active volcanic complex Puyehue-Cordón Caulle. Environmental and water physicochemical variables, including DOM and THg, were studied to identify the main drivers of Hg patterns in the lakes at high and low connectivity moments. THg was measured by cold vapour atomic fluorescence spectrometry (CVAFs). DOM sources, quality, and concentration were characterized through optical proxies and dissolved organic carbon concentration [DOC]. The lakes exhibited moderate to high [THg], which increased downward in the landscape, from Lake Pire (1.4-19.4 ng L-1) to BR (1.7-61.5 ng L-1). In the wet season, with higher hydrological connectivity, [THg] were related to terrestrial DOM prints, indicating their co-transport from the catchment. In the dry season, when lakes show a weak connectivity, [THg] was associated with enhanced mineralization (>dissolved inorganic carbon) and diffusion from the sediments, particularly in L. Pire. THg distribution and availability were related to [DOC] and quality, and the total suspended solids (TSS). High [THg] and low [DOC] determine elevated Hg availability in BR (2.81-165 ng mg-1) and Pire (1.14-14.3 ng mg-1). Moreover, the biotic contribution to TSS increased Hg fractionation into particles (BR>Pire). Food webs of Andean-Patagonian lakes have outstanding potential for Hg accumulation, because the moderate/high [THg] at extremely low [DOC] result in high Hg availability, promoting Hg binding to abiotic and biotic particles and its trophic incorporation.