Mercury patterns in lakes within a natural hotspot in the Southern Volcanic Zone of the Andes (Nahuel Huapi National Park, Patagonia, South America)

Abstract

Rationale. Mercury hotspots have been identified in pristine volcanic areas of the northern Patagonian Andes (South America). In this study, we investigated spatial and seasonal patterns of total mercury (THg) in two oligotrophic lakes in Nahuel Huapi National Park (Argentina), adjacent to the Puyehue Cordón Caulle volcanic complex (Chile). We hypothesise that THg levels in the lakes are linked to seasonal terrestrial inputs and that in-lake processes influence its distribution and availability. Methodology. Water samples were collected seasonally in the connected lakes Pire (shallow) and Brazo Rincón branch of L. Nahuel Huapi (BR, deep). The concentration of THg was studied together with physicochemical variables, including the concentration and quality of dissolved organic matter (DOM). THg was measured by cold vapour atomic fluorescence spectrometry (CVAF) and DOM was characterised through absorbtion and fluorescence spectroscopy. Results. Lakes showed moderate to high THg concentrations and remarkable Hg availability. THg increased downwards in the landscape and was associated with DOM terrestrial prints, indicating its co-transport from the catchment during high connectivity periods (from winter to early summer). In summer, THg levels were associated with higher mineralisation (higher dissolved inorganic carbon, DIC) and diffusion from sediments, especially in the shallow lake. THg availability and distribution were related to the quality of the DOM pool and the total suspended solids (TSS) year round. The high biotic contribution to TSS (higher chlorophyll-a:TSS) in BR indicated Hg binding to phytoplankton and incorporation into lake food webs. Discussion. Pristine oligotrophic lakes influenced by natural Hg sources have a high potential for Hg accumulation in food webs because their typically high Hg and low DOC concentrations result in high Hg availability, promoting Hg binding to abiotic and biotic particles.