Mercury in aquatic environments of Nahuel Huapi National Park, Patagonia, Argentina

Full text: Hg contents were determined in abiotic and biotic compartments of five lakes of Nahuel Huapi National Park, Patagonia, Argentina. Historically, this area was mostly protected from anthropogenic heavy metal contamination due to its remoteness, low population density and access difficulty. National Parks within the region also provided additional protection for whole or specific parts of particular watersheds. Even though no local source was identified, rather high Hg concentrations were measured in suspended load collected from lake Nahuel Huapi in previous work. The water bodies studied were lakes Nahuel Huapi, Moreno, Escondido, Espejo Chico, and Traful, the latter chosen as a reference lake. Hg concentration profiles of short sediment cores, dated by 210Pb and 137Cs techniques, were analyzed by INAA, as well as suspended load collected from three sites of lake Nahuel Huapi. Water samples were collected from the surface (1-3 m depth) and at 30 m depth (1m above the sediment) at lake Nahuel Huapi, and Hg concentrations were determined in unfiltered samples by CVAFS. The biota studied was the native mussel Dip lodon chilensis (digestive gland and total soft tissues pooled samples) and five species of fish, two native and three introduced (liver and muscle pooled samples). Hg contents were determined by INAA. Hg concentration profiles of sediment cores sampled from sites with higher productivity (OM contents of upper most layer ranging from 17 to 24%) showed a peak at 1 to 2 cm below lake bottom, with concentration values ranging from 0.6 to 2.8 mg.g-1, revealing modifications of Hg inputs in recent times. The water body studied with lower productivity (6 % organic matter contents) does not show this peak in the concentration profiles, and Hg contents range from 0.2 to 0.3 μg.g-1 in upper core layers. Two background levels were determined from deep layers measurements; the lower associated with pre- industrial time according to core dating, ranging from less than 0.07 to 0.2 μg.g-1, and the other associated with modern times, ranging from 0.1 to 0.3 μg.g-1. Hg flux to core sediments was computed when relevant enrichment in upper core layers over background was observed. Hg fluxes to core sediments ranged from 30 to 110 μg.m-2.y-1. Hg contents of suspended load ranged from 1 to 7 μg.g-1. Hg contents of water samples from lake Nahuel Huapi were higher near Bariloche city (0.1 to 1 μg.l-1) most relevant human settlement in the region with 100,000 inhabitants, and near La Angostura village (up to 0.6 μg.l-1), while other lake sites ranged from less than 0.01 to 0.2 μg.l-1 suggested that human activities in the region are a local source of Hg to the aquatic environment. Hg highest concentration values measured in total soft tissues pooled samples from lakes Nahuel Huapi and Moreno, were found to be similar to those observed in some Hg contamination situations, ranging from 0.5 to 0.9 μg.g-1 dw, and 3 to 5 times higher than reference samples collected in lake Traful. No correlation was observed between Hg concentration in digestive gland and total soft tissues. Hg in total soft tissues is considered to be more representative of Hg mussel content distribution in the lakes studied. Hg contents in fish liver and muscle tissues are higher than those expected for pristine lakes, however the values are not unusual in remote lakes (Hg concentration in liver pooled samples ranged from 0.06 to 4 μg.g-1 dw, and concentrations in muscle ranged from 0.07 to 2.5 μg.g-1 dw). The most remarkable result is that no bio-magnification phenomena were observed. (author)