Biogeochemistry of the volcanic acidic waters at the Andean Volcan Copahue (Argentina)

WENDT-POTTHOFF, K.; BAFFICO, G.; KOSCHORRECK, M.

Colonia

Congreso; Goldschmidt 2007; 2007

The naturally acidic water system near the active Volcan Copahue consists of the Copahue Crater Lake, the Upper Rio Agrio which flows into Lake Caviahue, and the Lower Rio Agrio below Lake Caviahue. The crater lake and most of the Upper Rio Agrio are extremely acidic (pH = 0-1.6) with oxidizing conditions and poor colonization, mostly by well-known acidophilic bacteria and algae. In the lower part of the system before inflow into Lake Caviahue, microbial biomass, oxygen penetration and oxygen consumption in the sediments were similar to non-extreme aquatic habitats [1]. Lake Caviahue has a pH similar to acidic pit lakes (pH 2.2 - 2.7), but lower iron and higher phosphorus concentrations [2]. Sedimentation of algae is higher than in acidic pit lakes due to different algal communities and volcanic ash input. This results in better degradable organic carbon compounds in the sediments compared to pit lakes. Microbial sulfate reduction proceeds in Lake Caviahue sediments at pH<3 [3], and methanogenesis occurs in deeper layers below 25 cm. Below Lake Caviahue, the Lower Rio Agrio is gradually diluted and neutralized by tributaries, thereby shifting from a chemotrophic system dominated by Fe-S-Al biogeochemistry to an organotrophic system dominated by heterotrophic processes. Epilithic biofilms are an important habitat throughout, and we assumed that respiration would increase with increasing pH. In contrast to this, we obtained a minimum curve for biomass, respiration and photosynthesis with lowest values at a site with massive mineral precipitations. This indicates that physical stress is a more important regulatory factor for the biocoenosis than pH. We also observed considerable interannual variations in physicochemical conditions and high diurnal fluctuations of temperature and discharge in this part of the river. These fluctuations may represent a stronger stress for the biota than more acidic but stable conditions.References[1] Wendt-Potthoff K. and Koschorreck M. (2002) Microb. Ecol. 43, 92-106.[2] Pedrozo F. et al. (2001) Hydrobiologia 452, 129-137.[3] Koschorreck M., Wendt-Potthoff K., and Geller W. (2003) Environ. Sci. Technol. 37, 1159-1162.