the río agrio-lago caviahue system which constitutes a unique case in south america of waters of extreme acidity (ph<4), has been studied during the last 10 years. the high acidity is originated by the magmatic effusions of the volcano copahue which feeds the sources of the río agrio. the water system neutralizes after crossing an extension of 50 km. this situation defines a natural gradient of acidity, concentration and transport of nutrients and metals. the system shows low species diversity in the lacustrine plankton as well as in the stream epilithon. the low ph allows the dissolution of the majority of the present elements, determining environments rich in metals and phosphorus. unlike other andean lakes of the patagonia, the lake caviahue experiments short periods of variations in ph and conductivity, situation that is controlled by the volcanic activity. the sediments of the lake are rich in organic matter (om) being the om who controls the phosphorus availability.the extreme conditions of the lake, the ammonium and dissolved inorganic and organic carbon concentrations maintain a high planktonic density, mainly represented by keratococcus raphidioides. the replacement of algal species (seasonal succession) not occurs in this lake and the small changes observed are related to the geochemistry of the basin. the epilithic biomass along the río agrio inferior exhibits a strong distribution linked to the changes in ph. the conspicuous algae developments are mainly composed by filamentous green algae (e.g., ulothrix spp.).