Periphyton algal community along a gradient of pH: Agrio River from pH 2 to 7

BAFFICO, G.; MAGES, M.; PEDROZO, F.

Caviahue, Argentina

Workshop; International Workshop Geogenically Acidic Water Systems: Volcanic Waters, Mining Lakes and Rivers; 2004

CRUB - UNComahue

The periphyton structure was surveyed along an acid-neutral gradient in a natural acidic river (Agrio River, Neuquén, Argentina) and its tributaries. In each sampling site temperature, pH and conductivity were measured and water samples were collected for chemical analysis. Periphyton samples were taken by mean of a sampler of known surface (10 cm2) in order to determine algal biomass (chlorophyll a concentration), algal density (Utermöhl counts) and species diversity. Results showed that pH increased and conductivity decreased as the Agrio River receives tributaries of different chemical characteristics. In the Upper Agrio River changes were slight as a consequence of Jara River entrance, but in the Lower Agrio River changes were more marked because of Trolope and mainly Ñorquín entrance. Jara River is a small tributary with low pH and conductivity that produce a slight change in Agrio River. Trolope River is a freshwater, neutral river of flow comparable to Agrio River at this point. The effects on Agrio River of this freshwater incoming were a slight increase in pH and decrease in conductivity, as well as precipitation of a red material on the stones of the channel. Periphyton chlorophyll and density increase markedly at this point. Ñorquín River is an alkaline river of minor flow compared with Agrio River. But the effects of this incoming on Agrio River were a marked increase in pH and decrease in conductivity, as well as precipitation of a grey-white material on the stones of the channel. Periphyton chlorophyll and density decreased markedly at this point. Some selected elements (SO4, Cl, Ca, Fe, Al and TP) change along the Agrio River. In general, there are a dilution along the river, but in some cases a precipitation of a certain element occur as a consequence of the pH increase. For example, Fe decrease markedly between RA Outflow (16.8 mg L-1) and RA Salto (Trolope River entrance) (6.2 mg L-1). The Iron concentration continues decreasing until RA Ruta 21 (0.2 mg L-1). Red precipitates are present along this portion of the river. A similar behaviour is observed in the Al concentration. In the Lower Agrio River, is evident a sequence in the dominance of the buffer system from the outflow to the place where the waters are neutralized.