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

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

Caviahue, Argentina

Workshop; 6th Meeting of the IAVCEI Committee on volcanic lakes (CVL); 2004

The pH often is understood as the main variable that affects the biota of acidic waterbodies. However, the pH is a key factor controlling many other physical and chemical factors. In general, the decrease of the pH alters the species composition of attached algae because only are present acid-resistant species. In some acidic habitats, the high metal concentrations in the water seems to limit algal development due to toxicity. The ecological effects of the deposition of metal hydroxides has been less studied than the toxicity by metals or by the acidity. The deposition of Iron or Aluminium hydroxides affect the periphyton and invertebrates more than the pH or the dissolved metals (McKnight & Feder, 1984). It was also noted that metal hydroxide deposition could change the physical conditions of the substrate, leading to a reduction of periphyton growth on rocks. The periphyton structure was surveyed along an acid-neutral gradient in a natural acidic river (Agrio River, Neuquén, Argentina). In each sampling site temperature, pH and conductivity were measured and water samples were collected for chemical analysis. Periphyton samples were taken 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 (Trolope and mainly Ñorquín 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. The dominant algae species present in the periphyton were Ulothrix spp and Klebsormidium sp, while 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. Despite that the dominant algae species present in the periphyton still were Ulothrix spp and Klebsormidium sp, chlorophyll and density decreased markedly at this point as well as diversity. Some selected elements (S, Cl, Ca, Fe, Al and P) 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 (6.2 mg L-1), because Trolope River entrance. 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.