Limnetica 39

Discharge (Q) is an essential variable to understand how fluvial ecosystems function. To this aim, we assessed the hydrological and physico-chemical dynamics of two contrasting streams in Andean Patagonia: Casa de Piedra (CP) and Gutiérrez (G). CP originates in a small lake (0.15 km²) situated at high-elevation, whereas the source of G is a large (17 km²) piedmont lake. There are other differences between the drainage basins of these streams: CP drainage basin covers 63 km² and slopes steep (33.9 m/km), while that of G is bigger (162 km²) and gentler (5.9 m/km). The current research was carried out over a period of 1 year. Variables measured were precipitation and temperature, as well as hydrological data (Q = discharge, EC = electrical conductivity, water temperature, turbidity and pH). The climate and topography of the region led to 3 distinct hydrological periods: stormflow, meltflow and baseflow. Discharge presented a clear seasonal pattern with higher values at meltflow, due to snow melting from the mountains. Stream CP was very flashy (high variation in flow regime), unlike stream G. The greatest flashiness was observed during the stormflow period. Baseflow was characterised by low values of Q. The temperature of water fluctuated between the different hydrological periods and between the study streams, whereas pH varied seasonally only. Great turbidity in the streams was observed during the stormflow period, as a result of high run-off. Throughout the year, the relation between Q and EC in CP revealed different hydrological flowpaths towards the stream: flows derived from melting snow, lateral flows through the landscape and groundwater flows. In contrast, EC in stream G was high and constant during the whole recording period, independently of Q. This indicates that the large headwater lake is its main source of water. This lake clearly affected the physico-chemical dynamics of stream G. In brief, the current research brought new knowledge into the ecological aspects of hydrological processes acting on the Andean-Patagonian aquatic ecosystems. On the basis of the results presented here and on expected future trends, we believe that the hydrological and physico-chemical dynamics of these ecosystems will be highly affected by climate change.