Land-use change versus natural controls on stream water chemistry in the Subandean Amazon, Peru

Abstract

Abstract This study evaluates the effects of deforestation and land-use change, as compared to natural controls, on stream water chemistry in the Subandean Amazon. Dissolved major and trace elements were determined near the stream outlet of 48 independent watersheds with varying morphology, bed rock composition and intactness of forest cover (pristine to highly exploited). Geomorphological characteristics were derived from a digital elevation model, geological formations from digitalized maps and forest cover from digital classification of SPOT satellite images. Partial least square regression and multiple linear regression showed that watershed average elevation, which ranged between 396 and 1649 m, was the strongest control on stream water chemistry, explaining >70% of the variation in K and a considerable part also for Mn, U, Mg and HCO 3 with near exponential concentration increases down the altitude gradient. Forest cover, which ranged between 7% and 99%, correlated strongly with average elevation (Spearman correlation coefficient, r s = 0.8), but had no statistically significant impact on stream solute concentrations. Thus, in the studied Subandean region, watershed scale deforestation has not resulted in measurable impacts on stream water chemistry which is dominated by the spatial variation in natural controls.