Riparian Buffer Zone and Whole Watershed Influences on River Water Quality: Implications for Ecosystem Services near Megacities

Water quality is one of the most altered ecosystem services in watersheds at the periphery of megacities, where point sources and non-point sources of pollution derived from domestic and agricultural activities are in many cases not well regulated. Despite widespread needs for information, riparian ecosystem service management of megacities in developing countries is largely absent from the literature. To begin to understand connections between surface water quality and landscape factors (i.e. land use and topography), we examined physicochemical and biological properties of water quality at 37 sub-watersheds of the Guadalupe Dam Watershed (GDW) that lies to the northwest of Mexico City. The influences on water quality of sub-watershed versus riparian zone at 50, 100 and 200 m were compared. We applied multiple linear regressions and redundancy analysis. The latter was also used to partition the variance explained by topography (i.e. elevation and slope), land use, and their combined variation. Our results showed that elevation had the greatest influence on water quality, likely because it represents a gradient of urban influence and consequently environmental pollution. Agriculture was not a dominant predictor of water quality, and its direct impacts were somewhat masked given the interaction with other land uses and landscape factors. The shared variation of topography and land use exerted more influence on water quality than either alone, showing that topographic factors constrain certain human activities in the area. Landscape factors accounted for more variation (52 %) at 100 m of riparian zone, giving new information important for the management of riparian ecosystems. Our findings are a step forward in the understanding of the effects of human activities and natural landscape features in providing water of good quality. They will also assist natural resource managers in the challenge of managing water-related ecosystem services through scientifically-based management actions and policies, a gap faced by regulatory authorities of megacities in the developing world.