Effects of urbanization on nutrient biogeochemistry of aridland streams

Land-use and land-cover change affect the biogeochemistry of stream ecosystems in numerous ways, both direct and indirect. Changes result from hydrologic modifications, including direct alterations of flow regimes and hydrologic flowpaths and indirect changes in hydrologic patterns via increased impervious cover in contributing areas of watersheds. Direct changes to channel morphology (i.e., reduced complexity) and to floodplains of streams and rivers also influence biogeochemistry, for example, by eliminating surface water–groundwater exchange. The nature of and strength of connections between the stream and its watershed may be altered by large-scale changes such as those brought about by urban and suburban development. Finally, in urban and agricultural areas, elevated nutrient loading is exacerbated by land-cover changes that increase the potential for erosion or overland flow, and decrease the opportunities for nutrient retention. Using comparative analysis based on published work, mined public data, and new research, we evaluate urbanization effects on stream ecosystems in the Sonoran Desert region of central Arizona. Five key characteristics of non-urban desert streams—nitrogen limitation, a flashy hydrologic regime that initiates succession, extensive groundwater-surface water interaction, episodic terrestrial–aquatic interactions, and high capacity for nutrient retention—are both dramatically altered and scarcely affected by urbanization. The similarities exhibited by aridland streams and their urban counterparts arise from large-scale constraints (e.g., episodic terrestrial-aquatic interaction is imposed by the climatic regime in both stream types), whereas the differences, like interrupted flowpath continuity in the urban landscape, likely result from the myriad direct modifications of streams and catchment land cover in cities.