Unintended Consequences of Urbanization for Aquatic Ecosystems: A Case Study from the Arizona Desert

H and geomorphology are important determinants of the structure and function of fluvial ecosystems. Streamflow quantity and timing are often viewed as “master variables” limiting the distribution and abundance of riverine species (Poff et al. 1997) and the flux of nutrients into streams (Grimm 1987, Welter et al. 2005). Geomorphic structure—particularly channel form—is known to influence community composition (Vannote et al. 1980) and trophic interactions (Doyle 2006), as well as ecosystem processes (Alexander et al. 2000).Nutrient dynamics are sensitive to a stream’s fluvial geomorphology (Valett et al. 1994, Doyle and Stanley 2006), because as materials move across a landscape or through a stream, certain locations or“hotspots” account for a disproportionate amount of nutrient removal or processing, and these hotspots are linked to geomorphic structure (Peterjohn and Correll 1984, McClain et al. 2003). The spatial arrangement of and connections among hotspots influence how materials move between patches, the relative availability of materials to different patches, and the export of materials to adjacent ecosystems. The origin, development, and persistence of these patches as well as their interactions are largely under geomorphic control (McAuliffe 1994). At large scales, work on lake districts (Webster et al. 2000) and lake chains (Kling et al. 2000) has demonstrated that lake chemistry is affected both by the strength of hydrologic linkages and by the type of flowpaths between lakes (i.e., whether they are connected via surface streams or subsurface flows). At smaller scales, research on desert streams has shown that nitrogen (N) cycling is affected by vertical hydrologic exchange between interstitial hyporheic flows and surface waters, which link subsurface patches with high nitrification rates to surface patches with high algal growth and high N-uptake rates (Holmes et al. 1994, Valett et al. 1994). Indeed, it is becoming increasingly clear that stream ecosystems can be understood only in light of their geomorphology and hydrology (Doyle and Stanley 2006).