Down by the riverside: urban riparian ecology

many landscapes (Naiman and Decamps 1997; Ewel et al. 2001; McClain et al. 2003). Because they occur at the land–water junction alongside natural waterways, they form unique ecosystems that act as “buffer zones” between uplands and streams (Hill 1996; Lowrance 1998). Research on riparian ecology has been diverse, and includes analysis of the habitat requirements of rare plant and animal species, prevention of pollutant movement from uplands into streams, and the regulation of stream temperature and physical structure (Sweeney 1992; Cirmo and McDonnell 1997). Human influences are also pronounced in riparian zones, ranging from extreme manipulation for transportation, energy generation, and agricultural needs to a desire to maintain the ecological functions listed above. The vast majority of riparian ecological research has concentrated on agricultural and forested watersheds, and there has been particular interest in the ability of riparian zones to prevent the movement of nitrate (NO3 ) from agricultural uplands into coastal waters (Lowrance et al. 1997; Mitsch et al. 2001). Nitrate, a by-product of fertilizer use, human waste treatment, and fossil fuel combustion, is a highly mobile ion and is the most common groundwater pollutant in the US (USEPA 1990; Boyer et al. 2002). Coastal (salt) waters are especially susceptible to NO3 induced eutrophication, the excessive growth and decomposition of algae that leads to low oxygen levels in bottom waters (Diaz 2001). In this paper we review the effects of urbanization on the structure and function of riparian ecosystems. Urbanization is one of the most dramatic and dynamic global human alterations of ecosystems (Grimm et al. 2000; Pickett et al. 2001). The most marked effects are on hydrology and streams, with impervious surfaces altering the rates and pathways of water movement into, through, and out of ecosystems (Arnold and Gibbons 1996; Paul and Meyer 2001). Although there have been many studies on the effects of urban hydrologic changes on the physical, chemical, and biological properties of streams, very few have addressed riparian ecosystems. While several authors have suggested that urbanization leads to “disconnection from and loss of floodplains” (Riley 1998; Booth et al. 2001; CWP 2003), there have been no systematic analyses of effects on riparian ecosystem structure and function. Much of the research that we review in this paper comes from the Baltimore Ecosystem Study (BES) (http://beslter.org), one of two urban study sites in the US 315