Urbanization and stream ecosystems: Predictable responses and inevitable socioeconomic conflict

Urbanization is a pervasive, increasing form of human land use, and more than half of the world’s population now lives in urban areas.  Urban growth typically includes human population concentration, specialized labor, and changes in political and human institutions.   It also requires a subsidy of energy and materials to maintain. This intensification of population comes with a predictable impact on natural systems, including streams.  All aspects of stream ecosystems are altered from their non-urban state as a result of urbanization physically, chemically, and biologically, from ecosystems to individual organisms.  To date, little evidence suggests that natural processes and community structure can be maintained in urbanized systems.  Metabolism, nutrient cycling, and decomposition are core ecosystem processes and all exhibit altered rates in urban streams.  Similarly, algal, invertebrate and fish assemblages exhibit almost universally altered diversity and composition.  In the presence of these effects, the US Clean Water Act requires a decision: rehabilitate these systems to their intended aquatic life uses or acceptance that these uses are no longer possible in these systems.  This process pits socioeconomic desire against ecological condition and will continue to play out across the US as urban systems spread or until technology improves rehabilitation.