Beyond the urban stream syndrome: Understanding the ecology IN, OF, and FOR sustainable urban stream ecosystems
A recent addition to paradigms in stream ecology is the “urban stream syndrome,” which suggests that hydrological changes associated with impervious surfaces in cities drive the loss of ecological integrity. Here, we synthesize findings from investigations in urban streams around the U.S. that encompass a wide range of city sizes and biogeoclimatic contexts. This synthesis provides new insights to further explore the ecology in, of, and for developing approaches for sustainable streams.
Our data suggest the following three conclusions about urban stream ecosystems that build upon the framework proposed in the Urban Stream Syndrome. 1) Although urbanization causes predictable declines in the diversity of macrofauna, we find no evidence that urbanization degrades microbial diversity, including algae and bacteria. Across the geographic gradient the microbial communities are as diverse in highly urbanized streams as less urbanized streams. Although conditions in the highly urbanized streams are “stressful” to invertebrates and fishes, microbial communities with fast generation times can develop resistance, and can tolerate urban conditions. These findings have implications for the biodiversity in urban streams and the capacity of urban streams to provide microbially-mediated ecosystem services. 2) There is variation around the mean response of streams to urbanization that has been inadequately explored. Our data suggest that the "plumbing" of the urban network may be more important than the coverage of impervious surfaces in the waters. In a comparison of 20 similarly developed (10% impervious surface cover) watersheds in the Raleigh-Durham metropolitan area, NC, the configuration and connection of pavement and roofs to urban streams can generate as much variation in stream hydrographs and chemical enrichment as observed across the full urbanization gradient. In arid western cities under investigate, water diversion for agricultural use and hazard protection are the strongest factors degrading water quality, driving dynamics of ecosystem metabolism, and simplifying diversity of native animals in the river segments flowing through urban areas. Reducing impervious surface coverage to improve water quality may not be adequate in light of these findings. 3) Urban streams differ greatly in their capacity to produce, retain and process C. Our data suggests that urban streams are producers, reactors or transporters of carbon depending on their spatial configuration, hydrologic connections and geologic setting. Combined, these three conclusions have implications for how urbanization might impact ecological processes and how these systems might be better managed to promote future sustainability of urban stream ecosystems.