Watershed urbanization and ecological thresholds in streams: influences of physiography, watershed size and spatial arrangement of impervious cover.
King, Ryan*,1, Baker, Matthew2, Whigham, Dennis 2, Weller, Donald2, Kazyak, Paul3, Hurd, Martin3, 1 Center for Reservoir and Aquatic Systems Research, Waco, TX, USA2 Smithsonian Environmental Research Center, Edgewater, MD, USA3 Maryland Department of Natural Resources, Annapolis, MD, USA
ABSTRACT- Urbanization of watersheds predictably results in degradation of stream ecosystems. Stream biological assemblages often exhibit a threshold response once a critical level of imperviousness is reached. We used macroinvertebrate assemblage data collected from nearly 2000 streams in the Mountain, Piedmont, and Coastal Plain of Maryland (USA) and estimated the percentage of impervious cover that resulted in ecological thresholds among these physiographic provinces and three watershed size-classes. We also evaluated the effect of spatial arrangement on thresholds by weighting impervious cover by its inverse distance from the sampling station and stream, a technique that weighted land cover close to a feature of interest more heavily than cover located farther away. Results from changepoint analysis revealed that the percentage of impervious cover in a watershed that resulted in a threshold was lowest in the high-gradient Mountain province and highest in low-gradient streams of the Coastal Plain. Within provinces, thresholds varied by watershed size-class, with lower threshold levels of imperviousness in large than in small watersheds. Spatial proximity of impervious cover to sampling stations also influenced thresholds, but had a greater influence in small watersheds. Here, apparently higher thresholds observed using watershed-scale percent impervious cover were mostly caused by low levels of urbanization near stations. Collectively, these results imply that the effect of urbanization can be highly dependent upon many spatial factors and that there is no universal critical level of percent impervious cover for predicting ecological thresholds in streams.
Key words: ecological thresholds, watersheds, urbanization, streams
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