Not all pavements lead to streams: Variation in impervious surface connectivity affects urban stream ecosystems

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Watershed urbanization leads to chemical and thermal pollution of urban streams and significant declines in aquatic biodiversity. Most investigators have focused on variation in total watershed impervious surface cover (ISC) as the primary driver of urban stream degradation. We asked instead whether the degree of connectivity between ISC and urban stream channels alters its effect. We compared 7 streams in the Raleigh–Durham metropolitan area of the southeastern USA that drained watersheds with similar amounts of pavement (ISC 5 7–16% of watershed area) but spanning a wide range of hydrologic connectivity between these pavements and their receiving streams via both subsurface (pipe density range: 1.1–6.9 km/km2) and surface (road density range: 5.8–10.7 km/ km2) flowpaths. Despite draining watersheds with similarly low levels of development, these 7 streams exhibited remarkable variability in their hydrologic and thermal regimes and varied in their macroinvertebrate diversity from a low of only 11 taxa to a high of 22. Both macroinvertebrate community composition and the tissue concentrations of Cu, Pb, and Zn in 3 stream invertebrate taxa (Cambaridae, Tipulidae, and Hydropsychidae) found across all sites were correlated with watershed hydrologic connectivity. These results suggest that the connectivity of ISC may drive considerable variation in the magnitude of ecosystem degradation associated with the same level of watershed development, with less connected or disconnected impervious surfaces having proportionally lower negative effects on aquatic organisms.

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