Hydrologic disturbance reduces biological integrity in urban streams

The adverse effects of urbanization on lotic ecosystems (degradation of chemical water quality, physical habitat, and biological communities) are linked by the interaction of land use practices and land cover changes (increased impervious area) to the alteration of watershed hydrology, the natural flow regime of urban streams, and physical disruption of stream biota and their habitats. Seasonal floods are part of the natural disturbance regime of many streams, but urbanization increases the frequency and magnitude of these pulse disturbances. This study evaluated the impact of hydrologic disturbance on fish and macroinvertebrates in 81 Ohio streams (56 urban and 25 reference streams). Hydrologic variables included annual and monthly 24-hour rainfall maxima and computed annual peak discharge. Estimation of annual peak discharge involved GIS-based drainage area delineation and land cover characterization (using National Land Cover Data, NLCD, from the early 1990s), followed by computation of runoff and discharge. Ohio EPA biological parameters (the Index of Biotic Integrity, IBI, the modified Index of Well-being, M_Iwb, and their component metrics for fish, and the Invertebrate Community Index, ICI, and component metrics for macroinvertebrates) were negatively impacted by annual peak discharge in urban streams as compared to reference streams. For fish communities in urban streams, community attributes such as IBI and M_Iwb, species richness, relative abundance, and number of sculpin and darter species were negatively impacted, while abundance of tolerant and pioneering fish species, disturbance-indicators, increased with discharge magnitude. For macroinvertebrates, community attributes including ICI and taxa richness were negatively impacted. Study results support the application of runoff/discharge management practices (BMPs) as part of urban stream restoration efforts to mitigate hydrologic disturbance impacts to fish and macroinvertebrates. The method presented here could be applied to all Ohio EPA stream survey sites and associated biological data for which corresponding 24-hour rainfall data are available and for which land use characterization from early 1990s Landsat satellite imagery is valid.