When do macroinvertebrate communities of reference streams resemble urban streams? The biological relevance of Q critical

• Published in: Freshwater science Vol. 35; no. 3; pp. 778 - 794
• Main Authors: Hawley, Robert J., Wooten, Matthew S., MacMannis, Katherine R., Fet, Elizabeth V.
• Format: Journal Article
• Language: English
• Published: Society for Freshwater Science (SFS) 01.09.2016
• DOI: 10.1086/687808
• ISSN: 2161-9549

The threshold discharge for streambed mobilization (Q critical) has been proposed as a mechanistically relevant management target for geomorphic stability and biological integrity. The geomorphic relevance of Q critical (Q c)is well established, but its influence on biological communities is less documented. In urban watersheds, where increased frequency of Qc exceedance is nearly ubiquitous (i.e., the urban disturbance regime), excessive streambed disturbance typically co-occurs with other well-established drivers of the urban stream syndrome, such as habitat degradation and poor water quality. Thus, the specific influence of excess streambed disturbance on aquatic communities is not clear. We used a 7-y study of biotic integrity, geomorphic stability, and Q c exceedance at a reference site (Middle Creek [MDC 5.5]) with excellent habitat and water quality to isolate the effects of streambed disturbance. At MDC 5.5, streambed disturbance was the dominant driver of biotic variability through time, and biological indices and geomorphic stability were significantly correlated with time since a Q c event. During 2011, a year with particularly high frequency and magnitude of Qc events, the Macroinvertebrate Biotic Index (MBI) at MDC 5.5 fell to its lowest score on record. In the context of 73 monitoring sites across a gradient of urbanization, the 2011 MDC 5.5 MBI of 30 (poor) was more similar to that of communities in streams draining watersheds with ∼30% total impervious area than to reference-stream scores (∼60) during more typical sampling years. Our study underscores the contribution of excess Q c exceedance to poor biological communities. We suggest calibrating stormwater management to maintain the natural streambed disturbance regime in addition to the more common management objectives of water quality and flood control.