The Role of Patterns of Urban Development in Stream Benthic Index of Biotic Integrity Score

Abstract

The "Urban Stream Syndrome" describes the consistent, negative changes that streams in urbanized watersheds experience. Increasing numbers of people living in urban areas around the world imply increasing pressure on stream health. Understanding the mechanisms that cause stream degradation and the factors that can mitigate the impact of urbanization are more important than ever. Here, existing data on an index of macroinvertebrate communities, the Benthic Index of Biotic Integrity (BIBI), were used to assess stream health in twenty-two moderately-urbanized drainage basins (13 to 15 percent impervious area or ~ 30% urban land cover) in the Puget Sound lowlands. Land cover patterns in the basin and the riparian area (100 m) were calculated using Fragstats v4 and correlated with stream BIBI score and the metrics that comprise the score. Infrastructure intensity (road density, number of road crossings, and number of stormwater outfalls) was also correlated with these response variables. A subset of streams - the four highest and four lowest-scoring streams - were visited to collect additional habitat data (sediment size, large woody debris, and channels structure). No landscape metrics were correlated with BIBI score. However, patch density of urban land was weakly positively correlated with intolerant taxa richness, while number of road crossings and number of stormwater outfalls were weakly negatively correlated with intolerant taxa richness. No macroinvertebrate metrics were related to land cover in the riparian area. The variability in BIBI score remained unexplained by any metric in this study, and it is possible that BIBI scores in this study were within the natural range of variation of the BIBI. This index may only be able to provide a coarse level of information on stream condition. The results of this study imply that more fragmented urban land is associated with higher numbers of intolerant taxa. Fewer road crossings and fewer stormwater outfalls are also associated with more intolerant taxa. Exploring these relationships further and with other measure of stream condition is recommended.