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Jerry McMahon: Over the last 10 years the US Geological Survey’s National Water Quality Assessment Program has examined the effects of urban development on stream ecosystems. In this study, the two primary objectives were first of all to look at the physical, chemical and biological responses of streams to urban development. And also, to look at how those responses varied across the country. OK, our first finding was that all three biological communities we examined, algae, invertebrates and fish responded to urban development. Urban development significantly affected one or more biological communities in eight of the nine metropolitan study areas with Denver being the exception.
In Portland, which is shown in this slide, all three communities responded negatively to urban development. Because invertebrate communities showed the strongest and most consistent response to urban development across the country, we will focus on the response of macroinvertebrate. The second finding is that declines in aquatic insect communities are noted at the early stages of urban development. Unlike the hypothesized response, even small levels of urban development had an immediate negative effect on aquatic insects. There’s no period of resistance to the effects of urban development. This is illustrated by the immediate decline in the aquatic insect community composition as the level of urban development in the Boston study watersheds increases. This response is continuous over the entire range of urban development. The aquatic insect communities studied in the Boston watersheds never reach a state of exhaustion.
Our third finding is that urban development is often accompanied by a loss of pollution sensitive species and a shift toward communities that are dominated by pollution tolerance species. EPT Richness represents the sum of sensitive insect species in the order of Ephemeroptera - mayflies, Plecoptera – stoneflies and Trichoptera – caddisfly. This figure indicates the difference in the number of sensitive EPT invertebrate species found in watersheds at the high end of the urban gradient and at the low end. A substantial decline in EPT species richness occurred in all but the Denver, Dallas and Milwaukee study areas. Measures of EPT Richness are commonly used to assess biological condition of stream in state biomonitoring program.
A biological community that includes sensitive species is often an indication of healthy stream ecosystem and therefore changes in the presence of sensitive species provides useful information about the biological condition of the stream. Our fourth finding is that important regional differences existed in the types of land cover that were being converted to urban uses. Across the nine regions of the country that were studied, urban development occurred primarily through the conversion of either agricultural or forested lands. Forest is the dominant pre-urban development land cover in Portland, Salt Lake City, Birmingham, Atlanta, Raleigh and Boston; whereas, in the other three study areas land being converted to urban uses is associated with some form of agricultural activity. The characteristics and activities associated with these two land cover types may mask or escalate the influence of urban development on stream ecosystems.
For example, although agricultural practices have evolved dramatically in the last hundred years, nutrient enrichment, soil erosion, monocultural practices and the loss of natural habitat are still major concerns. Watersheds where the predominant pre-urban development land cover is agricultural land already have some degree of water quality impairment prior to the urbanization that can obscure the effects of urban development. This may help explain the very small loss of sensitive taxa in areas where urban development occurs on land that previously was in agricultural uses. Sensitive species had already been lost before urban development occurred. So how can this information be used?
The information we’ve developed on the response of stream ecosystems to urban development will help urban planners and other stakeholders clarify the most appropriate strategies in managing, protecting and restoring urban streams. Two findings are of particular importance. First, the fact that macroinvertebrate community conditions starts degrading almost immediately once urban development begins suggests that a great deal of caution should be exercised in thinking that there is a safe zone of urban development at least for a stream’s macro invertebrate communities. Second, we now know that streams in different regions of the country respond differently to urban development. This is due to regional differences in the overall template of factors that affects stream ecosystems such as climate and the types of land that are being developed for urban uses. These regional factors also affect the response of hydrology, habitat, water chemistry and stream biota. Management approaches have to be shaped by an understanding of how this regional template helps determine what is possible to achieve in terms of water quality, criteria and standing.
For additional information about our project, please visit the project website where you can obtain both reports as well as the data used in the project. I’m Jerry McMahon and on behalf of all my colleagues, I’d like to thank you for your interest in understanding the effects of urban development on stream ecosystems.
Title: The Effects of Urbanization on Stream Ecosystems (extended) Part III: Study Results
Part three of a three part series on how development can have negative effects on streams in urban and suburban areas. As a watershed becomes covered with pavement, sidewalks, and other types of urban land cover, stream organisms are confronted with an increased volume of storm water runoff, increased exposure to fertilizers and pesticides, and dramatic changes in physical living spaces within the stream itself.
Location: Raleigh, NC, USA
Date Taken: 6/3/2010
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