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Effects of urbanization on small streams(CRCFE project E301: Biological Assessment and Management of Urban streams)This project consisted of two components. The first was a broad survey of macroinvertebrate and diatom community composition in small streams of the Melbourne Region in south-eastern Australia (the subject of this page). The second component involved an experimental assessment of placing artificial riffles (thereby increasing habitat diversity) into degraded, channelized urban streams. This page deals with the first component. Further information on the riffle experiment. The Melbourne metropolitan area is almost completely sewered, with sewage being transported to treatment plants which discharge to the sea. During the twentieth century, Melbourne was served by a powerful Board of Works, which enthusiastically ensured that urban areas were well protected from floods by installing efficient drainage systems which quickly drain stormwater runoff into the streams of the region. The primary urban impacts on streams flowing through the Melbourne metropolitan area therefore most likely result from stormwater drainage. In the surrounding area (the hinterland) there are a number of urban settlements, with expanding populations as Melbourne spreads. These centres tend to be less intensively drained (either localized piping of stormwater, or drainage via open earthen drains), and sewage is either treated by septic tanks or by small sewage treatment plants that drain into some of the study streams. All sites sampled in the metropolitan area supported extremely degraded macroinvertebrate communities, dominated by a small number of species tolerant to pollution. Diatom communities of the metropolitan area were also distinct from those of surrounding areas (the hinterland), although they were no less diverse than many hinterland sites. Variation in community composition among metropolitan sites was not well explained by any environmental variables. In contrast, macroinvertebrate communities of the hinterland were much more diverse, even in catchments with comparable levels of catchment urbanization to some of the metropolitan sites. Among these hinterland sites, community composition was highly correlated to catchment imperviousness (a neutral measure of urban density): sites with higher catchment imperviousness (up to 12% impervious) supported fewer species. Diatoms also showed correlations with catchment imperviousness, but this relationship was complicated by increased nutrient concentrations downstream of sewage treatment plants in the hinterland. (The primary urban-associated environmental factor that explained diatom community composition was concentration of total phosphorus, which itself was highly correlated to some other nutrients.) From this work, we have developed a model of urban stormwater impacts on small stream ecosystems. As a catchment becomes more impervious, impacts arising from the hydrological and polluting effects of stormwater will have an increasingly degrading effect on in-stream communities. However, these impacts are much greater when the impervious areas of a catchment are connected to the streams by efficient stormwater drainage systems. These effects underlie the ideas behind 'Best Management Stormwater Practices', which reduce the connection between impervious surfaces and receiving waters. The project also produced some findings of importance to assessment procedures using community composition. Project teamPeter Breen, project leader Further readingWalsh, C.J., Sharpe, A.K., Breen, P.F. & Sonneman, J.A. (2001). Effects of urbanization on streams of the Melbourne region, Victoria, Australia. I. Benthic macroinvertebrate communities. Freshwater Biology 46(4), 535-551. (abstract) Sonneman, J.A., Walsh, C.J., Breen, P.F. & Sharpe, A.K. (2001). Effects of urbanization on streams of the Melbourne region, Victoria, Australia. II. Benthic diatom communities. Freshwater Biology 46(4), 553-565. (abstract) Walsh, C. J. (2000). Urban impacts on the ecology of receiving waters: a framework for assessment, conservation and restoration. Hydrobiologia 431(2/3), 107-114. Walsh, C. J. and Breen, P. F. (1999). "Urban stream rehabilitation through a decision-making framework to identify degrading processes and prioritize management actions." In Rutherfurd, I. and Bartley, R. (eds.) Proceedings of the Second Australian Stream Management Conference, Vol. 2. Adelaide, South Australia. pp. 673-678. (abstract) VSC (1999) Urban Stormwater: Best Practice Environmental Management Guidelines. 268 pp. Victorian Stormwater Committee, CSIRO Publishing, Melbourne. More information Dr Chris Walsh |
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