Effects of urbanization on streams of the Melbourne region, Victoria, Australia. I. Benthic macroinvertebrate communities

1. Macroinvertebrate community composition was assessed in small streams of the Melbourne region to test the effects of (a) urban density (catchment i mperviousness 0-51%) and (b) stormwater drainage intensity (comparing the i ntensively drained metropolitan area with urban areas of the hinterland, wh ich had open drains and some localized stormwater drainage). 2. Hinterland communities separated into two groups of sites correlating st rongly with patterns of electrical conductivity (EC), basalt geology and an nual rainfall. Community composition varied little in the high-EC, western group (imperviousness 0.2-1.2%), but in the eastern group it was strongly c orrelated with catchment imperviousness (0-12%), with lower taxon richness in more impervious catchments. 3. Metropolitan communities (imperviousness 1-51%) were all severely degrad ed, with high abundances of a few tolerant taxa. Community composition was poorly correlated with patterns of geology, rainfall or imperviousness. Dif ferences between metropolitan and hinterland communities were well explaine d by patterns of biochemical oxygen demand and electrical conductivity, whi ch were postulated to indicate the more efficient transport of pollutants t o receiving streams by the metropolitan stormwater drainage system. 4. Degradation of macroinvertebrate community composition was well explaine d by urban density but intensive urban drainage increased degradation sever ely at even low urban densities. Quantification of relationships between im perviousness, drainage intensity and stream degradation can better inform t he assessment, conservation and restoration of urban streams.