Effects of urbanization on streams of the Melbourne region, Victoria, Australia. II. Benthic diatom communities

1. Epilithic and epiphytic diatom community composition were assessed in sm all streams of the Melbourne region to test the effects of (a) urban densit y (sub-catchment imperviousness 0-51%) and (b) stormwater drainage intensit y (comparing the intensively drained metropolitan area with urban areas of the hinterland, which had open drains and some localized stormwater drainag e). 2. Communities separated into three groups: eastern hinterland, western hin terland and a metropolitan group. Separation of eastern and western hinterl and groups, and of eastern and western sites within the metropolitan group were best explained by patterns of electrical conductivity, basalt geology and annual rainfall. Separation of metropolitan and hinterland groups, and patterns within the hinterland groups were best explained by nutrient gradi ents (phosphorus, ammonia and total nitrogen). 3. Nutrient concentrations were not only apparently influenced by urban den sity but also by effluents from small sewage treatment plants and agricultu ral activities at a few sites. 4. Species richness did not vary consistently between the metropolitan and hinterland groups but within the western hinterland, sites with low nutrien t concentrations tended to be more species-rich than mildly enriched sites. 5. Composition of both diatom and macroinvertebrate communities (assessed i n a concurrent study) were sensitive indicators of urban-derived impacts. H owever, diatoms were better indicators of nutrient enrichment, while macroi nvertebrates were better integrative indicators of catchment disturbance.