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 small streams of the Melbourne region to test the effects of (a) urban density (sub-catchment imperviousness 0-51%) and (b) stormwater drainage intensity (comparing the intensively drained metropolitan area with urban areas of the hinterland, which had open drains and some localized stormwater drainage). 2. Communities separated into three groups: eastern hinterland, western hinterland and a metropolitan group. Separation of eastern and western hinterland 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 gradients (phosphorus, ammonia and total nitrogen). 3. Nutrient concentrations were not only apparently influenced by urban density but also by effluents from small sewage treatment plants and agricultural 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 nutrient concentrations tended to be more species-rich than mildly enriched sites. 5. Composition of both diatom and macroinvertebrate communities (assessed in a concurrent study) were sensitive indicators of urban-derived impacts. However, diatoms were better indicators of nutrient enrichment, while macroinvertebrates were better integrative indicators of catchment disturbance.