1. We assessed the relative importance of different scales of spatial and temporal variability on benthic macroinvertebrate assemblage structure in six unpolluted streams in monsoonal Hong Kong using ordination and complementary multivariate analyses. The spatial scales were regions, sites (streams) and sections (riffles) within sites. The temporal scales were years (three, including one with unusually high rainfall), seasons (dry versus wet) and days within seasons.
2. Significant differences in assemblage structure were manifested at all temporal scales. Those at the site scale were most obvious, whereas demarcation of assemblage structure at the section (riffle) scale was smaller, and there was no significant regional differentiation in assemblage structure. Seasonal variability in assemblage structure was greater than that among years or days.
3. Inter-year differences in assemblage structure were recorded at all sites, and were noted among all years at some sites but not at others. They were recorded more frequently during the dry season, although their occurrence (in pair-wise comparisons between years) appeared to be related to differences in the monsoonal (wet season) rainfall.
4. Seasonal differences in assemblage structure were strongly evident at all sites. Inter-site differences were more apparent during the dry season when local (site-scale) influences on assemblages were stronger. By contrast, wet-season samples were more variable because of spate-induced disturbance, and inter-site differentiation was less distinct.
5. Differences among days at all sites were relatively minor, but shifts attributable to repeated spate-induced disturbance were evident at some sites during the wet season.
6. Differences at the section scale were recorded more frequently during the dry season, when the extent of within-site variability among sections was higher, reflecting increased patchiness within sections resulting from increased substratum heterogeneity and/or greater intensity of biotic interactions.
7. Seasonal shifts in macroinvertebrate assemblage structure at a variety of scales in Hong Kong streams are likely to be attributable to monsoonal rains affecting the relative intensity of abiotic disturbance and biotic interactions in accordance with the harsh-benign hypothesis.