1. The monsoon causes major flood events in some Himalayan streams, but their seasonal predictability might reduce the resulting disturbance. We assessed seasonal change in the benthos of 16 streams in central Nepal over a gradient of declining rainfall and increasing altitude from 600 to 3800 m. All sites were surveyed on four occasions, two in winter (November) and two pre-monsoon (June), with additional sampling during the monsoon (August) at four low altitude sites. Invertebrate abundance, taxon richness and persistence were assessed at all sites, and density and meso-habitat distribution at the four low altitude sites only. 2. Strong seasonal variation among invertebrates was confined primarily to streams at low altitude (600-800 m) where monsoon rainfall was greatest and catchments were dominated by terraced agriculture. At these sites, a significant reduction in benthic density (on average by 77%) and taxon richness (by 20%) occurred between the winter and pre-monsoon periods, so that invertebrate numbers were already low before the monsoon. A further significant decline occurred in all meso-habitats during the monsoon, but the change in density was small in absolute terms. 3. Persistence in rank abundance was equally low at all sites, but turnover in composition was significantly lower at sites in semi-natural forest than in catchments managed for terracing or alpine pasture. 4. These data provide no evidence that monsoonal floods represent major disturbance, instead supporting the view that the ecological response might reflect an adjustment to predictable flow pattern. However, catchment land use in the Himalaya appears to be a significant source of ecosystem instability, and confounds the simple interpretation of monsoon effects.
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