DIATOMS AS INDICATORS OF RIVER QUALITY IN THE NEPALESE MIDDLE HILLS WITH CONSIDERATION OF THE EFFECTS OF HABITAT-SPECIFIC SAMPLING

1. Using a replicated survey design at the catchment scale, we compare d the composition and diversity of diatom communities in three stream groups in the Middle Hills of Nepal: semi-natural reference (Arun Vall ey), enriched by agricultural runoff (Likhu Khola) and grossly pollute d by sewage (Kathmandu Valley). We sampled riffles at all sites; in th e Kathmandu Valley and Likhu Khola we also sampled diatoms in pools an d on vegetation. 2. Species richness and diversity H' were significant ly higher in agricultural streams than in either organically polluted streams or references. Community composition, as shown by principal co mponents analysis, varied significantly between all three stream types due to differences in the abundances of species characteristic of org anic enrichment (Kathmandu Valley), moderate enrichment (Likhu Khola) or undisturbed hillstreams (Arun Valley). 3. Few species varied in rel ative abundance between pools, riffles and vegetation so that variatio ns in community composition and diversity were stronger between stream groups than between habitats. Samples from any one habitat produced o nly 75.7% (+/- 8.6 SD) of the species from three habitats, so that sur veys aimed at recording biodiversity may need more comprehensive habit at coverage than surveys for biomonitoring. 4. We conclude that diatom communities can indicate different sources of pollution in Nepalese s treams, and advocate further studies to develop this indicator potenti al over a wider area of the Himalaya. Comparisons between replicate gr oups of streams selected a priori helped to clarify effects which were sometimes obscured by other survey designs.