MACROINVERTEBRATE COMMUNITIES IN STREAMS IN THE HIMALAYA, NEPAL

1. Macroinvertebrates were sampled in the riffles of fifty-eight strea ms from three regions of the Himalaya (Anapurna, Langtang and Everest) in Nepal. A semi-quantitative method with identification to family le vel was used to describe communities on-site. 2. Stream physicochemist ry was assessed and the community structure of macroinvertebrates was related to chemistry, physiography (substratum composition, altitude a nd size), geographical location and the dominant land use in each catc hment (terraced agriculture, forest or scrub). Community data were ana lysed by ordination (DECORANA) and classification (TWINSPAN). 3. The c oncentration of cations in stream water decreased significantly with a ltitude. Chemistry also differed between regions; sites from Anapurna had a higher pH and conductivity than those in the other two areas. 4. Communities were dominated by aquatic insect larvae, with Ephemeropte ra, in particular the Baetidae, most numerous across sites. 5. There w ere, nevertheless, differences in community structure between sites, w hich were related closely to stream physicochemistry. Ordination score s were strongly correlated with altitude, magnesium concentration and substratum composition. Classification was also linked to altitude and chemistry, differentiating high-altitude sites with low silica concen trations from others. Sites from the Anapurna and Everest regions, wit h their contrasting chemistry, were also separated. 6. Community struc ture was also related to land use: streams draining catchments dominat ed by terraced agriculture had different communities from those in scr ub or forest. This result was confounded, however, by the strong relat ionship between land use, altitude and chemistry; sites in terracing w ere at lower altitude, had higher concentrations of silica and a highe r proportion of fine sediments than those in the other land-use types. 7. Overall, our data indicate that natural features of the relief and geology in the Himalaya create strong gradients in their invertebrate faunas, but that activities of man may have an effect on stream struc ture and ecology through catchment management.