ALTITUDINAL TRENDS IN THE DIATOMS, BRYOPHYTES, MACROINVERTEBRATES ANDFISH OF A NEPALESE RIVER SYSTEM

1. Hydrobiological changes were assessed along an altitudinal transect of eighteen to twenty-three tributaries from 600 to 3750 m in two adj acent river systems in east-central Nepal. The transect incorporated c atchments under terraced agriculture at the lowest altitudes in the Li khu Khola, through streams in forest, alpine scrub and tundra at highe r altitudes in Langtang. 2. Diatoms, bryophytes, macroinvertebrates an d fish all showed pronounced altitudinal changes in assemblage composi tion as shown by TWINSPAN and DECORANA. A few taxa were restricted to streams at high altitude, but many more occurred only at lower altitud es where taxon richness increased substantially despite catchment dist urbance by terraced agriculture. 3. Diatoms characteristic of lower al titude streams were mostly motile, epipelic or episammic Navicula and Nitzschia spp., which occur typically at greater electrolyte and nutri ent concentrations. Those characteristic of higher and steeper sites i ncluded attached Fragilaria spp. and prostrate Achnanthes spp., tolera nt of turbulent flow. 4. Cover by bryophytes varied within catchment t ype; high altitude springs supported dense mats, unlike streams fed by ice and glaciers. Taxa confined to low altitudes included those chara cteristic of humid subtropical conditions. 5. Invertebrate families oc curring only at lower altitudes included a range of burrowers and pool dwellers. Numerically, filter feeding Hydropsychidae and Simuliidae d ominated streams in terraced and forested catchments, whereas grazing baetid mayflies dominated higher altitude streams in scrub and tundra. 6. The combined density and biomass of at least six fish species in t he Likhu Khola were 23-250 (per 100 m(-2)), and 86-1282 g wet mass (pe r 100 m(-2)), respectively. No fish were found in Langtang streams, pr obably because torrential headwaters prevented colonization. 7. Our da ta confirm that altitudinal transitions in stream biota are pronounced in the Himalaya of Nepal, but are likely to reflect a wide array of p otential influences.