Altitudinal trends in the diatoms, bryophytes, macroinvertebrates and fish of a Nepalese river system

SUMMARY 1. Hydrobiological changes were assessed along an altirudinal transect of eighteen to twenty-three tributaries from 600 to 3750m in two adjacent river systems in east-central Nepal. The transect incorporated catchments under terraced agriculture at the lowest altitudes in the Likhu Khola, through streams in forest, alpine scrub and tundra at higher altitudes in Langtang. 2. Diatoms, bryophytes, macroinvertebrates and fish all showed pronounced altitudinal changes in assemblage composition as shown by TWINSPAN and DECORANA. A few taxa were restricted to streams at high altitude, but many more occurred only at lower altitudes where taxon richness increased substantially despite catchment disturbance by terraced agriculture. 3. Diatoms characteristic of lower altitude streams were mostly motile, epipelic or episammic Navicula and Nitzschia spp., which occur typically at greater electrolyte and nutrient concentrations. Those characteristic of higher and steeper sites included attached Fragilaria spp. and prostrate Achnanthes spp., tolerant of turbulent flow. 4. Cover by bryophytes varied within catchment type; high altitude springs supported dense mats, unlike streams fed by ice and glaciers. Taxa confined to low altitudes included those characteristic of humid subtropical conditions. 5. Invertebrate families occurring only at lower altitudes included a range of burrowers and pool dwellers. Numerically, filter feeding Hydropsychidae and Simuliidae dominated 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 the Likhu Khola were 23–250 (per 100m−2), and 86–1282 g wet mass (per 100 m−2), respectively. No fish were found in Langtang streams, probably because torrential headwaters prevented colonization. 7. Our data confirm that altitudinal transitions in stream biota are pronounced in the Himalaya of Nepal, but are likely to reflect a wide array of potential influences.