MACROINVERTEBRATE DRIFT IN STREAMS OF THE NEPALESE HIMALAYA

1. Macroinvertebrate drift was investigated in seven small headwater s treams along an altitudinal gradient (600-3350 m) in two adjacent rive r systems in east-central Nepal. Study streams in the Likhu Khola were at the lowest altitude and flowed through terraced agriculture. At hi gher altitudes, Langtang streams flowed through catchments of forest a nd alpine scrub. Samples were collected every 3 h over a 24 h period i n each stream. 2. Terrestrial macroinvertebrate drift was greatest in streams with semi-natural catchments, but was scarce in streams where trees were absent. 3. At lower altitudes drift was dominated by Hydrop sychidae and Baetidae, which were most abundant in night samples, and Hydracarina, which tended to be day active. Baetidae also dominated dr ift at high altitudes, but here drift was increasingly aperiodic or di urnal. The composition of the drift reflected a significant correlatio n between the percentage abundance of taxa in the drift and benthos. 4 . As in other mountain regions of the world, drift was aperiodic in fi shless streams (high altitude), but strongly nocturnal in streams wher e insectivorous fish were present (lower altitude). However, a wide ar ray of potentially important variables along the altitudinal gradient, such as temperature, climate and community structure, might be import ant influences on these patterns.