Patterns of macroinvertebrate distribution in relation to altitude, habitat structure and land use in streams of the Nepalese Himalaya

Abstract

Macroinvertebrate communities, environmental variables and stream habitat structure were assessed in riffles and margins in twenty streams along an altitudinal transect (600-4320 m a.s.l.) in east-central Nepal during the summer monsoon. Streams flowed through terraced agricultural land at the lowest altitudes (Likhu Khola), with forest, alpine scrub, and tundra occupying catchments at increasingly higher altitudes (Langtang). We asked: how do the structural attributes of Nepalese streams change with altitude and land use, and how do macroinvertebrate communities respond? Streams at the lowest altitudes had shallower gradients, higher water temperatures, greater proportions of sandy substrata and more reaches of locally reduced velocity than streams at higher altitude. The percentage of the substratum covered by organic matter was significantly greater in forested and alpine streams, partly reflecting their riparian character. Marginal habitat character also reflected riparian land use, with softer vegetated margins in forested and alpine streams, and harder stone margins in terraced and tundra streams. Macroinvertebrate communities were dominated everywhere by Baetidae and Chironomidae. Taxon richness generally declined with increasing altitude, but in contrast to previous studies in Nepal, maximum taxon richness did not occur at the lowest altitudes, where monsoon effects are most intense. Some taxa were restricted to low altitudes (e.g. Hydropsychidae, Hemiptera and Ephemeridae), or to streams with forest and alpine catchments at mid-altitudes (e.g. Limnephilidae and Ephemerellidae). The distribution and abundance of macroinvertebrate taxa reflected stream physiography, marginal habitat structure and substratum composition, particularly accumulations of detritus. In turn, these patterns reflected the meso-habitat distribution of macroinvertebrates, with eleven out of twenty-three taxa showing significant preference for either riffles or margins. Taxa typical of marginal habitats were common in streams with terraced catchments, but were absent at higher altitudes where taxa which showed no significant habitat preference were dominant. Sandy habitats, in terraced streams, held relatively few macroinvertebrates. Nevertheless, some taxa which were restricted to lower altitudes were burrowing and neustonic forms, and may be influenced by the high diversity of substratum particle size and areas of locally reduced current velocity which existed there. We conclude that substratum composition and marginal habitat structure, which are closely associated with land use type and altitude, were important correlates with macroinvertebrate distribution down the Himalayan profile. These data add to the continuing debate about the relative roles of natural and anthropogenic factors on the distribution of aquatic macroinvertebrates in Nepal.