Macroinvertebrate drift in streams of the Nepalese Himalaya

Abstract

1. Macroinvertebrate drift was investigated in seven small headwater streams along an altitudinal gradient (600-3350 m) in two adjacent river systems in east-central Nepal. Study streams in the Likhu Khola were at the lowest altitude and flowed through terraced agriculture. At higher altitudes, Langtang streams flowed through catchments of forest and alpine scrub. Samples were collected every 3h over a 24h period in 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 Hydropsychidae and Baetidae, which were most abundant in night samples, and Hydracarina, which tended to be day active. Baetidae also dominated drift at high altitudes, but here drift was increasingly aperiodic or diurnal. The composition of the drift reflected a significant correlation between the percentage abundance of taxa in the drift and benthos. 4. As in other mountain regions of the world, drift was aperiodic in fishless streams (high altitude), but strongly nocturnal in streams where insectivorous fish were present (lower altitude). However, a wide array of potentially important variables along the altitudinal gradient, such as temperature, climate and community structure, might be important influences on these patterns.

Impact of season-long water abstraction on invertebrate drift composition and concentration

“…However, they are similar to other estimates (e.g., Brewin & Ormerod, 1994;Leung et al, 2009). Early in the season, the drift assemblage was dominated by mayflies, chironomid larvae and pupae, and water mites, which are all taxa that are frequently found as dominant members of drift assemblages in other systems (Brewin & Ormerod, 1994;Musselwhite & Wipfli, 2004;Hay et al, 2008). As the irrigation season progressed, additional noninsect taxa (e.g., microcrustaceans, flatworms, and Physa snails) became dominant members of the overall drift assemblage and mayfly nymphs declined in dominance.…”

“…However, they are similar to other estimates (e.g., Brewin & Ormerod, 1994;Leung et al, 2009). Early in the season, the drift assemblage was dominated by mayflies, chironomid larvae and pupae, and water mites, which are all taxa that are frequently found as dominant members of drift assemblages in other systems (Brewin & Ormerod, 1994;Musselwhite & Wipfli, 2004;Hay et al, 2008).…”

Temporal consistency in the long-term spatial distribution of macroinvertebrate drift along a stream reach

“…Numerous studies have shown distinct circadian patterns with maximum drift usually occurring in low light level periods or at night (Cowell & Carew, 1976;Barnes & Shiozawa, 1985;Benson & Pearson, 1987;Brewin & Ormerod, 1994;Hieber et al, 2003). Temporal differences in flow may also affect drift, with an increase in drift occurring during spates or periods of increased water flow (Crisp & Robson, 1979;Neves, 1979;Bird & Hynes, 1981).…”

“…At large scales, the density and abundance of drifting invertebrates may be determined by altitudinal gradients, hydraulic patterns and climate (Brewin & Ormerod, 1994;Crosa et al, 2002). At fine scales, macroinvertebrate drift may be determined by current velocity, substrate composition, vertical position in the water column and food availability Rabeni & Minshall, 1977;Peckarsky, 1980;Ciborowski, 1983;Lancaster et al, 1996;Fenglio & Bo, 2004).…”

Effect of riparian land use on contributions of terrestrial invertebrates to streams

“…For example, contrasts in riparian vegetation among streams can result in significant contrasts in light and thermal regimes (Weatherly & Ormerod, 1990 ;Tait et al ., 1994), sources of nutrients and detritus (Bilby & Bisson, 1992 ;Vought et al ., 1994), and input of terrestrial invertebrates (Brewin & Ormerod, 1994) . Terrestrial invertebrates may at times be important components of the diet of stream fishes (Chapman, 1966 ;Hunt, 1975 ;Garman, 1991) .…”