Benthic Insect-Substratum Relationship Along an Altitudinal Gradient in a Himalayan Stream, India

Takoli Gad is a small spring-fed tributary of the River Alaknanda (a tributary of River Ganga) in Tehri district of Uttarakhand (India). Insect-substratum relationship was studied at five sampling sites representing an altitudinal gradient in Takoli Gad during January 2000 to February 2002. The swift flowing stream is largely dominated by pebbles and boulders (>32 mm). In general, the substrate composition ranged from coarse sand (phi=0) to small pebbles (phi=-4). The benthic fauna comprised of 34 genera belonging to 09 orders and 25 families of insects. It included mostly the nymphs and larvae of Ephemeroptera, Plecoptera, Trichoptera, Coleoptera, Lepidoptera, Odonata, Neuroptera, Diptera and Hemiptera. The total benthic density was maximum during winter III (3408.0 ind. m 2 ) at downstream site, and minimum during monsoon I (283.0 ind. m 2 ) in the middle stretch. The Simpson Index of Diversity (D) for substrate heterogeneity was low for headwater region of the stream. The Index of Representation (IR) revealed that the heterogeneous substratum was the preferred habitat of most of the taxa during the winter. The distribution of the benthic insects varied slightly along the longitudinal gradient. Many genera were seasonally absent in different sections of the stream. Seasonal variation in the density of benthic insects was correlated with the change in the substrate composition, notably during the rainy season when the mean grain size changes to cobbles (Md=8). A relatively stable substrate composition during winter together with low to moderate current velocity (0.4-0.6 m s 1 ) and shallow water depth (0.14-0.20 m) along with abundant detritus favours a rich and diverse insect community. Further, clustering method also shows the substratum during winter to be preferred by majority of the taxa.