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This is the pioneering attempt to study the spatial patterns in structure of lotic ecosystems that form the Ganga River system in the Himalaya. The diversity of source (glacier-fed [GF], snow-fed [SN] and spring-fed [SF]) and stream-size (both interrelated) across the altitudinal panorama, create numerous habitats that contribute to structural diversity. The spatial patterns in richness, density and taxonomic composition and distribution of benthic diatoms are less affected by source compared with macro-invertebrates but shows strong influence on the distribution of fish fauna that are poikiliotherms, because a glacier-fed river carries ice-cold water (usually <20°C) in contrast to normal waters in spring-fed system (22°C near snowline, 32°C in foothills). The abundance patterns of biota of lower organisation grade (diatoms, macro-invertebrates) do not differ sharply even across distant river basins as they are more influenced by proximate factors; thus the sub-basins of the Alaknanda resemble more by virtue of one basin and there is notable resemblance between distant SF Bemunda (lower Ganga basin) and SF Gomti (East Rāmgangā basin) and even the farthest Yamuna and Rāmgangā. Fish are more sensitive to temperature and current velocities that are related to altitude and hence longitudinal rather than the spatial gradients in the mountains. The lotic ecosystem of Doon Valley harbour rich and diverse diatom flora, macroinvertebrate fauna and ichthyofauna. The examination of trophic, saprobic and ecological status shows that organic pollution, degradation and anthropogenic eutrophication are non-existent in the Lesser Himalayan rivers and streams, but the fragile Doon Valley is under severe anthropogenic stress. This and habitats fragmented by hydropower projects in the major rivers has threatened the iconic game fish Himalayan mahseer in the Ganga.

Large-scale distributional pattern for the benthic macro-invertebrate fauna was determined in the glacier fed Himalayan and Trans-Himalayan rivers and streams of India at the elevation range of 2000-3000 m asl. In Trans-Himalaya the family Heptageniidae (Ephemeroptera) alone (Chandra and Bhaga) or in combination with Chironomidae (Diptera) in similar proportions (Chenab) or Diptera alone (Miyar) dominated the assemblages. Its influence seems to extend to Rupin drainage in the Himalaya where Chironomidae alone dominated the assemblages. Except for this Himalayan river. Various families of Trichoptera attained highest abundance in other rivers of the Himalaya. Thus, Leptoceridae in combination with Limnephilidae (Alaknanada at Tapovan) and Heptageniidae and Baetidae (Alaknanada at Mana) is the only instance of similarity in abundant taxa by virtue of same river. The Mandakini was partially similar to Alaknanda by virtue of abundant Limnephilidae. The Bhagirathi was characterized by abundance of Philopotamidae. Thus, assemblages exhibit greater variability in the Himalayan rivers than Trans-Himalayan rivers and are hence entirely different, as also evident from the cluster analysis. This present hypothesis is not applicable to explain the macro-invertebrate assemblages in Himalayan and Trans-Himalayan region.

Benthic macroinvertebrates play important ecosystem roles in the cycling and outflow of nutrients.  The benthos transforms organic detritus from sedimentary storage into dissolved nutrients that can be mixed into overlying waters and used by rooted plants (macrophytes) and algae (phytoplankton) to enhance primary productivity.  This study examined the distribution pattern of benthic macroinvertebrates in a lesser Himalayan foothill stream from the headwaters (2,200m) to mouth (375m).  Five stations (S1 to S5) were established along the 43-km course of the stream.  Samples were collected at bi-monthly intervals from January to December 2009.  The total density of the benthic macroinvertebrate community increased with decreasing altitude and differed significantly among stations. Dominant orders were Diptera at S1 (Simulidae, 27%) & S5 (Chironomidae 24%), Trichoptera at S2 (Limnephilidae 16%) & S3 (Hydropsychidae 9.9%), and Ephemeroptera (Heptageniidae 9.2%) at S4.  Principal component analysis revealed that the characteristic taxa were Simulidae at S1, Limnephildae at S2, Hydropsychiidae, Rhyacophilidae, Tipulidae, Perlodidae, Dryopidae & Notonectidae at S3, Heptageniidae at S4, and Chironomidae, Siphlonuridae, & Agrionidae at S5.  Cluster analysis showed one large cluster comprising S1 and S2 as sub-groups with resemblance to S3-S4, and S5 as an outlier.  The similarity between the stations S3-S4 was attributed to similar land-use pattern (agriculture) and stream order (II Order), while S1 and S2 were slightly similar due to partial similar forest type (oak forest at S1, pine-oak forest at S2) and stream order.  At S5, however, the considerable change in forest type (mixed forest) land-use and stream order (III Order) caused S5 as an outlier in cluster.  The variations in the abundant and characteristics taxon at different stations were attributed to change in substratum and land-use patterns. 

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