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.
The purpose of the study is to examine the difference in diatom community, assemblages and their diversity with respect to the natural and regulated section of montane river Tons (Yamuna basin) in Himalaya. Diatom samples were collected at 4 stations from upstream to downstream; TR1 to TR4 at regular monthly basis and
preserved in 4% formaldehyde solution. The permanent mounts were prepared in naphrax and examined for recording the flora under the bright field using Nikon Japan 80i trinuclear research microscope. The number of diatom taxa decreased from natural to regulated section. The continuity was impaired in this stretch as only 42 of 116 species and 14 of 24 genera occurred at all the stations. The species richness was highest in the natural upstream section TR 1(96 from 21 genera). It declined at TR2 (59 species-18 genera) and increased mildly at TR3 (63 species-15 genera) and TR4 (69 species,15 genera). Four genera (Denticula, Hantzschia, Hippodonta and Adlafia)
were totally absent in downstream section. Didymosphenia and Melosira were recorded only at the dam location, exhibiting restricted distribution. Dominant taxa varied and amounted to 4 taxa for TR1 and TR2, only 2 taxa for TR3. No diatom taxon was at dominant at TR 4. Cymbella was recorded as the most species rich genera. Species
richness decreased from TR1 to TR4. Consequently, the assemblages differed in natural, impounded and regulated sections of the Tons R.Vast variations in the flora, species richness and assemblages suggest a severe impact on the Tons R. ecosystem.
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