Ecology and biogeography of Himalayan diatoms: distribution along gradients of altitude, stream habitat and water chemistry

Himalayan streams, an important natural resource, are subject to numerous threats such as pollution and hydrological changes with deteriorating water quality in some areas. In order to develop diatom-based monitoring tools, we investigated diatom species composition at 206 sites in relation to variation in hydrochemistry, habitat characteristics (land use, shading, substrate), altitude and geographical position across a ca. 900 km east-west distance and a 4300 m altitudinal range in Nepal and north-west India. A total of 261 species were found, of which naviculoid (55), monoraphid (47), Gomphonema and fragilarioid (30), cymbelloid (26) taxa and Nitzschia (22) were most abundant. Many species were cosmopolitan, but some had restricted distributions. Of these, most were confined to the eastern Himalaya. More than one third of the species could not be identified. Richness and diversity increased in streams with higher Si concentrations and smaller substratum size, but were not correlated with altitude as a single factor. Canonical correspondence analysis and non-metric multidimensional scaling ordination analyses showed that different regions had distinct diatom assemblages. Important environmental gradients included basestatus, reflecting differences in geology, concentrations of Si (0.5-11.5 mg/L) and Na (0.2-9.0 mg/L), reflecting impacts from agriculture, and habitat, such as substratum size, land use and stream size. The importance of these factors varied in different climatic zones. Across the east-west range and altitudinal gradient hydrochemistry and habitat characteristics explained more of the variation in species composition than altitude and geographical position, but these factors were inter-correlated and their effects difficult to disentangle. Some species had restricted distributions along the altitudinal gradient, and particularly in the eastern Himalaya a change in species composition occurred most clearly at 2000 m a.s.l. Consequently, areas above and below this altitude should be considered separately when developing diatom-based monitoring methods