Diatom assemblages in high mountain streams of the Alps and the Himalaya

Despite their remote location high alpine freshwater ecosystems are subject to environmental change due to acidification, eutrophication, global warming and increasing UV-B radiation. Diatoms, which together with cyanobacteria usually dominate the benthic primary producers of high mountain running waters are powerful indicators of environmental change. To establish a baseline data set, diatom assemblages in high altitude streams from the Alps and the Himalaya were compared. Epilithic diatom assemblages were sampled at 40 sites: 20 were located in the Southern Alps (Natural Parks of Trentino, Italy) and 20 in the Himalaya (northwest India, Nepal). Environmental variables were measured at the time of sampling. Species of cosmopolitan distribution were most abundant. However, several taxa characteristic of one or other mountain range were among the subdominant or rare diatoms. Multivariate analyses (TWINSPAN, CCA) showed that, in the absence of major anthropogenic impacts and extreme physical conditions (desiccation, discharges), the structure of the diatom assemblages is mainly determined by geochemical factors, such as the level of mineralisation, pH and sulphates. It is suggested that the high altitude running waters of the Himalaya may be useful reference sites for monitoring comparable ecosystems in mountain regions subject to more pronounced anthropogenic pressures, such as the Alps.