Invertebrate drift and physico-chemical variables in the tributaries of the Mutt, a Swiss glacial stream

The study of the invertebrate assemblages in a small glacial catchment of t he Swiss Alps (the Mutt) was designed to categorise sites in relation to th eir water sources and assess the contribution of small headwater tributarie s to the biodiversity of such Alpine catchments. Physico-chemical characteristics and macroinvertebrate drift were monitored in seven streams during three contrasting hydrological periods in June, Au gust and September 1997. Physico-chemical characteristics clearly differentiated kryal (glacial) and krenal (groundwater-fed) streams. The glacial waters showed a broader ampl itude in environmental variation, depending on altitude, geology and season . Macroinvertebrate drift composition was significantly correlated with the environmental conditions. In kryal streams, where harsh conditions prevail ed, diversity and abundance were low, while the krenal streams showed highl y diversified and dense macroinvertebrate communities. favoured by physico- chemical constancy. However, the low number of taxa found at the same time in these groundwater-fed tributaries and in the main Mutt stream indicated a low contribution from these small streams to the communities of the main stream. Nevertheless, the habitat heterogeneity generated by different water origin s in the tributaries favoured the coexistence of diversified macroinvertebr ate communities in a rather small catchment area. Therefore these tributari es played an important part in the biodiversity of the glacial catchment.