Evidence of rock glacier melt impacts on water chemistry and diatoms in high mountain streams

• We compare rock glacier impacted streams with non-impacted reference streams.
• We study biological and geochemical indicators of water quality in mountain streams.
• We relate elevated ions and heavy metals to melt waters from active rock glaciers.
• Diatoms in rock glacier impacted streams show pronounced species variability.
• Diatoms species composition and diversity respond to increased acidity.

A first study of high alpine freshwaters at Hochebenkar and Krumgampen (Oetztal Alps, Tyrol, Austria) revealed pronounced differences in the concentration of major ions, heavy metals, species composition and biodiversity of epilithic diatoms in streams emerging from two active (i.e. ice containing) rock glaciers and in adjacent unaffected reference streams. The clear-water streams impacted by active rock glaciers are characterized by high electrical conductivity (EC) values, but differ in acidity, heavy metal concentrations and by the proportion of circumneutral and acidobiontic diatoms. On the contrary, all reference streams exhibit low EC and circumneutral to slightly acidic pH values characteristic for surface waters on bedrock composed of paragneiss and micaschist, with no detectable heavy metals and a diatom composition typical for high altitude softwater streams. The high ion concentrations in streams emerging from rock glaciers are attributed to a seasonally increasing release of melt waters from active rock glaciers.