Freshwater molluscs in mountain lakes of the Eastern Alps (Austria): relationship between environmental variables and lake colonization

This study examines the colonization of twelve mountain lakes of the Central Alps in Austria by freshwater gastropods and bivalves. Of the twelve lakes studied, seven are in the montane vegetation level (600 – 1400 m), and five are subalpine (1400 – 2200 m). At each lake, species diversity and abundance was measured by taking 10 samples along the shore, for each parameter. In addition to recording of molluscs, I measured 12 environmental factors, namely, altitude, water temperature, shading, water pH, conductivity, oxygen saturation, biological oxygen demand after 5 days (BOD5), nitrate concentration, total hardness, mean grain size of the bottom substrate, and percentage of submerged vegetation and algal cover, at each sample site. I then used multivariate statistical analysis, specifically Canonical Correspondence Analysis, to detect any relationships between these physico-chemical factors and the distribution of molluscs. The study detected a total of 13 mollusc, 8 gastropod and 5 bivalve species in the lakes, with lakes at the montane vegetation level generally exhibiting higher species diversities than lakes at the subalpine vegetation level. Pisidium casertanum, Galba truncatula, P. subtruncatum, and Radix balthica exhibited the highest species abundance, while Acroloxus lacustris and Musculium lacustre, both typical of lower altitudes, were very rare. Multivariate analysis revealed an important influence of shading, altitude, nitrate, BOD5, oxygen saturation, substrate grain-size and submerged vegetation on the distribution patterns of most mollusc species. In contrast, the distribution patterns of P. subtruncatum and P. obtusale were highly independent of the measured environmental factors.