Oligochaete assemblages of Swiss Alpine lakes

The present paper describes the oligochaete assemblages of 12 Swiss Alpine lakes (1700-2500m above sea level) in Canton Ticino. The lake catchment geology is dominated by gneiss. The scarcity of carbonate rocks leads to a low buffering capacity, increasing the lakes' sensitivity to acidification. Because of their very low phosphorus concentrations, they are defined as ultraoligotrophic. Oligochaetes were identified from kick-samples taken from the outflow and littoral zone of the lakes one to three times per year in 1991-1994, 2003 and 2007, when the lake water chemistry was also characterized. Oligochaete assemblages consisted of 19 species in total, 18 of which were found in the littoral zone, and 10 in the outlets. Amongst them, Cernosvitoviella goodhui Healy, 1975 was recorded in Switzerland for the first time. Principal component analysis (PCA), performed excluding the highly alkaline Lake Bianco (pH=7.8 and alkalinity=566 mu eqL(-1)) due to its atypical chemical composition, divided the remaining lakes into two groups with different sensitivity to acidity: the first group of four lakes was characterized by a higher average pH (6.4) and alkalinity (32 mu eqL(-1)), whilst the second group of seven lakes was characterized by a lower average pH (5.7) and alkalinity (5 mu eqL(-1)). Multivariate analysis performed on data collated from the littoral zone highlighted geo-lithology as a key driver in determining the species distribution among lakes. When applied to the lake outlet data, a similar distinction between acidic and calcareous waters was implied. Precipitation influenced the oligochaete assemblage in the littoral zone. During years with higher annual rainfall, the relative abundance of Enchytraeidae increased, probably because their semi-aquatic nature allows them to colonize the littoral zones that dry out periodically.