Modern and pre-Industrial Age distributions of Cladocera in Italian and Swiss Alpine lakes

We compared contemporary and pre-Industrial Age (before 1850 AD) fossil assemblages of Cladocera in sediment cores from 25 lakes in the Italian and Swiss Alps to investigate the impact of mounting anthropogenic stresses over the last 150 years on community composition. In addition, we sought relationships between specific species and their environments by measuring nutrients, major ions, pH, alkalinity, conductivity, chlorophyll, and lake and catchment morphological features at the time of core collection for comparison with surface sediment fossil assemblages. The modern (surface sediment) communities of the study lakes consisted mainly of benthic chydorids, primarily Alona affinis, A. quadrangularis, Acroperus harpae, and Chydorus sphaericus-type, with Daphnia as the sole planktonic genus. Principal component analysis (PCA), relating the modern Cladocera assemblages to environmental variables at the time of sampling, indicated that A. affinis and A. quadrangularis are influenced by altitude and dissolved inorganic nitrogen (PCA axis 1) whereas Acroperus harpae and C. sphaericus-type are more influenced by potassium (PCA axis 2). Redundancy analysis, however, identified lake water pH and potassium as the measured variables most impacting modern cladoceran assemblages, and especially the distribution of C. sphaericus-type and A. harpae. Pre-Industrial Age samples contained more Daphnia (longispina-type primarily) than modern samples, and some harbored Eurycercus lamellatus, which has since been extirpated from these lakes. The directional shifts in Cladocera assemblages from the pre-industrial period to the present, illustrated as changes in the sample scores along the PCA axes 1 and 2, were associated with the secondary PCA gradient in almost all lakes, and thus were probably the indirect responses to a cascade of limnological alterations, perhaps initiated by large anthropogenic forcing factors such as atmospheric pollution or fish introductions.