Phytoplankton assemblage structure and dynamics as indicator of the recent trophic and biological evolution of the western basin of Lake Como (N. Italy)

We studied the phytoplankton assemblage of the western basin of Lake Como (Northern Italy) during 1997. The phytoplankton assemblage was composed of 65 taxa, belonging to six taxonomic groups. Chlorophyta were represented by the highest number of taxa (28) followed by Bacillariophyceae (17), Cyanoprokaryota (9), Dinophyceae (6), Chrysophyceae (3) and Cryptophyta (2). The total assemblage density and biomass ranged from 902 ind ml(-)1 and 134.5 mm(3) m(-3) in February to 58 766 ind ml(-) (1) and 9360 mm(3) m(-3) in October. The density and biomass variation at three stations showed a common pattern, with higher values in the southern part of the basin where TP concentrations were always greater. The phytoplankton succession was analysed by cluster analysis (average linkage clustering) and non-metric multidimensional scaling (NMDS) ordination, both applied to a dissimilarity matrix obtained from a calculation of the Bray-Curtis index. In general, the seasonal succession followed a simple pattern, with a clear spring phase. These results are discussed considering the trophic evolution of the lake and its recent colonization by Dreissena polymorpha.