Summer spatial variations in phytoplankton composition and biomass in surface waters of a warm-temperate, deep and oligo-holomictic lake: Lake Iseo, Italy

The spatial distribution of phytoplankton in the surface waters of typical deep sub-alpine lake was investigated by analyzing water samples collected at three depth ranges (0-1 m, 9-13 m, 15-19 m) at seven sites. A total of 21 samples were analyzed by a Generalized Linear Model. The biovolume of different phytoplanktonic taxa significantly differed between depth ranges. Indeed, different algal taxa showed maximum population density at different depths, as a result of different optima of light, physical conditions and nutrients. The layer of 9-13 m depth showed the greatest abundance of phytoplankton, mainly represented by Bacillariophyceae and Cyanobacteria. Conversely, Chlorophyceae, which require few nutrients and are not inhibited by the high light energy, were most abundant at the surface. The vertical heterogeneity in phytoplankton distribution closely paralleled that of nutrients, which in turn depends on the stable thermal stratification of the lake water column. Biovolume of phytoplankton taxa and chemical parameters did not differ significantly between sampling sites that were some kilometers far away from each other. This is due to large horizontal movements of water that occur in the lake and that determine rather uniform conditions throughout the basin.