Occurrence, Dynamics and Production of Picoplankton in Hungarian Shallow Lakes

Phototrophic picoplankton were detected in 105‐106 cells/ml concentrations in seven water bodies of differing chemistry and trophic level. Dominant picoplankters were, at all sites, coccoid cyanobacteria of 0.8‐1.2 μm dimensions, exhibiting red or yellow autofluorescence. Apart from the effects of water temperature their quantitative dynamics were significantly influenced by the nitrogen supply and the herbivorous zooplankton (Cladocera). Their maximum contribution to the total planktonic primary production was about 50%, both in mesotrophic and hypereutrophic environments. However during the bloom of filamentous nitrogen‐fixing blue‐greens their role became negligible. In phytoplankton communities the significance of picoplankton is overestimated several times, when based on the cell count, while it is underestimated on the basis of biomass. The most useful characteristics of phytoplankton size groups was the total surface area of their cells.