Auto- and heterotrophic picoplankton in a lowland river (Warta River, Poland)

The structure of plankton in a lowland river was studied with a particular focus on autotrophic as well as heterotrophic picoplankton cells. Water samples were collected in the middle, meridional section of the Warta River in 2005. Heterotrophic picoplankton (HPP) were represented by bacteria varying in size and form, but mainly by cocci and rods. HPP abundance ranged from 3.5 × 10⁶ to 9.5 × 10⁶ cells ml^-1. Autotrophic picoplankton (APP) densities were considerably lower and ranged from 1.2 × 10³ to 92 × 10³ cells ml^-1. They were represented by prokaryotes (picocyanobacteria) and eukaryotes (small chlorophytes). Both components occurred together but eukaryotic APP usually prevailed in terms of biomass. In terms of biomass, APP seemed to play a minor role in this ecosystem in comparison with heterotrophic picoplankton.