The smallest phototrophic organisms in a lowland river: comparison with other components of the phytoseston
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The phytoseston of the Warta, a polluted eutrophic river in western Poland, was studied from spring to autumn 2003. The focus of the study was autotrophic picoplankton (APP). APP abundance fluctuated from 1.3?106 to 1.05?108cells L1 and was characterized by a maximum in late July. APP biomass was low, ranging from 0.5 to 19.7 mug C L1. Both cyanobacteria and eukaryotic organisms were found within the picoplankton. The picocyanobacterial community was often dominated by colonial forms, mostly Aphanocapsa spp. and Cyanogranis ferruginea (Wawrik) Hindak. Eukaryotic picoplankton included chlorophytes of the genera Chlorella, Choricystis, and Pseudodictyosphaerium jurisii (Hind?k) Hindak. Among the phytoplankton larger than 2 mum, the most abundant species were diatoms and chlorophytes, except in late August, when filamentous cyanobacteria dominated. The contribution of APP to the total phytoplankton biomass was usually below 1%, and the mean (0.6%) was close to the lower limit of values found in lakes.
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E. Szelag-Wasielewska, Department of Water Protection, Adam Mickiewicz University, Drzymaly 24, 60-613 Poznan, Poland