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The present study documents the findings that picophytoplankton is one of the fundamental contributors to primary biomass production not only in aquatic environments with very low content of inorganic nutrients, but also in lakes with higher trophicity. The autotrophic picophytoplankton (APP) comprises bacterialsized (0.2-2μm) picocyanobacteria and eukaryotic phototrophs (mainly green algae). These organisms can provide up to 90% of the primary production of shallow soda lakes. Shallow soda pans are very characteristic of the Pannonic Ecoregion (Austria, Hungary and Serbia) and they represent a considerable part of the lakes present in this region. Between 11 May and 20 June 2009, the authors determined the total biomass of the phytoplankton (based on chlorophyll-a) in 65 soda pans (with an average depth of 17 cm) of the Pannonic Ecoregion. They also estimated the biomass and abundance of the picophytoplankton, using epifluorescence microscopy. The physico-chemical characteristics of the water, relevant for the phytoplankton, were also investigated. The chlorophyll-a (Chl-a) concentration ranged between 1 μg l-1 and 1400 μg l-1 with an average of 115 μg l-1. The abundance varied between 1000 cells ml-1 and 88 million cells ml-1. Picoplankters were represented by 1.5 μm-sized green algae and 1.0 μmsized phycocyanin-rich bacteria. Phycoerythrin-rich picocyanobacteria have not been detected. Based on the present findings, in these water bodies, the contribution of picoplankton to total phytoplankton did not show a decreasing tendency with increasing trophic state, as it was recorded earlier by other authors in different types of aquatic environments. The present results reveal that in soda lakes the picophytoplankton has a crucial contribution to the total primary production not only under conditions of nutrient deficiency, but also in lakes with higher trophicity. The total phosphorus (TP) concentration in the soda lakes studied varied between 70 μg l-1 and 59 000 μgl-1. Based on TP, four of the lakes were considered eutrophic and 61 hypertrophic. Based on phytoplankton biomass, half of the lakes were classified as oligotrophic, respectively mesotrophic, while the rest were considered eutrophic, respectively hypertrophic. [ABSTRACT FROM AUTHOR]