Three years of taxonomic and environmental studies (2008–2010) on the green algal genus Pediastrum Meyen confirmed its occurrence at 50 of 71 localities situated in northern, central and southern regions of Poland. The genus commonly occurs in the littoral of eutrophic water bodies, especially coastal lakes and fish ponds. Pediastrum co-exists mainly with other taxa of Chlorophyceae and its share in algal communities is low.
All together, 32 Pediastrum taxa belonging to 19 species occur in Poland, 23 of which were found in the field studies described here: P. simplex Meyen var. simplex, P. simplex var. echinulatum Wittrock, P. musteri Tell et Mataloni, P. patagonicum Tell et Mataloni, P. kawraiskyi Schmidle, P. orientale (Skuja) Jankovská et Komárek, P. integrum Nägeli var. integrum, P. boryanum (Turpin) Meneghini var. boryanum, P. boryanum var. pseudoglabrum Parra, P. boryanum var. cornutum (Raciborski) Sulek, P. boryanum var. perforatum (Raciborski) Nitardy, P. boryanum var. longicorne Reinsch, P. boryanum var. brevicorne A. Braun, P. boryanum var. forcipatum (Corda) Chodat, P. duplex Meyen var. duplex, P. duplex var. gracillimum W. West et G.S. West, P. duplex var. rugulosum Raciborski, P. duplex var. asperum (A. Braun) Hansgirg, P. alternans Nygaard, P. angulosum (Ehrenberg) Meneghini var. angulosum, P. privum (Printz) Hegewald, P. tetras (Ehrenberg) Ralfs and P. biradiatum Meyen var. biradiatum.
Eight of these taxa are rare on the world scale: P. musteri, P. patagonicum, P. privum, P. alternans, P. duplex var. asperum, P. integrum var. integrum, P. kawraiskyi and P. orientale. Nine other taxa previously recorded in the Polish literature were not confirmed in the present study: P. simplex var. clathratum (Schröter) Chodat, P. simplex var. sturmii (Reinsch) Wolle, P. subgranulatum (Raciborski) Komárek et Jankovská, P. argentinense Bourrelly et Tell, P. obtusum Lucks, P. longecornutum (Gutwiński) Comas, P. biradiatum var. glabrum (Raciborski) Parra, P. braunii Wartmann and P. taylori Siemińska.
Study of a newly created culture collection composed of 12 isolates representing 9 Pediastrum taxa, together with observations of field material, suggest that P. simplex, P. boryanum, P. duplex, P. orientale and P. tetras are the most variable Polish Pediastrum species. The morphological variability of Pediastrum creates problems in determining its varieties. For correct identification of taxa, morphological analyses of field and cultured material should be supplemented by molecular studies. The present study is offered as a contribution to world ecological and biogeographic studies on Pediastrum.