BackgroundThe permanent dominance of Planktothrix-like сyanobacteria has been often reported for shallow eutrophic\hypertrophic lakes in central Europe in summer\autumn. However studies on phytoplankton growth under ice cover in nutrient-rich lakes are very scarce. Lake Nero provides a good example of the contrasting seasonal extremes in environmental conditions. Moreover, the ecosystem underwent a catastrophic transition from eutrophic to hypertrophic 2003–05, with dominance of filamentous cyanobacteria in summer\autumn. Towards the end of the period of ice cover, there is an almost complete lack of light and oxygen but abundance in nutrients, especially ammonium nitrogen, soluble reactive phosphorus and total phosphorus in lake Nero. The aim of the present study was to describe species composition and abundance of the phytoplankton, in relation to the abiotic properties of the habitat to the end of winters 1999–2010. We were interested if Planktothrix-like сyanobacteria kept their dominant role under the ice conditions or only survived, and how did the under-ice phytoplankton community differ from year to year.ResultsSamples collected contained 172 algal taxa of sub-generic rank. Abundance of phytoplankton varied widely from very low to the bloom level. Cyanobacteria (Limnothrix, Pseudanabaena, Planktothrix) were present in all winter samples but did not always dominate. Favourable conditions included low winter temperature, thicker ice, almost complete lack of oxygen and high ammonium concentration. Flagellates belonging to Euglenophyta and Cryptophyta dominated in warmer winters, when phosphorus concentrations increased.ConclusionA full picture of algal succession in the lake may be obtained only if systematic winter observations are taken into account. Nearly anoxic conditions, severe light deficiency and high concentration of biogenic elements present a highly selective environment for phytoplankton. Hypertrophic water bodies of moderate zone covered by ice in winter and dominated by Planktothrix - like сyanobacteria in summer/autumn may follow several scenarios in the end of winter. It may be intense proliferation сyanobacteria normally dominating in summer, or the switch to the other species like the euglenoids and cryptomonads flagellates, or almost total depletion of phytoplankton.