The 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.

Although the water bodies of southern Russia experience the most extreme effects of cyanobacterial blooms, molecular genetic data on the composition of toxigenic cyanobacteria in this region have been absent. Screening for the genes responsible for the synthesis of hepatotoxins (microcystins and cylindrospermopsin) and neurotoxins (anatoxin-a and saxitoxins) in cyanobacteria from the Tsimlyansk Reservoir on the Don River was carried out. The presence of microcystin-producing Microcystis and Planktothrix populations, as well as of cyanobacteria capable of synthesis of a neurotoxin anatoxin-a, was revealed by polymerase chain reaction (PCR). A hypothesis of the presence of anatoxin-a-producing Planktothrix rubescens population in the phytoplankton of the Tsimlyansk Reservoir is proposed. The obtained PCR data were confirmed by the results of enzyme-linked immunosorbent assay (ELISA) and liquid chromatography/mass-spectrometry (LC/MS). Anatoxin-a and five microcystin variants were identified in the phytoplankton biomass.