The phytoplankton in the inner Neva Estuary is described from data obtained from 1996 to 2000.T he seasonal dynamics of the phytoplankton biomass are characterized by a bimodal curve with a summer maximum.T he average seasonal biomass was approximately 3 mg l−1, the maximum biomass was 8–11 mg l−1. The species composition and quantitative parameters were compared to those observed in the 1980s.A notable, nearly 1.5–2 fold, increase in the biomass in the summer–autumn period and the predominance of Oscillatoria species among the blue-green algae were observed.A decline in the nutrient load in the water body at the end of the 1990s appeared to be insufficient to bring about a decrease in the proportion of Oscillatoria algae in the total species composition or a decline in the biomass of the entire phytoplankton community. In 2000 a certain change in the structural composition of the phytoplankton complex was noted.S pecies that had been predominant in the 1980s and had lost their advantage in the early 1990s, regained their earlier status.
The occurence of toxic freshwater blooms of cyanobacteria (blue-green algae) has been reported in many countries. These toxic water blooms have caused the deaths of domestic animals and wildlife. Cyanobacteria may produce acute toxins such as hepatotoxic peptides (microcystins, nodularins and cylindrospermopsin) and neuro-toxic alkaloids (anatoxin-a, anatoxin-a(s), homoanatoxin and aphanotoxins). Very important toxins are microcystins (MCYST) with high biological activities and wide distribution all over the world. They have been produced by Microcystis, Oscillatoria, Anabaena and Nostoc. All MCYST can play the role of tumor-promoting activators. This paper for the first time presents results showing the presence of microcystin hepatotoxin in Polish lakes and drinking water.
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