Cyanotoxin diversity and food web bioaccumulation in a reservoir with decreasing phosphorus concentrations and perennial cyanobacterial blooms

• In eutrophic dam reservoir a gradual decrease in phosphorus concentration was accompanied by reduction of the biomass of toxigenic cyanobacteria.
• Significant increase in species richness of harmful cyanobacteria and higher production of microcystins than anatoxin-a were also found.
• Diversity of microcystin variants produced by cyanobacteria increased over time.
• Transfer of microcystins and anatoxin-a to edible fish and diverse tissue distribution of MC variants were found.

In a shallow multifunction dam reservoir, perennial water blooms formed by several toxin-producing cyanobacteria (Anabaena spp., Aphanizomenon spp., Planktothrix agardhii and Microcystis spp.) were observed. Over a seven-year period, concomitantly with a gradual decrease in phosphate and total phosphorus concentrations in the water and an increase in the DIN to DIP ratio, a reduced biomass of cyanobacteria was noted. Simultaneously, a twofold increase in cyanobacterial species richness was found. The concentration of intracellular anatoxin-a was positively correlated with the total cyanobacterial biomass, but the concentration of intracellular microcystins was significantly negatively correlated with the level of phosphorus in the water. Therefore, in a period with a very low (2.3–3.6) DIN:DIP ratio, intracellular ANTX prevailed in the reservoir, while in the following years (at DIN:DIP = 23–36) much higher MC levels were noted. The highest total concentrations (22.2 μg L−1) of intracellular MCs (MC-LF > -LY > -LR > -LA = -LW) and ANTX (14.4 μg L−1) were found in 2010. In the following year, eight MC iso-forms were detected (MC-LF > -LY > -LA > -LR > -LW > -WR > -YR > -RR). The number of MC variants was positively correlated with the increased contribution of Anabaena planctonica/A. affinis and Microcystis spp. to cyanobacteria biomass. The indigenous bentho-pelagic fish Abramis brama L. accumulated in their tissues relatively high amounts of both ANTX (e.g. 6.2–18.4 μg g−1 FW of liver) and different variants of MCs (up to 4.4 μg g−1 FW of liver). Cyanotoxin tissue contents decreased in the following order: gills > liver > muscles. These observed strong changes in the species structure of cyanobacteria assemblages, even at their considerably smaller biomass, appeared to be an undesirable phenomenon due to the predominance of the efficient MC and ANTX producers, such as Anabaena spp., which is easily digested by fish. The variability of the profile of cyanobacterial blooms that depends on nutrient fluctuations and may account for the diverse toxin accumulation and tissue distribution in freshwater ichthyofauna is noteworthy, especially in water bodies used for fishery.