Replacement of chroococcales and nostocales by oscillatoriales caused a significant increase in microcystin concentrations in a dam reservoir

A two-year study on cyanobacterial development and the dynamics of intracellular microcystins was carried out in a shallow dam reservoir. Potentially toxic cyanobacteria (Microcystis wesenbergii, M. aeruginosa, Woronichinia naegeliana, Anabaena spp., Planktothrix agardhii) were observed to be the main component (70-94% total biomass) of the phytoplankton community, in which species composition was unstable and was very different between the 2005 and 2006 summer seasons. Generally, total phytoplankton, cyanobacterial biomass and total microcystin (MC) concentrations in the reservoir were much higher in 2006 than in 2005. The highest MC concentration (173.8 μg MC-LR equivalent dm^-3) was seen in 2006 during P. agardhii (Oscillatoriales) domination (max. fresh biomass 50.3 mg dm^-3; above 91% of phytoplankton biomass). Positive correlations between microcystin concentrations and cyanobacterial biomass suggest that populations of Nostocales and Oscillatoriales in 2005 and Oscillatoriales (P. agardhii) in 2006 may have been the main producers of MCs in the reservoir. The strong increase in P. agardhii biomass concomitant with a decrease in the total biomass of Chroococcales and Nostocales was responsible for the increase in MC concentration in the Siemianówka dam reservoir.