Cyanobacterial bloom intensity in the ecologically relevant state of lakes – an approach to Water Framework Directive implementation

This study focused on the determination of the cyanobacteria development in terms of the probabilistic recognition of the bloom intensity level in the ecologically relevant state of lakes. Assuming the possibility of using once-a-year sampling frequency, the ecological status or potential was assessed based on the modified Polish phytoplankton-based method (PMPL_MOD) in 23 Masurian lakes. The summer cyanobacteria biomass reached 52.3 mg l^-1. The biomass did not exceed the WHO-defined low risk threshold of 2 mg l^-1, and was usually observed in lakes with at least good ecological status/potential. This threshold may be sufficient to propose it as a good/moderate cyanobacterial bloom intensity threshold. The average values of 3.3, 9.9 and 22.4 mg l^-1 were recorded in the lakes with a moderate, poor and bad ecological status/ potential, respectively. The WHO-defined moderate risk threshold of 10 mg l^-1 was exceeded only in lakes with a poor or bad ecological status/potential. The structure of cyanobacteria assemblages changed along with the deterioration of the ecological status/ potential and a decrease in Chroococcales biomass and an increase in Oscillatoriales and Nostocales biomass. Nevertheless, Planktolyngbya limnetica, Pseudanabaena limnetica and Aphanizomenon gracile had a significant presence in the total cyanobacterial biomass in all surveyed lakes.