Drivers of the Ceratium hirundinella and Microcystis aeruginosa coexistence in a drinking water reservoir

The spatial and temporal patterns of phytoplankton in relation to physical and chemical properties were studied in a drinking water reservoir - the Lazberc Reservoir - located in north-east Hungary. The main objectives were to determine the role of the hydrologic regime in structuring algal growth and the effects of physical and chemical variables on the coexistence of a dominant species: cyanobacteria, Microcystis aeruginosa (Kutzing) Kutzing and an eukaryotic dinoflagellates, Ceratium hirundinella (O.F. Muller) Dujardin. The results suggest that nutrients (TP, NH4-N and NO3-N), turbidity, and the hydrologic regime play an important role in regulating the occurrence of investigated species. The rainy summer period resulting in a mixing of the water column, and the low level of nitrogen coupled with the internal release of phosphorus from the lake sediment under brief periods of anoxia promote cyanobacterial bloom. During this period, the water column was characterised by high turbidity, which created favourable conditions for developing a huge C. hirundinella biomass and the simultaneous occurrence of M. aeruginosa. This study also demonstrated - that contrary to what was previously known, deep standing waters in the temperate zone are not only dimictic (mixing twice a year during the spring and the fall), but they can also be mixed during the summer period due to continued rainfall, which equalizes the temperature in the whole water column. This can promote the occurrence of cyanobacterial and eukaryote planktonic organisms.