Biological control of undesirable cyanobacteria in culturally eutrophic lakes.

The relationship between Oscillatoria agardhii and one of its natural grazers, the gymnostomid ciliate Nassula ornata was studied in laboratory experiments. Ingestion of Oscillatoria by Nassula increased exponentially over the temperature range 5-20°C, and the grazer was able to suppress Oscillatoria at all temperatures. Laboratory studies showed further that the predatory copepod Cyclops strenuus was able to suppress Nassula, and that juvenile roach (Rutilus rutilus) also fed on Nassula. In an enclosure experiment in an Oscillatoria-lake, Nassula reduced Oscillatoria concentration with more than 50% as compared to the control. The scarcity of Nassula in many Oscillatoria-lakes may be due to predation by cyclopoid copepods and juvenile fish; predominantly cyprinids. A proposed model describes well the population dynamics of Oscillatoria and Nassula in mixed cultures.