STUDIES ON THE TOXIC EFFECTS OF FILTERED CYANOBACTERIA CULTURE MEDIA OF TWO DIFFERENT STRAINS (MICROCYSTIN-PRODUCING AND MICROCYSTIN-FREE) OF MICROCYSTIS AERUGINOSA ON DAPHNIA MAGNA

Negative effects of cyanobacteria on crustacean zooplankton Daphnia can be documented widely and several main explainary causes have been proposed ( i.e., toxicity, poor nutritional value, bad taste, and morphology). But most of the previous studies concentrated on the direct effects of cyanobacteria on Daphnia by adding Microcystis s. alone or in mixtures ( such as with Scenedesmus s.), information on the effects of culture media of Microcystis s. on Daphnia was little. This paper was to explore the harmful effects of info-chemicals released by cyanobacterium Microcystis aeruginosa during growth and to compare the differences in effects of different strains of M. aeruginosa on the crustacean zooplankton Daphnia magna. Filtered cyanobacteria culture media of two different strains (microcystin-producing and microcystin-free) were used in this study. The results were as follow: (1) Both of the culture media of the two strains selected affected the growth and production of D. magna and the results showed that inhibitory effect occurred at high concentrations, while stimulatory effects were detected at low concentrations; (2) Microcystin-producing strain affected the survival of D. magna much more than the microcystin-free strain did; (3) No significant differences in growth of Daphnia were detected between the two strains, but microcystin-producing strain showed stronger inhibitory effects on production than the microcystin-free strain did. Therefore, it can be predicted that, when cyanobacterial blooms were dominated by microcystin-producing strains M. aeruginosa reduction in abundance and/or biomass of large filter-feeding cladocerans ( such as Daphnia) could probably be explained by the high lethal effects of microcystin; but when the phytoplankton community was dominated by microcystin-free strains, the foregoing phenomenon could not be explained by that, and then it should be attributed to other factors, such a