Experimental Studies On The Toxic Effects Of Microcystis Aeruginosa On Cladocerans And Bivalves

The toxic influences of Microcystis aeruginosa PCC7820 on three cladoceran species and two bivalves were studied through laboratory experiments.Toxic M. aeruginosa PCC7820 not only decreased the survival rate and population increase rate but also delayed maturation of Daphnia pulex. A rise of the experimental temperature from 20°C to 30°C caused an obvious increase in the toxicity of M. aeruginosa PCC7820 to D. pulex. Population increase rate of A pulex significantly increased when good quality food (Scenedesmus obliquus) was added to toxic M. aeruginosa PCC7820. The present results suggest that zooplankton community in a natural bloom may be affected not only by toxic cyanobacteria (toxin level and abundance of toxic algae) but also by the abundance of edible algae.M. aeruginosa PCC7820 decreased the survival rate of the two small cladocerans Moina micrura and Ceriodaphnia cornuta. The survival rates were in the order of non-toxic M. aeruginosa > starved > toxic M. aeruginosa. M. micrura was more sensitive to M. aeruginosa PCC7820 and starvation than C. cornuta. M. aeruginosa inhibited the feeding rates of the two cladocerans. S. obliquus significantly reduced the toxicity of M. aeruginosa PCC7820. Population of both cladocerans significantly increased when Scenedesmus obliquus was added to the food.When S. obliquus, toxic and non-toxic M. aeruginosa were offered as single food to Unio douglasiae, there was no significant difference in clearance rate, but the clearance rate of Corbicula fluminea on toxic M. aeruginosa was significantly lower than that on the other two algae. Clearance rates of both two bivalves were higher on mixed food than on single food. The excretion products (faeces and pseudofaeces) of two bivalves contained mainly S. obliquus. The ratio of S. obliquus to M. aeruginosa in both mixed-food treatments increased in the excrete products of both bivalves, compared with the ratio of these phytoplankton species in the food. It seems that both bivalves prefered M. aeruginosa to S. obliquus.In conclusion, toxic M. aeruginosa PCC7820 exerted significant toxic effects on...