Study On The Promoting Effect Of Non-toxic Microcystis Additions On The Growth And Reproduction Of Daphnia Magna And Its Mechanism

In recent years,cyanobacteria blooms and its harm has drown highly attention worldwide.Due to lack of key nutrients such as polyunsaturated fatty acids,cyanobacteria is a low-quality food for crustacean zooplankton that cannot support their normal growth and reproduction.Therefore,primary productivity cannot be transferred to the food web during lake blooms.However,large numbers of crustacean zooplankton survived during cyanobacteria blooms in many natural lakes.Field stable isotope studies revealed that cyanobacteria could be an important food source for zooplankton in serveral lakes,which is contradictory to the view that cyanobacteria was not conducive to zooplankton growth.Notice that cyanobacteria contains the general nutrients needed by crustacean zooplankton,such as proteins,fatty acids and polysaccharides in spite of the lack of key biochemicals.Thus,this study hypothesized that the addition of cyanobacteria can promotes zooplankton growth and reproduction while certain amounts of high-quality food was provided.In order to test the hypothesis,The feeding experiment of Daphnia magna was designed in this study,the diet of the D.magna was provided with insufficient quantity of Scenedesmus(0.7mg C·（L·d）^-1)which was labeled by ¹³C stable isotope mixed with Microcystis with different concentration gradient,to observe the effects of Microcystis addition on the growth,reproduction,excretion,egestion and respiration of D.magna,and the carbon budget of D.magna was calculated by ¹³C stable isotope labeling.The main results are as follows:1.Microcystis is rich in carbohydrates and proteins,but don’t contain long-chain polyunsaturated fatty acids,which is a poor-quality food.D.magna tends to retain essential fatty acids,e.g.eicosapentaenoic acid（20:5ω3）,in response to changes in the dietary environment.2.The addition of Microcystis was beneficial to the growth and reproduction of D.magna when Scenedesmus was simultaneously supplied(0.7 mg C·（L·d）^-1).With the increase of the addition concentration of Microcystis,the promotion of the growth and reproduction of D.magna was significantly,The maximum growth rate was 6.29μg·d^-1,and the average number of offsprings of D.magna was up to 13.2 per individual.The rate of respiration,ingestion,egestion and excretion of D.magna showed increased accordingly.3.The proportion of Microcystis used for growth by zooplankton was lower than that in food,while in respiration,egestion and excretion was higher than that in food.With the increase of Microcystis concentration,the proportion of egestion per ingestion,respiration and excretion per assimilation decreased first and then increased,the assimilation efficiency and growth efficiency of D.magna to carbon in food increased first and then decreased with the increase of Microcystis concentration.These results suggested that Microcystis can subsize zooplankton from both structural and metabolic aspects,and promote zooplankton growth and reproduction by regulating carbon source pathways when given a certain amount Scenedesmus(0.7 mg C·（L·d）^-1).This study can explain the contradiction between the results of field investigation and laboratory study,deepen our understanding of the structure of food web during blooms and the elimination process of cyanobacteria,as well as provide theoretical basis for the control of cyanobacteria blooms.