Effects Of Znic And Iron On Microcystis In Taihu Lake Under Green Light And Disturbance

Trace elements are essential to the growth of phytoplankton,and also affect the community structure of phytoplankton.Different species of phytoplankton have different tolerance to various trace elements,which could be one of the main factors that a particular phytoplankton species becomes the dominant species in the lake.At present,most studies on the effects of trace elements on phytoplankton focus on the effects of trace elements on the growth of single species of Microcystis under laboratory conditions,but there are few reports on the effects of trace elements on the growth of Microcystis in natural water under field conditions.In this study,field experiments were performed.The effects of zinc and iron on the growth of Microcystis in Taihu Lake were compared under disturbance and green light conditions.Besides,the experiments focused on the effects of different zinc concentrations on the growth of Microcystis in Taihu Lake.The main conclusions of this study were as follows:（1）Under the condition of disturbance and green light,the effects of zinc and iron on Microcystis growth and phytoplankton community were compared.During the whole experiment stage,the average number of Microcystis in the control,iron treatment,and zinc treatment group were 1.33,1.40,and 1.24×10⁹ cells/L,respectively.The number of Microcystis in the iron treatment group was significantly higher than that in the control group and the zinc treatment group,and the number of Microcystis in the three groups was maintained from large to small as the iron treatment group,the control group,and the zinc treatment group.Compared with the control group,the number of Microcystis in the 0.4 mg/L zinc treatment group was reduced by 53.10%on 4th days,and the reduction rate was 70.03%on 7th days.While,2 mg/L Iron promoted the growth of most phytoplankton,including Microcystis,and led to the diversity of phytoplankton species.Compared with the control group,the growth rate of the iron treatment group was 29.08%and 9.91%on 4th and7th days,respectively,and the number of Microcystis was 2.14 times that in the control group on 16th days.The results showed that 0.4 mg/L zinc ion could effectively inhibit the growth of Microcystis.（2）Under the condition of disturbance and green light,the effects of different zinc concentrations on the growth of Microcystis in Taihu Lake were studied.The results showed that zinc ion of 0.5～4mg/L could effectively inhibit Microcystis,and the higher the concentration of zinc ion was,the faster the inhibition effect was,which was also shown in Anabaena.During the whole experiment,the average Microcystis density of the control group was 2.29×10⁹ cells/L,while that of the zinc treatment groups of 0.5,1,2,and 4 mg/L were 2.09,1.99,1.95,and 1.85×10⁹ cells/L,respectively.There was a significant difference between the zinc treatment group of 4 mg/L and the control group.In the whole experimental stage,the average density of Anabaena in the control group and the zinc treatment group of 0.5,1,2,and 4 mg/L were 6.486,5.164,4.356,3.882,and 2.276×10⁶ cells/L,respectively.The average density of Anabaena decreased with the increase in zinc concentration,and there was a significant difference between the control group and the zinc treatment group of 4 mg/L（P=0.038）.Zinc ion of 0.5～4 mg/L can effectively promote the growth of green algae such as Scenedesmus and Chlorella.The number of dominant species of Navicular in Bacillariophyta basically showed a positive correlation with zinc ion concentration.In the early stage of this experiment,it promoted the growth of Cyclotella and inhibited its growth in the later stage.The experiment showed that a certain concentration of zinc could significantly inhibit the growth of Microcystis and change the community structure of phytoplankton.（3）Under the condition of disturbance and green light,the inhibition effect of zinc on different concentrations of Microcystis was investigated.The results showed that when 4 mg/L zinc ion was used to treat Microcystis with initial algae density of 0.785,1.595,3.219,and 6.217×10¹⁰ cells/L,the removal rates of Microcystis were 98.75%,90.24%,75.59%,and 62.14%,respectively.It indicated that the lower the initial algae density,the more significant the inhibitory effect of zinc ion on Microcystis.At the end of the experiment,the dominant species in the 0.785×10¹⁰ cells/L treatment group changed from Cyanobacteria to Diatoms.The proportions of Cyanobacteria and Diatoms in the1.595×10¹⁰ cells/L treatment group were 41.7%and 40.25%,respectively,while those in the 3.219and 6.217×10¹⁰ cells/L treatment groups were 79.08%and 90.34%,respectively.The results showed that the inhibitory effect of zinc ion on Microcystis was influenced by the initial concentration of Microcystis.The results of this study show that zinc ion can effectively inhibit the growth of Microcystis at a certain concentration,which provides a new method for controlling Microcystis bloom.