| Shanghai City located in the Yangtze Delta Plain area,where abundance of rivers which played important roles in flood protection,irrigation,navigation,landscape and recreation.With the growth of urban population and the rapid development of economic,rivers which accommodated large quantities of industrial and agricultural waste water and domestic sewage suffering from serious water pollution problems,especially the eutrophication of water.The structure and function of the river ecosystem that appearing cyanobacteria blooms have changed significantly with the eutrophication of water suppling a certain material basis for the frequent outbreaking of cyanobacteria blooms in rivers and the increasing of temperature in river caused by global warming,promoting the proliferation of cyanobacteria and prolonging the duration of cyanobacterial blooms.Filamentous cyanobacteria easily took advantage in eutrophic rivers with the characteristics of variety of species,producing toxin and chain-like cells morphology that making them difficult to be grazed.In addition,filamentous cyanobacteria also have a strong invasion capabilities in river water bodies that make water bodies which have not caused filamentous cyanobacterial blooms are also threatened potentially.In recent years,a number of rivers according to the monitoring in Shanghai have been outbroken Planktonthricoides raciborskii?P.raciborskii?blooms in the summer,which caused the declining of water transparency,the deterioration of water quality and seriously affecting of residents'lives.In view of the above issues,in this paper,the relationship between population dynamics of P.raciborskii and environmental factors in 36 urban rivers which outbroke blooms in Shanghai have been studied from the aspects of temperature,nutrients,algae competition and allelopathy.This study included field surveys and indoor simulation experiments which was benefit for providing guidance for the scientific prediction of blue-green algae in urban rivers.The details of the study are as follows:1.The filed investigation data in summer from 36 urban rivers in Shanghai City were used to explore the relationship between P.raciborskii and environmental factors.The results of generalized additive model?GAM?showed that the biomass of P.raciborskii.significantly increased when the water temperature ranged between 25?and 35?and the pH values ranged between 6.5 and 9.5.Contrarily,P.raciborskii didn't show significant correlation with total nitrogen and total phosphorus?P>0.05?when the concentrations ranged from 0.04 to 7.33 mg·L-1 and from 0.04 to 4.10 mg·L-1,respectively.It may indicate that the high concentration of nutrients in Shanghai rivers was no longer the restricted factors for the growth of P.raciborskii.Furthermore,significantly positive correlation were observed between the biomass of rotifer,protozoan and cladocera with P.raciborskii,while the latter showed a negative correlation with copepod.This correlation pattern between zooplankton and P.raciborskii may be explained by the abundance of small sized edible algae in the water column and the tolerance capability of different sized zooplankton to filamentous cyanobacterium.2?In order to investigate the optimal growth temperature of P.raciborskii,different temperature gradients?15?,20?,25?,30?,32?,35??was setted.The results showed that the chlorophyll-a content of P.raciborskii growthed with the increasing of temperature,the specific growth rate and the photosynthesis efficiency was high when the temperature ranged between 20?and 32?.It was showed that the optimum growth temperature of P.raciborskii ranged between 20?and 32?,and exceeding this threshold was not suitable for the growth of P.raciborskii.The average water temperature of Shanghai in summer was 25-35?,which reached the optimum temperature range of the growth of P.raciborskii.3.Based on the high average concentration of nitrogen and phosphorus in water of Shanghai rivers in summer,the relationship between the different concentrations of nitrogen and phosphorus and the growth characteristics of P.raciborskii was studied under the laboratory conditions.Results showed that when the concentration of nitrogen was less than 1.5 mg·L-1,the biomass of P.raciborskii did not increase significantly with the increase of nitrogen concentration,while the growth of P.raciborskii was unlimited when the concentration of N was above 1.5 mg·L-1 under the condition of sufficient phosphorus concentration.The trend of P.raciborskii increased with the increase of phosphorus concentration,especially when the phosphorus concentration exceeded 0.1mg·L-1,The growth of planktonic algae is not limited under the condition of sufficient nitrogen concentration.Under the condition of sufficient phosphorus concentration,the higher of TN/TP ratio was,the larger of the biomass of P.raciborskii was.The nutrients of Shanghai rivers in summer were adequate when the average concentration of nitrogen and phosphorus was 2.62 mg·L-1and 0.46 mg·L-1 respectively,provided important material basis for the outbreak of P.raciborskii blooms.4.The field investigation showed that the small-sized algae Scenedesmus quadricauda?S.quadricauda?,Chlorella ellipsoidea?C.ellipsoidea?,Microcystis aeruginosa?M.aeruginosa?and Pseudanabaena limnetica?P.limnetica?were also dominant species when P.raciborskii was dominant in rivers.In order to explore the competition mechanism between P.raciborskii and other four dominant species.S.quadricauda,C.ellipsoidea,M.aeruginosa,P.limnetica and P.raciborskii were respectively mixed with 25%:75%,50%:50%,75%:25%ratios in different proportions and cultured under suitable conditions.In addition,S.quadricauda,C.ellipsoidea,M.aeruginosa and P.limnetica were cultured in the filtrate of P.raciborskii.Both of the results showed that S.quadricauda,C.ellipsoideaand M.aeruginosa took more competitive advantage than P.raciborskii and P.raciborskii took more competitive advantage thanP.limnetica.It was mainly due to the fact that when P.raciborskii predominates in rivers,and large-scale zooplankton will cause greater grazing pressure on small-sized algae such as S.quadricauda,C.ellipsoidea compared with the indoor simulation experiment without predation pressure,which may be one of the reason that P.raciborskii possessed the competitive superiority of other four microalgaes.On the other hand,P.raciborskii is more adaptable to the higher water temperature in summer than C.ellipticus,S.quadricosa,M.aeruginosa,and P.limnetica.This may be one of the reasons for the competitive advantages of P.raciborskii.Moreover,P.raciborskii can produce microcystin which did not inhibit the growth of them,Therefore,the production of microcystin of P.raciborskii was not the reason that P.raciborskii predominating in water. |
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