| With the intensification of eutrophication in the East China Sea（ECS）,phosphorus nutrient has become the main limiting factor for its primary productivity and the biomass of different algae has changed accordingly,which has affected the results of interspecific competition and the species composition of phytoplankton community.In order to investigate the mechanism of generation and disappearance of the harmful algal blooms（HAB）in the ECS,three typical dominant species,Skeletonema costatum,Prorocentrum donghaiense and Karenia mikimotoi,were selected to carry out the experiments of bi-algal culture and co-algal culture under different phosphorus concentrations with reference to the actual proportion of DIP:DOP（1:1）and temperature changes（16-24℃）in spring and summer in this paper to simulate the algae interspecific competition process in the ECS.Besides,the Lotka-Volterra competition model and relative advantage Dt were used to evaluate the mutual inhibition of algae in bi-culture and co-culture,and the effects of different phosphorus sources,different concentrations and temperature changes on the interspecific competition were compared to explore the competitive inhibition mechanism among different algae and provide theoretical basis for the prevention and control of the HABs in the future.The main research results of this paper are summarized as follows:（1）The results of bi-algal culture experiment under under P-sufficient condition showed that S.costatum could gain advantage in the interspecific competition between diatom and dinoflagellate because of its higher growth rate(2.298 d-1),and the relative advantage（Dt）could reach 0.22～0.78 and 0.07～1.15 in the bi-algal culture between S.costatum and P.donghaiense（or K.mikimotoi）.However,the Lotka-Volterra model showed the inhibition intensity of P.donghaiense（or K.mikimotoi）on S.costatum was about 21 and 25 times higher than that of S.costatum on P.donghaiense（or K.mikimotoi）respectively,so the survival strategy of P.donghaiense and K.mikimotoi was better than that of S.costatum in bi-algal culture.This was different from the previous results of competition inhibition,indicating that two phosphorus sources of DIP and DOP played a key role in the interspecific competition between diatom and dinoflagellate.In addition,P.donghaiense had a competitive advantage in the bi-algal culture with K.mikimotoi because the optimum growth temperature of P.donghaiense in the ECS was consistent with the experimental temperature（20℃）.The competitive inhibition parameter BPK(5.950×10-6 ml cell-1s-1)of P.donghaiense on K.mikimotoi was slightly higher than the inhibition parameter AKP(6.010×10-6 ml cell-1s-1)of K.mikimotoi on P.donghaiense,and the Dt was positive（0.18～1.55）throughout the experimental period.（2）Considering the phosphorus nutrients in seawater were often scarce after the blooms of S.costatum,the bi-algal competition experiments were also carried out under P-deficient condition.The results showed that the competition results in the P-deficient environment were different from those in the P-sufficient environment,dinoflagellates could have competitive advantage mainly because they had a larger cell phosphorus pool and could express more alkaline phosphatase（AP）to utilize DOP efficiently.In the later stage of the experiment,the Dt between S.costatum and P.donghaiense（or K.mikimotoi）was only-0.09～-0.90 or-0.07～-0.57 respectively,and the inhibition parameters APS and AKS(3.045×10-5 ml cell-1 s-1 and 2.762×10-5 ml cell-1 s-1,respectively)of P.donghaiense and K.mikimotoi to S.costatum were significantly higher than those of BSP and BSK.For the interspecific competition between two dinoflagellates,K.mikimotoi was more efficient in using DOP and had stronger allelopathic effect under P-deficient condition compared with P.donghaiense,thus K.mikimotoi was the winner of interspecific competition finally,and the competitive inhibition parameter AKP(3.205×10-5 ml cell-1 s-1)of K.mikimotoi on P.donghaiense was about 6 times of the inhibition parameter BPK(5.609×10-6 ml cell-1 s-1)and its Dt was negative（-0.04～-1.42）on day 4-8.（3）In order to further study the mechanism of competition among various algae species,a co-culture experiment of three algae was set up based on the temperature change in spring and summer（16-24°C）and the actual DIP and DOP structure ratio（1:1）of the ECS.The results showed that S.costatum was able to win the competition among three algae with higher growth rate and lower optimum growth temperature in the early stage of the experiment.The inhibition parameters ASP and ASK(0.071×10-5 ml cell-1 s-1and 0.065×10-5 ml cell-1 s-1)of S.costatum on P.donghaiense and K.mikimotoi were significantly higher than the inhibition parameters BPS and BKS(-0.539×10-5 ml cell-1 s-1and-0.797×10-5 ml cell-1 s-1)of the two dinoflagellates on S.costatum,and the Dt of S.costatum-P.donghaiense group and S.costatum-K.mikimotoi were 0.33～1.32 and0.02～3.16 respectively.In addition,S.costatum also won significantly in the middle of the experiment（day 41-60）due to the nutrients provided by the pulsed algae solution.However,P nutrients showed a trend of scarcity in the early stage（day 21-40）and the end of the experiment（day 61-75）,P.donghaiense could occupy a dominant position in the competition due to its stronger ability to store and utilize DOP.The inhibition parameter BPS of P.donghaiense on S.costatum was about 38.69 and 4.35 times than the BSP of S.costatum on P.donghaiense and the Dt was reduced to-0.22～-1.75 and-0.35～-1.49.As for K.mikimotoi,we believed that it could not grow under the dual inhibition of low temperature at the early stage of the experiment and the possibly existing allelopathy.In summary,this paper provided a new perspective for simulating the algae competition process in the ECS and further exploring the generation and extinction mechanism.Subsequent studies should focus on the effects of temperatures and allelopathy on the occurrence and disappearance of red tide,as well as the effects of cell phosphorus pool and polyphosphate on the interspecific competition of algae,so as to clarify the interspecific competition mechanism of HABs. |
