The Effect Of Allelopthy On The Species Competition Between Skeletonema Costatum And Prorocentrum Donhaiense

In order to investigate the inter-specific competition between Skeletonema costatum and Prorocentrum donghaiense, and verify the effects of allelopathy on the growth and interaction of algae, we designed two parts of experiments, which were conducted under high nutrient conditions and relatively low nutrient conditions, respectively.Under high nutrient conditions, bi-algal culture experiments of P. donghaiense and S. costatum were conducted, with initial Chla ratios set at1:5,1:2,1:1,2:1,5:1,10:1 and 15:1 for P. donghaiense to S. costatum. The results showed that, S. costatum was completely dominant and P. donghaiense was significantly suppressed in all case. In order to quantify the competition in bi-algal cultures, the growth of S. costatum and P. donghaiense was simulated using Lotka-Volterra model, and the simulative results indicated that S. costatum would always outcompete P. donghaiense. Thus, we suggest that the interactions between these two species are not dependent upon the initial Chla ratios or combination of cell densities, S. costatum exhibits a superior survival strategy to P.donghaiense. The filtrates of S. costatum were used to examine the effect of allelopathy on the interaction between S. costatum and P. donghaiense. And the results showed that, S. costatum filtrates induced no significant effect on the growth of P. donghaiense and its own. That might suggest allelopathy was not the main method for S. costatum to achieve dominance, when cultivated with P. donghaiense under high nutrient conditions.Under low nutrient conditions, culture filtrates of S. costatum were tested to verify the effects of allelopathy on the growth and competition of algae. S. costatum growth was inhibited, especially in P limiting nutrient filtrate cultures. The filtrates of S. costatum in exponential phase exhibited significantly inhibitory effect on its own compared with the controls, which was stronger than that in declining phase. Hence, allelopathic effect is connected with the growth phase of allelopathic algae. The filtrates with different ratios showed different inhibitory degree to S. costatum itself, and the ratio was higher the inhibition was stronger. In degradation experiments of S. costatum filtrates, dramatic decrease of inhibitory effect on S. costatum was observed. A possible explanation for this might be that allelopathic compounds in S. costatum cultures could be degraded. In addition, during all experiments using S. costatum filtrates, the growth of P. donghaiense was not affected, that might because of the tolerance of P. donghaiense to these allelochemicals secreted by S. costatum. Furthermore, S. costatum and P. donghaiense were cultivated together in S. costatum filtrates, and the results showed that, P. donghaiense could outcompete S. costatum and achieve dominance in the end. Consequently, allelopathy may have important effect on the competition and succession between S. costatum and P. donghaiense, when the nutrient is low and deficient.