Prorocentrum donghaiense and Skeletonema costatum are two most typical red tide species in the East China Sea. They can coexist in natural phytoplankton community, but during algal blooms, P. donghaiense blooms are frenquently observed following S. costatum blooms. Many studies have been conducted to clarify the relationship between these two species, but until recently, the mechanism controlling the bloom succession between S. costatum and P. donghaiense is still not resolved. In order to examine the competition between these two species and the effect of allelopathy, this study conducted series bi-algal culture experiments and filtrate experiments.
The bi-algal experiments of P. donghaiense and S. costatum were conducted in f/2 medium. The initial ratios of Chl-a of the two algae were 1:5,1:1 and 5:1, respectively. Experimental results showed that S. costatum suppressed P. donghaiense and become completely dominant in the bi-algal culture. The growth tendency and the maximum growth rate μ of in the bi-algal culture were similar. On the contrary, the growth of P. donghaiense was significantly suppressed by S. costatum from the 5th day and its average maximum cell density in the bi-algal culture was much lower than that in its mono-culture. The results showed S. costatum might have a superior survival strategy to P. donghaiense.
To invesgate the interaction betweem P. donghaiense and S. costatum in the bi-algale culture, Lotka-Volterra model was applied to simulate our results. The simulated results showed that the carrying capacity (K) and growth rate (r) of S. costatum were much higher than that of P. donghaiense, and S. costatum apparently inhibited growth of P. donghaiense in the bi-algal culture. The model indicated that S. costatum would always outcompete P. donghaiense in the bi-algal culture, and initial cell densities induce little effect on the competitive outcome, which matchedthe experimental results. In order to study whether allelopathy could affect the competition between these two species, the filtrates of S. costatum cultivated under f/2 medium were used to incubate P. donghaiense and S. costatum, The results indicated that under f/2 level, N/P=40 level and N/P=120 level, both P. donghaiense and S. costatum in filtrate cultures exhibited the same growth trends as their controls. Therefore, S. costatum filtrates induced little allelopathic effect on the growth of P. donghaiense and itself, and the result might suggest allelopathy was not the critical mechanism for S. costatum to achieve dominance.
In this study, concentration of nutrients in bi-algal and mono-algal cultures was analyzed. The results showed that in bi-algal cultures, nutrient uptake curves, especially the PO₄-P uptake curve, wersimilar to those of S. costatum in mono-algal cultures. This similarity might imply that, the advantaged physiological characteristics of S. costatum, such as higher growth rate, and superior adaptability to absorb nutrients, could contribute for it to outcompete P. donghaiense. In addition, as a sticky algae, S. costatum might inter-specificly coagulate and then remove P. donghaiense from the culture medium.