The inhibitory degree between Skeletonema costatum and dinoflagllate Prorocentrum donghaiense at different concentrations of phosphate and nitrate/phosphate ratios

Interactions between Skeletonema costatum (S. costatum) and Prorocentrum donghaiense (P. donghaiiense) were investigated using bi-algal cultures at different concentrations of phosphate (PO₄-P) and nitrate/phosphate (N/P) ratios. Experiments were conducted under P-limited conditions and the Lotka-Volterra mathematical model was used to simulate the growth of S. costatum and P. donghaiense in the bi-algal cultures. Both of these two species were inhibited significantly in bi-algal culture. The results of the simulation showed that the inhibitory degree of S. costaum by P. donghaiense was high when the concentration of PO₄-P was low (0.1 μmolL⁻¹/2 d), but that of P. donghaiense by S. costaum was high with increased PO₄-P supply (0.6 μmolL⁻¹/2 d). At low concentration of PO₄-P (0.1 μmolL⁻¹/2 d), or high concentration of PO₄-P (0.6 μmolL⁻¹/2 d) with high N/P ratio (160), the interactions between S. costatum and P. donghaiense were dependent on the initial cell densities of both species. At high concentration of PO₄-P (0.6 μmolL⁻¹/2 d) with low N/P ratio (25 or 80), S. costatum exhibited a survival strategy superior to that of P. donghaiense. The degree of inhibition of P. donghaiense by S. costaum increased with elevated N/P ratio when the medium was supplemented with concentration 0.1 μmolL⁻¹/2 d of PO₄-P. The degree of inhibition to P. donghaiense by S. costaum increased with elevated N/P ratio at low concentration of PO₄-P (0.1 μmolL⁻¹/2 d). This trend was conversed at high concentration of PO₄-P (0.6 μmolL⁻¹/2 d). However, the degree of inhibition of S. costaum by P. donghaiense increased with the increased N/P ratio at different PO₄-P concentrations (0.1 μmolL⁻¹/2 d and 0.6 μmolL⁻¹/2 d). These results suggested that both phosphate concentration and N/P ratio affected the competition between S. costaum and P. donghaiense: P. donghaiense is more competitive in environments with low phosphate or high N/P ratio and the influence of N/P ratio on the competition was more significant with lower phosphate concentration.