Interactions between the red tide-forming dinoflagellate Gyrodinium instriatum Freudenthl et Lee and the diatom Skeletonema costatum (Greville) Cleve were investigated in bialgal cultures. The growth rates and maximum cell densities of G. instriatum decreased dramatically in cultures containing S. costatum. In addition, morphologically abnormal cells of G. instriatum were observed in these cultures, and many cells lost their motility. Conversely, the growth of S. costatum was generally not influenced by the presence of G. instriatum, although small etiolated cells of S. costatum were occasionally observed when the initial cell density of G. instriatum was high. The growth of G. instriatum and S. costatum in the bialgal cultures was simulated using a mathematical model. This interactive model indicated that G. instriatum and S. costatum steadily approach a stable equilibrium point of about 5 × 10 3 cells ml-1 and 12 × 105 cells ml -1 respectively, when the two species coexist.
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