Elucidating the seasonal dominance replacement mechanism of harmful raphidophytes Chattonella marina and Heterosigma akashiwo using the Lotka–Volterra model

Phytoplankton growth can cause red tide, lowering water quality and damaging fisheries. The simple Lotka–Volterra model (LV model), estimating the population changes in different species, is often used to analyze the growth and competition of causative species. However, the LV model has not been sufficiently applied to examining the actual occurrence and development of red tides in the field. Therefore, the mechanism of seasonal dominant species replacement and applicability of the LV model were studied to analyze the competition between the harmful raphidophytes Chattonella marina and Heterosigma akashiwo. Sensitivity analysis was performed, and optimization method of competition coefficient (α) was discussed by comparison with measured data from culture experiments. We found that it is crucial to evaluate in advance the initial cell density conditions under which the population size changes concerning α (the order of root mean squared logarithmic error ≥ 10−2). Using this as a reference, we pointed that the initial cell density difference between the two species in co-culture experiments should be as large as possible. In addition, simulation analysis using the model revealed that growth rate (r) and carrying capacity (K) determine H. akashiwo dominance in spring, and the increase in α due to rising temperatures causes C. marina dominance in summer. Comparing α of C. marina with α of C. antiqua cited from a previous study found that the formation of red tides of C. marina less than that of C. antiqua can be explained by α that is 0.42 times smaller than α of C. antiqua. Moreover, the increase in α with increasing temperature is related to the physiological and ecological characteristics of C. marina, which is more likely to dominate the space suitable for growth with temperature rise. We consider that investigations of α, in particular, contributes to modeling various competition and more improving accuracy in analyses. These results provide valuable scientific findings on the occurrence and development of red tide flagellates and their analytical method.