Recently, water pollution with high concentrations of organic matter has occurred frequently in Hakata Bay. It is said that a high level of primary production provides much organic matter and affects water quality of the bay, and it is proved by the appearance of phytoplankton throughout the year. In this study, we simulated variations of phytoplankton population with a box-type model using monthly data in a long-term measurement and we analyzed the final growth rate changes of phytoplanktons that explain the conditions for its appearance. Consequently, we found that the final growth rate varies with pattern of half-year periodicity and water temperature and/or ambient nutrient controls the final growth rate to be low every January and July.

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