The primary production of phytoplanktons provides organic matter in high concentration in eutrophicated Hakata bay in Japan, even during the winter season in spite of low water temperature. Phytoplanktons may have the biological capabilities to maintain activities of photosynthesis under unfavorable conditions, and these capabilities affect water quality in the bays. In this study, characteristics of primary production were analyzed with a simple box-type ecosystem model. We introduced a concept of efficiency for absorption of sunlight energy to our simulation model of water quality to explain rather high growth rates of phytoplanktons in low sunlight intensity. Through simulation with a box model, we found that the efficiency of primary production in winter is higher than that in summer. It was suggested that the organic pollution comes from dissolved organic carbon (DOC) throughout the year and that the DOC is originated from the primary production of phytoplanktons in biological response of the seasonal change of ambient conditions.

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