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Osaka Bay is the semi-enclosed bay and one of the eutrophic bays in Japan. Various red tide species are found in Osaka Bay over the year, and diatom dominates usually in the Yodo River estuary, but other harmful species appear sometimes. The transition of dominant species can occur even in shorter time but its reasons are indistinct. To clarify the short time variation of the marine environment and dominant phytoplankton species, the field observation was carried out during the 16 days from the end of June to July. Greatly diurnal change was found in all data. Thalassiosira spp. dominated throughout the period. On the other hand other diatom species decreased, and dinoflagellate and other species increased in later period. All nutrient concentrations decreased toward July 5, but DIN and DIP concentrations recovered and DSi concentration continued the low condition after July 7. From the results based on nutrient concentrations and those mole ratio, the growth of all species was limited by DIP before 2 July. Diatom has advantage in this case because can grow under the lower DIP concentration. After that, nutrient which limits the growth of diatom changed to DSi due to the decrease of DSi concentration. Then DIP and DIN concentration increased in favor of dinoflagellate.