In the nearshore region of the Seto Inland Sea off Fukuyama, where red water blooms of phytoplankton frequently occur, the vitamin B12 content of the sea water was determined at nearly monthly intervals except in winter during 1970 and 1971. Water samples were taken from a depth of 2 m. The vitamin B12 content of the sample water filtered through a membrane filter was determined by the microbiological assay depending on Lactobacillus leichmanii. The results can be summarized as follows:
1) Vitamin B12 was detected in all the samples. Its concentration varied between 0.42 and 6.43 mµg/l
2) The vitamin B12 content was not conspicuously low in the sea water samples taken within red water blooms. Though its seasonal variation was not conspicuous, it was generally high in the months of high water temperatures (ea. 18°C or above); the maximum occurred in June or July when the chlorinity of sea water dropped due to the rainy season; a secondary maximum occurred in September or October.
3) At the station most distant from the river mouth (viz., 6 km) the sea water showed somewhat lower contents of vitamin B12 than at other stations. Besides the river drainage, the bottom mud was also considered as source of vitamin B12 supply.
4) In the present waters the fluctuation and succession of phytoplankton blooms took place during warm months. It does not seem probable, however, that these phenomena were controlled by any paucity of vitamin B12, because the latter was present in these months usually in concentrations of 2 mµg/l or above, which is sufficient to grow some of the vitamin B12 requiring plankters to a population density of about 105 cells/ml