The seasonal bloom of the toxic dinoflagellate Alexandrium tamarense, with reference to the ambient oceanographic conditions in Hiroshima Bay, Seto Inland Sea, Japan is described. Long-term observations on the vegetative cells of A. tamarense were conducted biweekly to monthly at one fixed station in northern Hiroshima Bay, where recurrent paralytic shellfish poisoning (PSP) incidents have been occurring since 1992. Over the 5-year study period, from April 1994 to December 1998, vegetative cells of A. tamarense were detected each year within the period from January to June. Observed annual maximum cell densities of A. tamarense reached 10³-10⁴ cells/L, and mostly peaked at a depth layer of 5 m at the sampling station in April or May. Oceanographic conditions during the bloom period were as follows: water temperatures ranged from 10.2°C to 20.2°C, and thermal stratification gradually developed around April. Inorganic nutrient concentrations were markedly low throughout the bloom period. Particularly, the annual lowermost concentration of SiO₂-Si was observed each year during this period. These environmental features indicate that the occurrence of vegetative cells of A. tamarense seems to be explained by temperature and nutrient conditions and that the A. tamarense bloom developed subsequently to or concomitantly with the diatom spring bloom.

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