The horizontal and vertical distributions and abundance of resting cysts of Alexandrium tamarense and A. catenella were investigated in sediments of Hiroshima Bay in April, May and July 1993. Cysts were counted using fluorochrome primuline-staining and epifluorescence microscopy. Cysts of Alexandrium spp. were found at all stations examined. Higher densities were observed in coastal waters off Hiroshima City and Kure City. These horizontal distributions were almost identical throughout the investigation from April to July. The cyst densities ranged from 50 to 1304 cysts/cm3 in April, 16 to 1476 cysts/cm3 in May and 57 to 1912 cysts/cm3 in July, respectively. It was found that the cyst density has increased ca. 30 times within the last 6 years. The vertical distribution of the cysts indicated that about 80 to 98% of all cysts existed in 0-3cm depth. This suggests that mass deposition of the cysts has occurred in the past several years. The present investigation found that the cyst abundance in Hiroshima Bay is so high that shellfish poisoning should be carefully monitored to prevent a PSP incident.
