VERTICAL AND HORIZONTAL DISTRIBUTION AND ABUNDANCE OF RESTING CYSTS OF THE TOXIC DINOFLAGELLATE ALEXANDRIUM-TAMARENSE AND ALEXANDRIUM-CATENELLA IN SEDIMENTS OF HIROSHIMA-BAY, THE SETO-INLAND-SEA, JAPAN

The horizontal and vertical distributions and abundance of resting cys ts of Alexandrium tamarense and A. catenella were investigated in sedi ments 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/cm(3) in April, 16 to 1476 cysts/cm(3) in May and 57 to 1912 cysts/cm(3) in July, respectively. It was found that the cyst density has increased ca. 30 times within the last 6 years. The vertic al distribution of the cysts indicated that about 80 to 98% of all cys ts existed in 0-3 cm depth. This suggests that mass deposition of the cysts has occurred in the past several years. The present investigatio n found that the cyst abundance in Hiroshima Bay is so high that shell fish poisoning should be carefully monitored to prevent a PSP incident .