1Marine Fisheries Research Center, Research Institute of Environment, Agriculture and Fisheries, Osaka Prefectural Government, Misaki, Osaka 599-0311, 2Seikai National Fisheries Research Institute, Fisheries Research Agency, Nagasaki 851-2213, Japan
In late April 2007, a large-scale bloom (>104 cells mL−1) of the toxic dinoflagellate Alexandrium tamarense Balech occurred in Osaka Bay, Seto Inland Sea, Japan. In order to elucidate the ecological profile of the A. tamarense occurrence, we carried out an intensive investigation on diel vertical migration of A. tamarense, associated environmental factors, and cell toxin content from 29 to 30 April 2007, in Sakai Dejima fishing port, Osaka Bay, Japan. Vertical profiles of cell density showed that A. tamarense was distributed in the surface during the day, then dispersed and increased in density on the bottom at night. Before dawn, A. tamarense cells accumulated at the surface, probably due to phototaxis. There was no change in toxin composition, however, toxin content increased from sunset to sunrise. On the other hand, the rate of chain-forming cells decreased. These results suggest that cell toxicity is stored at night and subsequent migration to the bottom increases the risk of shellfish poisoning for edible bivalves.