Effects of winds, tides and river water runoff on the formation and disappearance of the Alexandrium tamarense bloom in Hiroshima Bay, Japan

Effects of winds, tides and river water runoff on the formation and disappearance of Alexandrium tamarense blooms in Hiroshima Bay, Japan were investigated using data from March to June of 1992–1998. The north wind at the initial growth phase of A. tamarense appeared to have prevented bloom formation by dispersing the organism offshore and/or through turbulent mixing. The decrease in the cell density at the end of the blooms was significantly affected by tidal mixing, indicating that the turbulent mixing induced by tidal excursions may be one of the factors terminating the bloom. Box model analyses applied to the data collected from the observations in 1996 and 1997 showed that river water runoff apparently dispersed the bloom, implying that stratification of the water column due to river water runoff is not necessary for the bloom formation. In conclusion, calm conditions with less wind and tidal mixing along with less river water runoff are considered to be important for the formation of the A. tamarense bloom in Hiroshima Bay, Japan.