1Laboratory of Marine Environmental Science, Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka 812-8581, 2National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Hatsukaichi, Hiroshima 739-0452, Japan
Results of a four-year study on phytoplankton in Hakozaki Harbor, Hakata Bay, Japan indicated that Akashiwo sanguinea (Hirasaka) Hansen et Moestrup was the cause of frequent blooms during the late fall period when centric diatoms such as Skeletonema costatum and genus Chaetoceros declined. In laboratory experiments, the growth of A. sanguinea was found to be strongly inhibited in bi-algal cultures with S. costatum and Chaetoceros didymum, but just slightly with Asterionella japonica. Furthermore, when A. sanguinea was cultured in nitrogen and phosphorus enriched filtrates with media on which S. costatum and C. didymum had grown densely, the growth rate of A. sanguinea in the filtrates was significantly lower than that in fresh control medium. These results suggest that in spring and summer allelopathy of bloomed centric diatoms was an important factor that inhibited the growth of A. sanguinea but in the late fall when the cell density of these diatoms declined, A. sanguinea could form blooms.