1Tajima Fisheries Technology Institute, Hyogo Prefectural Technology Center for Agriculture, Forestry and Fisheries, Mikata, Hyogo 669-6541, 2Plankton Laboratory, Division of Marine Bioresource and Environmental Science, Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The seasonal changes in cell density and cell size of the harmful diatom Eucampia zodiacus were investigated for seven years (April 2002-March 2009) in Harima-Nada, the eastern part of the Seto Inland Sea, Japan. Vegetative cells of E. zodiacus were detected year-round in water columns, and the restoration of cell size occurred once in autumn every year just after reaching the minimum cell size. Average cell densities of the E. zodiacus population in autumn had a significant negative correlation with the integrated number of days during the period from restoration cell size to peak of the bloom. It was considered that vegetative cells of E. zodiacus observed in autumn play an important role as seed populations in winter and early spring blooms. Therefore, it is proposed that the time of “nori” bleaching occurrence can be predicted by monitoring the cell size restoration and the mean cell density of E. zodiacus in autumn in Harima-Nada.