1Akashiwo Research Institute of Kagawa Prefecture, Takamatsu, Kagawa 761-0111, 2Kagawa Prefectural Fisheries Experimental Station, Takamatsu, Kagawa 761-0111, Japan
A large diatom Chaetoceros densus bloomed in the coastal area of Kagawa Prefecture from Harima-Nada to Bisan-Seto, the Seto Inland Sea, in February 2005. The maximum cell density of C. densus 90,000 cells/L was recorded in Harima-Nada in mid February. With the increase of this species, nutrients decreased rapidly in the area. Discoloration of Nori, Porphyra yezoensis, occurred mainly at Nori cultures in Harima-Nada. Nutrient levels were as high as or higher than those of typical years until the bloom event. Salinity was remarkably lower during the event than in typical year. This is the first report on the discoloration of cultured Nori caused by blooming of C. densus through nutrient depletion. Careful monitoring of plankton is needed for the prediction of blooming because even minor species may become dominant and cause bleaching of cultured P. yezoensis.