Trends in occurrence of phytoplankton in the Ariake Sea off Saga Prefecture during a Porphyra (nori) cultivation period (1989-2010)

To understand the trends in occurrence of phytoplankton in the Ariake Sea off Saga Prefecture (Japan) during a Nori cultivation period (during October to March), we made out the phytoplankton calendar based on phytoplankton composition data obtained from October 1989 to March 2010. We found that: 1) the dominant phytoplankton during a Nori cultivation period were Skeletonema spp., Chaetoceros spp., Eucampia zodiacus, Asteroplanus karianus, Thalassiosira spp., Dactyliosolen fragilissimus and Pseudo-nitzschia spp.; 2) during January to March, most of above dominant phytoplankton tended to grow more densely than during October to December; 3) in the sea area ranging from Rokkaku river estuary to Shiota river estuary, phytoplankton tended to grow more densely than other sea area of Ariake Sea off Saga Prefecture; 4) during the Nori cultivation period from October 1997 to March 2005, phytoplankton tended to grow more densely than other thirteen years.