1Prefectural University of Kumamoto, Tsukide, Kumamoto 862-8502, 2Kumamoto prefectural Fisheries Research Center, Kamiamakusa, Kumamoto, 869-3603, 3Saga University, Honjo, Saga 840-8502, Japan
In the Yatsushiro Sea, we investigated the population dynamics of phytoplankton and some environmental factors, such as water temperature, salinity and underwater visibility. The field investigations at 8 offshorestations and Himedo were conducted monthly from April 2008 to March 2009 and weekly from June to October 2008, respectively. Moreover, we examined the growth of main phytoplankton species under different conditions of temperature, salinity and light intensity in the laboratory. Diatoms such as Skeletonema costatum, Chaetoceros spp., Thalassiosira spp. and Asterionellopsis gracialis dominated throughout the investigation periods, except for a Chattonella antiqua bloom in August 2008 in the sea. However, the dominant species of diatom fluctuated over time. Laboratory studies indicated that the growth characteristics of main phytoplankton species under different conditions of water temperature and salinity vary among species, where as the growth characteristics under different light intensities are similar to each other; the growth of most phytoplankton species tested saturated at a light intensity of 80 μmol m−2 s−1. From statistical analyses with laboratory data, we found that the seasonal and temporal dynamics of dominant phytoplankton species are closely related to water temperature and salinity and underwater light intensity, respectively, in the Yatsushiro Sea.