Abstract
The seasonal and horizontal distributions of Skeletonema costatum (GREV.) CLEVE were observed five times in the surface water of Yatsushiro Sea, Kumamoto and Kagoshima Prefectures, from October 1983 to November 1984. Numerically, S. costatum was by far the most important phytoplankton, with two maxima in February and in August. In terms of cell number, this species amounts to more than 50% of the total phytoplankton whenever the community density is more than 105 cells/l. The first maximum in February seems to be triggered by the release from zooplankton grazing pressure, since the high cell density of S. costatum and the high concentration of DIP were observed widely in Yatsushiro Sea. The second maximum in August seems to be attributed to the nutrients supplied through R. Kuma, since a marked horizontal gradient of S. costatum strongly related to those of salinity and DIP.
