Relationship between the Bloom of Skeletonema costatum and Water Quality or Meteorological Factors in the Northeast Part of Osaka Bay

The relatioship between the bloom of Skeletonema costatum and water quality or meteorological factors was examined in the northeast part of Osaka Bay, by referring to the Meteorological Agency's data. Monthly changes in water quality factors and phytoplankton in the northeast part of Osaka Bay showed that the DIN : DIP (dissolved inorganic nitrogen : dissolved inorganic phosphorus) ratio was high in the spring and low in the autumn and that S. costatum occurred throughout the year (the range of water temperature : ca. 7-31°C), but suddenly decreased in July or August. The DIN : DIP ratio where S. costatum occurred with a high density of cells was very high (ca. 900) from April to August whereas the ratio was low (ca. 50) from September to February. These results suggested that S. costatum in the northeast part of Osaka Bay showed roughly the spring and autumn types and that both types alternated in July or August.