Phytoplankton changes effected by anxious water of enclosed

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In order to understand a relation between phytoplankton floral change and nutrient supply, investigations of formation of anoxic environments, mechanisms of nutrient supply, and distribution of nutrients were carried out in Omura Bay, West Kyushu, Japan. Results were follows ; intrusion of dense seawater forms stratification at the center basin, and then anoxic water is produced by microbial respiration. Under anoxic environments, NH_4 and PO_4 are selectively dissolved from organic-enriched bottom sediments. In a mesocosm experiment where we produced an anoxic condition artificially, a floral change from diatoms to dinoflagellates occurred after selective regeneration of NH^+_4 and PO^<3->_4. Historical change of dinoflagellate cysts recorded in core sediment of Omura Bay suggests that eutrophication reflected by increase of heterotrophic dinoflagellate cysts appeared around 1970s.