Ariake Sound is a shallow, eutrophic estuarine system, located in the western part of Japan. We conducted field surveys and experiments in late autumn in 2002, 2003 and 2004 to clarify plankton community structure and trophic interactions in this system. A large photosynthetic dinoflagellate (Akashiwo sanguinea) was persistent and the dominant phytoplankton species in all years. Diatoms and other photosynthetic flagellates were relatively minor components of the assemblage. The growth of

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... nthetic flagellates such as Prorocentrum micans and cryptophycean flagellates under nutrient-enriched conditions was almost balanced with the grazing losses caused by a microzooplankton population dominated by protozoans, rotifers and cyclopoid copepods. Thus, P. micans and cryptophycean flagellates could not propagate their populations in the field. Diatoms had high potential growth rates and were subjected to low grazing pressure by the micro-and mesozooplankton. However, diatom growth was severely limited by light in this highly turbid estuarine system, and light limitation probably prevented diatoms from becoming dominant. A. sanguinea was grazed on by the ciliate Tiarina fusus, but the growth rates of A. sanguinea exceeded grazing losses by T. fusus; other zooplankton species did not graze on this dinoflagellate effectively. A. sanguinea accumulated at the surface layer in the turbid water during daytime, and thus could utilize light effectively. Low grazing pressure by zooplankton and avoidance of light limitation seem to have led to the persistent dominance of A. sanguinea, even though the potential growth rate of this species is moderate to low.