Evaluation for the role of Noctiluca scintillans on the material circulation in the Coastal Sea

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Noctiluca scintillans are widely distributed throughout whole oceans in the world and are also known to live in the Seto Inland Sea throughout the year, especially from spring to summer. It was recognized that ammonia included in high concentration in cellular fluid was toxic to fish and an extra-ordinary mass development of this species can cause severe damages to coastal fish farming. On the other hand, this heterotrophic dinoflagellate may act as one of the most important organisms contributing to nitrogen and phosphorus cycle in the coastal sea because of its significantly high content of ammonium nitrogen and phosphate.Ammonium nitrogen, phosphate, and silicate concentrations in cell fluid were estimated at 2470,183, and 54 pmol ind^<-1>, respectively. Particulate C : N : P ratio of the natural cells of the species was estimated at 143 : 14 : 1. Ammonium nitrogen and phosphate concentrations at surface layr of the water column seemed to be related to cell abundance of N.scintillans at the sampling location. Patches of N.scintillans provided 16 and 25 times of ammonium nitrogen and phosphate, respectively, in the uppermost layr (0-10 cm depth) of the water column, higher than those which contained in the ambient sea water. Silicate concentration within patches, however, was close to that in surrounding water. Morphological of fecal pellet of N.scintillans fed on the diatom, Thalassiosira sp., was examined by a scanning electron microscope. Fecal pellet composed entirely of intact diatom cells. It could be inferred that cells excrete silica as fecal pellcts and thus no remarkably high silicate concentration within red tide patches was observed. Relcased nutrients by N.scintillans at the uppermost part of the water column could be subsequently utilized by other autotrophic species living at the sea surface where light and carbon sources are available.