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Recent Occurrences of Dinophysis fortii (Dinophyceae) in the Okkirai Bay, Sanriku, Northern Japan, and Related Environmental Factors
Authors:Kazuhiko Koike  Hirotaka Otobe  Minoru Takagi  Tadahisa Yoshida  Takehiko Ogata  Takashi Ishimaru
Affiliation:(1) School of Fisheries Sciences, Kitasato University, Sanriku, Kesen, Iwate, 022-0101, Japan;(2) Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, Otsuchi, Kamiheii, Iwate, 028-1102, Japan;(3) Iwate Prefectural Fisheries Technology Center, Heita, Kamaishi, Iwate, 026-0001, Japan;(4) Department of Ocean Sciences, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo, 108-8477, Japan
Abstract:Occurrence of Dinophysis fortii, a causative organism of diarrhetic shellfish poisoning, in the Okkirai Bay, Sanriku was surveyed in 1995, 1996, 1998 and 1999. In each year, its major occurrence was detected from the late May or early June and continued until the late June or early July. Seawater temperature, salinity and nutrients measurements suggested that inflows of offshore water into the bay played key role on the first major occurrence of D. fortii. With an analysis of continuous temperature data in the Otsuchi Bay which locates north of the Okkirai Bay, this influent was considered to be intermittent inflow of the offshore water by internal tidal waves which propagated from north to south. First occurrence peak of D. fortii was synchronous with phycobilin containing microalgae, synechococcoid cyanobacteria and cryptomonad, in all years. In vivo fluorescence measurement of D. fortii cells in 1995 and 96 showed that the cells in these microalgal-rich water contained more phycobilin pigment than those in the microalgal-poor water. The result may support a hypothesis that D. fortii acquires phycobilin by an uptake of these microalgae. After the first major occurrence in the bay, D. fortii sometimes occurred in rather inshore waters where showed elevated ammonium level possibly due to increased heterotrophic activity. Together with another finding that D. fortii is mixotrophic, it could be assumed that the environment being suited to heterotrophic nutrition also stimulates D. fortii growth in the bay.
Keywords:Dinophysis fortii  diarrhetic shellfish poisoning  Sanriku  Okkirai Bay  Otsuchi Bay  internal tidal wave  synechococcoid cyanobacteria  cryptomonad
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