大村湾南部水域海水による赤潮プランクトンGymnodinium sp.(′65年型種)の培養-1-1977,1978,1979年夏期の海況と生物学的分析結果

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  • オオムラワン ナンブ スイイキ カイスイ ニヨル アカシオ プランクトン Gy
  • Growth of Gymnodinium sp. (type-'65) Cultured in the Seawaters Sampled at the Southern Part of Omura Bay―I Bioassay Results and Environmental Condition during Summer seasons of 1977, 1978 and 1979.

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In summer of 1977, 1978 and 1979, the seawaters collected at the southern part of Omura Bay at weekly intervals were examined if the water had the ability to support the growth of Gymnodinium sp. (type-'65). Bioassay was carried out by two ways, one of which was axenic cultivation of Gymnodinium type-'65 in the sample waters at 10m depth sterilized by filtration, to compare the growth with that in the artificial culture medium (control). The other method was to maintain the unfiltered sample waters without any treatment under laboratory condition to compare the population change of Gymnodinium type-'65 with natural population change in situ at the sampling station. In three years, the visible red tide by Gymnodinium type-'65 had occurred four times. Three to eight days before the bloom, every red tide had a heavy rain and/or strong wind which might cause the environmental change. The extreme low level (under 1m ℓ/ℓ) of oxygen content at 1m above of bottom layer (18m) was reflected in the waters at 10m depth, most of which in such cases could not support the growth of Gymnodinium type-'65. In bioassay of the waters at 10m depth by culturing axenically Gymnodinium type-'65, the ability of the seawater to support the growth tended to change abruptly at the sampling day of the occurrence of visible red tide, i.e. in two cases of four red tides, the seawater sampled at 10m depth became suddenly unsuitable for the growth of Gymnodinium type-'65 at the sampling day of the occurrence while the seawater had been able to support the growth till the previous sampling day. In one case, the seawater which had not had the ability to support the good growth till the sampling day before red tide, became suddenly suitable for the growth at the sampling day of the occurrence. The bioassay to check the population change of Gymnodinium type-'65 in the unfilterd test water under laboratory condition showed that only in 2 to 3 weeks just before occurrence of red tide, the results of bioassay on upper layer water were similar to the population change in situ at the sampling station. This study was the first step to find the premonitary symptoms of red tide occurrence by the bioassay methods. The results obtained connected directly neither prediction of the occurrence nor discovery of any trigger of the development of causative organisms. The results were, however, suggestive to improve the bioassay method for the aim of this study.

長崎大学水産学部研究報告, v.52, pp.29-40; 1982

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