Study on Areal Expansion of Harmful Plankton

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Surface sediments were collected by using TU-1 piston corer at Tanabe Bay, Wakayama Prefecture, and Ofunato Bay, Iwate Prefecture, to investigate whether toxic dinoflagellates of the areas occurred long time before the first finding, by comparing cyst distribution in the sediment with plankton record.The sediment core was cut every 1 or 2 cm interval.Alexandrium cyst concentration, age of sediment, and compaction rate of each fraction were analyzed.The TU-1 corer was evaluated as the best among three types of corers including KK and Frager gravity corers regarding the amount of sediment collected and avoidance of mixture of layrs.For measurement of sediment age, the vertical distribution of lead 210 was analyzed by using alpha emission from polonium 210, in addition to the measurement of water contents of the fraction.In Tanabe Bay, Alexandrium cysts were found in all fractions from surface to 30cm deep, having maximum concentration of 175 cysts/cm^3 at3-4cm deep.The deepest layr of 28-30cm in which Alexandrium cysts were detected accumulated around 1945, taking into account of the sedimentation rate as 0.98 cm/y and the compaction rate.As the first finding of planktonic Alexandrium was made in 1980, the organism has already occurred about 35 years before the first observation.In Ofunato Bay, Alexandrium cysts were found in all fractions from surface to 14cm deep, having maximum consentration of 1,620 cysts/cm^3at2-3cm deep.The deepest layr of 12-14cm in which Alexandrium cysts were detected deposited between 1898 and 1914, taking into account of the sedimentation rate as 0.23cm/y and the compaction rate.As the first finding of planktonic Alexandrium was reported in 1963, the organism has already occurred about 50 to 60 years before the first observation.These findings indicate that Alexandrium has occurred a long time before the first observation, and invasion of the species might not occur recently.