Formation　of　Anoxic 　Bottom　Waters 　in Omura　Bay

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Other Title
  • 大 村湾 に お け る 無酸 素水 塊 の 形 成

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Abstract

Omura Bay, covering an area of 320km^2, which is located in the northwestern part of Kyushu District, is connected with a southern part of Sasebo Bay by two narrow straits. Therefore, a sufficient water change corresponding to the seawater volume of the bay does not take place even at spring tides. A tidal range of the bay drops to one-third (mean range: 0.54 meter) and the mean high tide interval is delayed by about three hours as compared with Sasebo Bay. Movement of the bay water is stagnant. Strong stratification and anoxic bottom waters are formed in summer season. An analysis of oxygen data for the recent ten years clarified that there are three characteristics on the anoxic phenomena of this bay. The first is that a variation of the dissolved oxygen content between a much productive year and a much consumable year is great in the surface zone above a layer of oxygen saturation content which is usually observed at the depth of 7.5 meters. The second is that a greatest scale anoxic phenomenon such as covering the whole central waters occurs at an interval of ten years scale. Benthic communities suffer a deadly blow by the phenomenon and also an outburst of Gymnodinium nagasakiense red tide might be accelerated in such a year. The third is that a swell phenomenon of the anoxic seawater was often observed in the bottom zone. The phenomenon is likely to be transient although the swell sometimes reached as high as five meters above the bottom floor. Details of the swell such as the spatial distribution and developmental mechanism are still unknown.

Journal

  • Bulletin on Coastal Oceanography

    Bulletin on Coastal Oceanography 26 (2), 75-86, 1989

    Coastal Oceanography Research Committee, the Oceanographic Society of Japan

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