燧灘東部海域における貧酸素層の形成

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In August of 1976, mass mortority of benthic animals, mainly composed of shell fish such as cockle (Fulvia mutica (REEVE)) occurred in the eastern water area of Hiuchi Nada which locate at the central part of Seto Inland Sea. This seemed to be caused by the development of anoxic water in the lower layer of the water column. In August 1977, the heavy depletion of oxygen in this water area was again observed. The survays on dissolved oxygen contents and water temperature were carried out on August 25, 1976 and August 6-8, 1977. In this paper, distribution of dissolved oxygen and the formation of thermocline in summer season are discussed on basis of the data mainly obtained in 1977. On August 5-7, 1977, surface water was saturated with oxygen in whole area survayed and oxygen maximum (120%) layer was found at the depth of 5-8m. Mean depth of this water area is estimated as about 20m. The anoxic water less than 1 mg/1 developed in lower layer off the line of 20m deep which runs along about 4km off Kanonji and 1.5km off Kawanoe. It occupied about one half of this water area (total area 400k㎡). Discontinuity of oxygen contents appeared between 6 and 15m deep. The layer was slightly below the thermocline which was formed between 4 and 10m deep. The maximum temperature gradient estimated near the shore reached to 3.2℃・m-1 and this caused the stratification of water from early summer to autumn. Water stream of this area seemed to be very weak and after the formation of thermocline the water became more stagnant. Organic matters which derive from both waste of paper making and pulp industries locating along the south west coast and plankton residues consume dissolved oxygen in lower layer than that of thermocline to make the water anoxic. COD inflow from the industries is now reported to be about 36 ton・day-1, but total inflow of organic matter including plankton residues still remains to be estimated.近年, 燧灘東部海域では夏期に貧酸素化が進行し, 時に魚介類が斃死し問題になっている. そこで1976年8月および77年8月に, 水温と溶存酸素量を重点的に調査し, 夏期の温度躍層の形成と溶存酸素分布について検討した. その結果, 両年とも表層水は酸素で飽和されているが, 20m以上の深さを有する海域の底層水では1mg/1以下しか検出できなかた. そしてこのような底層の貧酸素化を示す海域は東部海域の約1/2を占めていた. 温度躍層は水深4〜10mに形成され, 最高温度勾配は3.2℃・m-1を記録し, 成層は垂直的に極めて安定なものであった. 溶存酸素の躍層は温度躍層よりわずかに深い5〜15mに認められた. この海域は極度の停滞域であるために成層は初夏から秋まで続き, この間底層水の溶存酸素は紙・パルプ工場排水やプランクトン起源の有機物の分解によって消費される. 工場排水のCOD負荷は現在躍36 ton・day-1とされているが, 海で生産される有機物による負荷については今後さらに検討する必要がある.
香川大学の論文
1978-03-01

燧灘東部海域における貧酸素層の形成

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