Journal of Oceanography, Vol. 57 (No. 1), pp. 15-27, 2001
Tomoharu Senjyu1*, Hidekazu Yasuda2, Shigehiko Sugihara1 and Masato Kamizono3
1Department of Fishery Science and Technology, National Fisheries University, 2-7-1, Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan
2Department of Fisheries Information and Management, National Fisheries University, 2-7-1, Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan
3Fukuoka Fisheries and Marine Technology Research Center, Buzen-kai Laboratory, 76-30, Unoshima, Buzen 828-0022, Japan
(Received 20 April 2000; in revised form 25 July 2000; accepted 29 August 2000)
Abstract: A hydrographic survey and a 25-hour stationary observation were carried out in the western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated the upper layer water from the bottom water over the observational area except for near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M2 tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer water is controlled by the advection due to the M2 tidal current. On the other hand, the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due to the M2 tidal current. The steep thermocline and halocline were maintained throughout the observation period in spite of the rather strong tidal currents. This implies an active intrusion of the low temperature and high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a course of circulation.