About in the middle of the Seto Inland Sea extending east and west, a somewhat pear-shaped sea region is observed; its northern border (i.e., the tip of the pear-shape) being that part of the southern shore of Honshu where Fukuyama and Kasaoka cities are located; its southern border (the bottom of the pear) being the middle portion of the northern shore of Shikoku, where Kan-onji, Iyo-mishima, Niihama and Imabari cities are situated in an east to west line. This sea extension communicates with the shallower Bisan Straits in the northeast; the commonly accepted boundary between them runs from Konoshima on, south to Obi-shima, and then southeast to Misaki. In the west, this sea region is bordered by the eastern edges of the island chain (the Geiyo Islands) extending from Tomo southwest to Imabari, and communicates with the less neritic Aki-nada region by several straits.
This sea region measures nearly 1,900 km2 in area and contains about 36 km3 of water. Its diameters are ca. 58 km laterally (Imabari to Kan-onji) and ca. 60 km longitudinally (along the 133°30' E meridian) (Fig. 1).
This region can be subdivided into a northern and a southern portion by the arc of small islands including Uo-shima and Ibuki-shima; the former is called Bingo-nada and the latter, Hiuchi-nada.
As part of the efforts to clarify the dynamics of organic production in this sea region, the authors performed, in early February, 1968, the measurements of the oceanographical factors named in the title above as well as pH, dissolved oxygen and turbidity of the sea water and also the depth of disappearance of the Secchi disc ('transparency'). Eleven stations were surveyed altogether during a 2-day cruise (Fig. 2). The driest and coldest season of the year was chosen for the observation.
This paper presents the results of this examination (Table 1). The survey should be regarded as a preliminary attempt, since the authors did not yet succeed in elucidating the real mechanisms underlying the apparently complicated distributions of some of the measured factors.
Following are the noticeable features of the results.
Reflecting the convective cooling, the water temperatures proved vertically uniform at all stations except St. 6 (Fig. 3); chlorinity and dissolved oxygen were also nearly uniform vertically at all stations. Geographically, both temperature and chlorinity were highest along the western border of the sea region, and lowered towards the east (Figs. 4 & 5). Both elements reached a minimum near the Bisan Straits; they reached another minimum at the innermost part of Hiuchi-nada (near Iyo-mishima). This distribution pattern may point to the following tendency: the more saline and warmer water distributed to the westward was flowing into Bingo-nada and Hiuchi-nada through the straits of the Geiyo Islands and was then cooled and diluted while flowing eastward. It is also inferable that less saline and colder water was found in the Bisan Straits.
The percentage saturation of dissolved oxygen was vertically uniform at all stations. Along the western border it was somewhat lower (ca. 96%); it showed a general trend towards increase towards the east except for the considerably low values (ca. 93%) at St. 15 (Table 1).
Concentrations of inorganic nutrients, which were determined by absorptiometry using unfiltered water samples, showed to be within the following ranges: phosphate-P, 0.28-0.53; nitrite-N, 0.0-0.40; nitrate-N, 0.6-1.7 and ammonia-N, 0.0-5.3 µg-atoms/l (Figs. 7 & 8). It was difficult to find any pronounced regularity regarding the vertical and/or geographic variations in their concentrations. This difficulty might have been overcome to a considerable extent if simultaneous measurements had been taken of organic phosphorus and nitrogen of dissolved and particulate forms as well as of the activities of marine organisms in situ (particularly of phytoplankton).
The Secchi disc 'transparency' was about 7 m along the western border, about 5 m in the central part, and between 6 and 8 m in the eastern part of the surveyed region. The only exception was the unexpected high value (11.6 m) at St. 15 (Fig. 6). The light scattering capability (‘turbidity’) of the water samples from various depths (except the bottom layer) was comparable with that of kaoline suspensions of 0.7-2.6 ppm. In neither vertical nor geographic distribution of turbidity, any pronounced regularity could be detected.