Progress in Seto Inland Sea Research

Home , Akashi Strait, Inland sea (geology), Osaka Bay, Seto Inland Sea

Journal of Oceanography, Vol. 58, pp. 93 to 107, 2002

Review

HIDETAKA TAKEOKA*

Center for Marine Environmental Studies, Ehime University, Bunkyo, Matsuyama 790-8577, Japan

(Received 14 May 2001; in revised form 1 September 2001; accepted 5 September 2001)

The Seto Inland Sea is a representative coastal sea in Japan with a complicated geom- Keywords: etry and thus a variety of marine environments. This sea is, at the same time, one of ⋅ Seto Inland Sea, the most industrialized areas in Japan, and its marine environment has been signifi- ⋅ anthropogenic cantly affected by the anthropogenic impacts over the last four decades. The wide impacts, ⋅ range of marine environments in this sea and the serious environmental issues result- heat bypass, ⋅ nutrient trap, ing from these impacts have attracted the attention of Japanese coastal oceanogra- ⋅ tide and tidal phers. It is believed that the nature and scope of these studies might be an example of current, the progress of Japanese coastal oceanography. The historical changes in the Seto ⋅ red tide, Inland Sea environment in the last four decades are briefly summarized, and the ⋅ oxygen-deficient progress in the studies of the Seto Inland Sea is reviewed with reference to historical water mass, changes. Some recent research topics and activities are also mentioned. ⋅ fronts.

1. Introduction studies might be a good example of the progress of Japa- The Seto Inland Sea in Japan is a semi-enclosed nese coastal oceanography. This paper therefore presents coastal sea surrounded by Honshu (the main island of a review of these studies. In Section 2, historical changes Japan), Shikoku and Kyushu Islands (Fig. 1). It has a in the marine environment of the Seto Inland Sea are dis- length of 500 km, an average depth of 30 m and contains cussed as a background to the present paper. The various approximately 600 islands. The sea is divided by islands individual studies are then discussed in Section 3. Ad- and peninsulas into wide basins, some of which are called vances in Seto Inland Sea studies are also reviewed in “nada” in Japanese, and these basins are connected by Section 3, classified according to subject. Recent research narrow channels called “seto”. This complicated geom- topics and activities are mentioned in Section 4. etry results in wide variations in the marine environment. The Seto Inland Sea region is also one of the most 2. Historical Changes in the Seto Inland Sea Envi- industrialized areas in Japan. After the New Industrial City ronment Law was enacted in 1963, many facilities for heavy in- Figure 2 shows a chronological table of events af- dustry were built in the coastal areas surrounding the sea. fecting the Seto Inland Sea over the last four decades, Urbanization of the coastal area also increased. At present, along with a description of completed studies concerning approximately 35 million people live within the Seto In- to the region and the annual number of red tide occur- land Sea watershed. This industrialization and urbaniza- rences in the region. The number of red tide incidents tion required substantial reclamation of land. The marine can be used as an indicator of eutrophication. During the environment of the Seto Inland Sea has been significantly 1970s, eutrophication advanced rapidly due to an increase affected by these impacts over the last four decades. in the volume of industrial and urban waste, resulting in The wide range of marine environments in the Seto a frequent occurrence of red tides and oxygen-deficient Inland Sea and the serious environmental issues result- water masses. These occurrences had a great impact on ing from anthropogenic development in the region have the marine environment. In 1970, a mass mortality of fish attracted the attentions of Japanese coastal oceanogra- occurred in Hiuchi-Nada due to an oxygen-deficient wa- phers. It is believed that the nature and scope of these ter mass. Large-scale red tides of Chattonella often occurred in Harima-Nada. The red tides that occurred in * E-mail address: [email protected] 1971 damaged the fish farming industry up to the value Copyright © The Oceanographic Society of Japan. of 7.1 billion yen.

93 Fig. 1. Map of the Seto Inland Sea. Lines in the inland sea denote the routes of the ferry boats (see Subsection 3.8).

To solve these problems, the Environmental Agency 3. Progress in Seto Inland Sea Research of Japan enacted “The Interim Law for Conservation of the Environment of the Seto Inland Sea” in 1973 and “The 3.1 Basic descriptive studies Law Concerning Special Measures for Conservation of The Prefectural Fisheries Observatories around the the Environment of the Seto Inland Sea” in 1978. These Seto Inland Sea started monthly observations of water laws resulted in a COD reduction to some extent. In ad- temperature, salinity and transparency at 320 fixed sta- dition, the annual number of red tides gradually reduced tions in 1964 in a project supported by the Fisheries between the mid 1970s and the mid 1980s, but has re- Agency. The Maritime Safety Agency accumulated tide mained stable since then. and tidal current data. In the 1970s and 1980s, many de- Prior to 1973, land reclamation was intensive. In scriptive studies summarizing these data were completed. average, approximately 16 km2 was reclaimed annually Yanagi and Higuchi (1979) analyzed the historical tidal between 1965 and 1973. This was regulated by the In- current data measured by the Maritime Safety Agency, terim Law for Conservation of the Environment of the and produced a chart of residual current flow patterns. Seto Inland Sea, but still took place to some extent after Yanagi and Higuchi (1981) also produced charts of the the law was introduced. The total area of land reclaimed amplitude and phase lag of the M2 and K1 constituents of since 1965 is approximately 250 km2, which is approxi- tide and tidal current. Summarizing the historical mately 12% of the area with a depth of less than 10 m in hydrographic data obtained by fisheries observatories, the Seto Inland Sea. More than half of the marine forest Takeoka (1985) revealed the distribution of stratification that existed in the early 1960s has been lost by reclama- in the Seto Inland Sea. Takeoka (1987) also described the tion. transparency distribution and analyzed the seasonal and Another environmental issue that has recently be- spatial differences in the distribution. come acute is the dredging of sand and gravel from the Among the results of these studies, distributions of seabed. The great demand for building materials and the M2 tide and M2 tidal current are shown here, because the lack of suitable quarries in western Japan led to an in- tide and tidal current are the most basic and important crease in dredging activity. Sea-sand dredging increased factors characterizing the Seto Inland Sea and the M2 rapidly in the late 1960s, thereafter 2 × 107 m3 of sea- constituent is dominant over almost the whole Sea. Fig- sand has been dredged annually. However, some prefec- ures 3(a) and (b) indicate the distributions of the tidal tures surrounding the Seto Inland Sea prohibited dredg- range and the phase lag of the M2 tide, and Figs. 3(c) and ing due to public pressure. (d) the distributions of the amplitude and the phase lag of

94 H. Takeoka Fig. 2. Chronological table of the events related to the Seto Inland Sea and the themes and projects of the Seto Inland Sea studies.

Progress in Seto Inland Sea Research 95 Fig. 3. Distributions of (a) tidal range, (b) phase lag of tide, (c) current amplitude and (d) phase lag of tidal current of M2 constituent in the Seto Inland Sea (after Yanagi and Higuchi, 1981). Areas where the current amplitude is larger than 70 cm sÐ1 are hatched in (c).

the M2 tidal current (Yanagi and Higuchi, 1981). In the features can be seen from the tide and tidal current phase Pacific Ocean, south of the Seto Inland Sea, tidal waves distribution plots. The phase of the tide is almost equal in propagate from east to west at significant speeds due to the central region and differs spatially in the eastern and the great water depth. Hence the phase of the tides at the western regions. The phase difference between the tide mouths of the Kii and Bungo Channels (see Fig. 1) are and tidal current is approximately 90° in the central re- approximately equal (Fig. 3(b)). The tidal waves propa- gion and is much smaller in the eastern and western re- gate at a much lower speed in the Seto Inland Sea due to gions. As a result of such tidal features, the flood tidal the shallow water depth. The waves propagate from the currents are directed to the area into which the tidal waves two channels into the inland sea over a long period of assemble. Moreover, the transport volume of the tidal time and meet at the central part of the Seto Inland Sea current is larger in the outer regions, and almost vanishes between the Bisan Strait and Hiuchi-Nada where the phase in the assembling area. Therefore, except in the narrow lag of M2 tide is the largest. They are delayed by about straits, the amplitude of the tidal current is generally larger 150° from the mouths of the channels (Fig. 3(b)). Both in the outer regions and smaller in the eastern part of tidal waves entering from the channels are dissipated Hiuchi Nada (Fig. 3(c)). In the narrow straits the ampli- during propagation. Therefore, the amplitudes of the tidal tudes of the tidal currents are much larger than those in waves propagating eastward and westward are approxi- the basin interior. The amplitudes of the four major tidal mately equal in the central part of the inland sea, whilst current constituents in the main straits are shown in Ta- in the eastern and western regions the amplitude of the ble 1. The maximum tidal speeds in some straits, such as tidal wave from the adjacent channel is greater than that Naruto and Kurushima Straits, approach 5 m sÐ1 in the from the opposite channel. Thus the tidal wave in the cen- spring tides. These strong tidal currents play a signifi- tral region becomes a stationary wave, whilst those in the cant role in maintaining high biological productivity in eastern and western regions are progressive waves. These the Seto Inland Sea as described later.

96 H. Takeoka Table 1. Amplitudes of four major tidal current constituents in Sea, converting the values obtained by other researchers the main straits of the Seto Inland Sea (after Yanagi and into values under the same definition. Furthermore, Higuchi, 1981). Imasato et al. (1980) and Awaji et al. (1980) revealed that the Stokes Drift due to large gradients of amplitude M S K O 2 2 1 1 and phase difference of tidal current around the straits (cm sÐ1) plays an important role in the tidal exchange. Tomogashima Strait 105 25 40 35 The second group of studies examined the transport Naruto Strait 330 90 50 50 and renewal of water or materials filling the individual Akashi Strait 160 60 50 50 basins or the whole Seto Inland Sea. Takeoka (1984a) Bisan Strait 95 35 15 10 published a theoretical study of the time scales that rep- Kurusima Strait 250 100 40 30 resent the transport of materials and water renewal, and Tsurushima Strait 90 45 25 20 proposed some basic concepts such as residence, transit Ohbatake Strait 250 75 25 20 and turn-over times and the average age that can be ap- Hayasui Strait 155 70 35 25 plied to problems in coastal seas. Takeoka (1984b) ap- Kanmon Strait 225 80 65 45 plied this theory to the experimental results obtained with a 1/50000 scale hydraulic model of the Seto Inland Sea over some kinds of the various time scales including the average residence times of the river water, oceanic water 3.2 Horizontal transport and water exchange and total water. He concluded that the average residence Coastal oceanographic studies in the 1970s and 1980s time of the total water was approximately 14 months. focused on horizontal transport processes and their quantitative evaluations, because the horizontal transport proc- 3.3 Interdisciplinary studies in the 1980s esses expel pollutants from bays and were regarded as a Until the early 1970s, the interest of oceanographers purification mechanism against marine pollution. was focused mainly within their own disciplines, although Hayami and Unoki (1970) gave a significant impact a few collaborative studies with other disciplines were on horizontal transport studies. On the basis of salt budget done. Since the end of 1970s, with support of public analysis, they concluded that the apparent one-dimen- awareness of environmental conservation, several inter- sional diffusivity along the axis of the Seto Inland Sea is disciplinary study groups were formed. These were mainly 107 cm2sÐ1. Considering the scale dependence of horizon- supported by Grants-in-Aid for Scientific Research from tal diffusivity in marine environments, this value seems the Ministry of Education, Science and Culture of Japan. to be too large. However, the value itself was accepted Two groups with special interests on red tides were because it was obtained from a very simple budget model. established in 1978 for basic biological studies on red This large apparent diffusivity was due to dispersion pro- tides (headed by T. Yanagida), and studies on the physi- duced by the linked effect of eddy diffusion and current cal, chemical and biological mechanisms of red tide gen- shear, as proposed by Bowden (1965). On the basis of esis (headed by J. Ashida). These groups were the pio- this assumption, Murakami et al. (1978, 1985) revealed neers of basic biological and interdisciplinary studies of that 30 to 50% of this large apparent diffusivity can be red tides, whilst previous studies were carried out from attributed to density-induced vertical circulation. In ad- the viewpoints of fisheries biology. These interdiscipli- dition to these studies, the generation mechanisms of re- nary studies, with some changes to the research titles and sidual currents, especially tide-induced residual currents, the constituent members, continued until 1984 with par- and their contribution to horizontal transport have been ticipation of more than 30 physical, chemical, biological studied (e.g. Yanagi, 1976; Oonishi, 1977; Yasuda, 1980; and fisheries oceanographers. The surveyed areas were Yanagi and Yoshikawa, 1983). not restricted to the Seto Inland Sea but covered the many Since the Seto Inland Sea is divided by islands and coastal areas around Japan where red tides occur. Among peninsulas into several basins, water exchange through them, Harima-Nada was the major research area because the narrow channels and straits attracted particular inter- of the frequent occurrence of the red tides and the seri- est. The studies of water exchange can be classified into ous damage that affected the fisheries, as shown in Fig. two categories. 2. The hydrographic structures related to red tides, the One group of studies examined tidal exchange proc- relationships between nutrient budget and red tides, the esses through the straits by means of field observations transition of red tide species, the formation of cysts and and hydraulic and numerical experiments. Theoretical their germination conditions, and the modeling of red tide studies relating to the definition of the tidal exchange rate formation and other related topics were mostly studied in were also carried out. Kashiwai (1984) summarized the these projects. The results were summarized by Okaichi tidal exchange rates at major straits in the Seto Inland (1987).

Progress in Seto Inland Sea Research 97 An interdisciplinary study group focusing on the Seto mechanisms, the biological process in the frontal areas Inland Sea was formed in 1981 with about 30 members and the accumulation of pollutants in the frontal areas divided into three subgroups. The title of the first sub- were widely studied by means of field observations and group was “Basic studies aiming at the comprehensive numerical experiments. The results were summarized by evaluation of the Seto Inland Sea” (headed by K. Kosaka). Yanagi (1990). This group collected historical data on environmental factors in the Seto Inland Sea obtained by various institu- 3.4 Physical studies on fronts tions, and methods of data handling and environmental Many kinds of coastal fronts can be found in the Seto indices suitable for the evaluation of the Seto Inland Sea Inland Sea due to large variations in marine environment. were studied. The second subgroup studied “The biologi- The generation mechanisms of these fronts attracted wide cal processes and environmental dynamics in the estua- interest from physical oceanographers, and as a result, rine areas of the Seto Inland Sea—Focusing on the Ohta several studies were completed. River and Hiroshima Bay—” (headed by T. Hayashi). The The first front studied was a thermohaline front in distributions of pelagic and benthic biota and their rela- the Kii Channel. Yoshioka (1971, 1988) described in de- tionship with the gradient of environmental factors were tail the structure of this front on the basis of field obser- investigated. The third subgroup focused on “Studies of vations. Another thermohaline front was discovered in the marine structure and the generation mechanisms of Iyo-Nada by Yanagi (1980). These thermohaline fronts the oxygen-deficient water mass in Hiuchi-Nada” (headed are generated between cold, less saline coastal water and by H. Higuchi, later by H. Takeoka). The distribution of warm, more saline oceanic water during the winter pe- the oxygen-deficient water mass and its seasonal change, riod. Detailed generation mechanisms of the thermohaline its relationship with the hydrographic structure, its influ- front in the Kii Channel were studied by Oonishi et al. ences on benthic communities and biogeochemical proc- (1978) by means of numerical modeling procedures. esses in the benthic environments were studied by means Another form of front in the Seto Inland Sea is a of frequent and vigorous field observations (Ochi and tidal front formed mainly during the summer period. A Takeoka, 1986; Takeoka et al., 1986; Imabayashi, 1986). tidal front is a transition zone between stratified and In this study, STD was used probably for the first time in tidally mixed waters, the basic concept of which was in- the Seto Inland Sea region, and interesting thermal struc- troduced by Simpson and Hunter (1974). There are many tures were identified. In addition to the thermocline be- narrow straits with strong tidal currents in the Seto In- low the surface mixed layer, a thermocline that had not land Sea around which tidal fronts can be formed. Figure 3 been previously recorded was often observed at approxi- 4 shows the distribution of log10(H/u ) (H: water depth Ð1 mately 6 m above the sea-bottom, and oxygen-deficient (m), u: amplitude of M2 tidal current (m s )) after Yanagi water was found below this level. This benthic and Okada (1993). According to the energetics given by thermocline was named “the second thermocline” by Ochi Simpson and Hunter (1974), tidal fronts are aligned along and Takeoka (1986). Thereafter, similar multi-layered the contour of a critical value of this parameter. The criti- structures were often found in various areas in the Seto cal value ranges from 2.5 to 3.0 in the Seto Inland Sea Inland Sea. The findings of the three subgroups were sum- (Yanagi and Okada, 1993). This means that tidal fronts marized by Kosaka (1985). are expected at the margin of the hatched areas in Fig. 4. Other important interdisciplinary studies carried out Some of them were studied by field observations; for in the 1980s were those focused on coastal fronts. A front example, in the Bungo Channel (Yanagi and Ohba, 1985), is a surface convergence zone between two different wa- in the western part of Hiuchi-Nada (Yanagi and ter masses, according to the definition by Yanagi (1987) Yoshikawa, 1987; Takeoka, 1990), in Osaka Bay (Yanagi which classified “siome” (in Japanese: tide rips), streaks and Takahashi, 1988a; Yuasa, 1994) and in Iyo-Nada and fronts. As described in the next section, there are many (Takeoka et al., 1993a). In the case of tidal fronts in Eu- kinds of fronts in the Seto Inland Sea, and they are con- ropean coastal regions, the shallower areas are vertically sidered to play important roles in material transport and well mixed. In contrast, the deeper areas are usually well biological processes. A study group comprising 10 mem- mixed in the Seto Inland Sea, because the straits in the bers (headed by T. Yanagi) was organized in 1984 to pro- Seto Inland Sea are deeply eroded by strong tidal cur- mote interdisciplinary studies on the coastal fronts sup- rents. Therefore, the tidal fronts in the Seto Inland Sea ported by a Grant-in-Aid for Scientific Research from the are formed by the effects of horizontal geometry. Ministry of Education, Science and Culture of Japan (later Another kind of tidal front was found in the Bungo supported by the Nippon Life Insurance Foundation). Channel (Takeoka et al., 1997). As demonstrated in Fig. 3 They selected the tidal fronts in the Seto Inland Sea and 4, log10(H/u ) in the Bungo Channel is larger in eastern the thermohaline front in Tokyo Bay as the major sub- and western coastal areas than in offshore areas due to a jects for their research. The generation and maintenance smaller value of u (weaker tidal current) in the coastal

98 H. Takeoka 3 Fig. 4. Distribution of log10(H/u ) in the Seto Inland Sea (after Yanagi and Okada, 1993). The value is smaller than 2.5 in the shaded areas.

areas. This means that stratification is expected to be more ing carried back to the inner bay by the current in the developed in coastal areas. However, Takeoka et al. lower layer, and returns to the surface. Accordingly, the (1997) found that stratification in coastal areas is much nitrogen or phosphorus remains in the bay longer than weaker than in offshore areas, and tidal fronts were formed the river water, which stays mostly in the upper layer and between the coastal and offshore areas. They showed that flows rapidly out of the bay. The basic principles of this the strong vertical mixing in coastal areas is due to a high mechanism were provided in the earlier study of Redfield vertical mixing efficiency caused by the complicated hori- (1956). Takeoka and Hashimoto (1988) demonstrated by zontal current patterns. Therefore, it can be stated that means of simple modeling techniques that differences in these tidal fronts are induced by the horizontal contrast the average residence times in Osaka Bay can be explained of vertical mixing efficiency. by this nutrient trap mechanism. Estuarine fronts and shelf fronts are also formed in Vertical transport processes were not the subject of the Seto Inland Sea. Yanagi (1987) classified all the sur- any significant research in earlier studies, because pol- face discontinuities including fronts and streaks, and sum- lutants are not expelled by the processes from the con- marized their generation mechanisms. cerned area. However, the need to study such processes increased during the interdisciplinary studies, because red 3.5 Transport of bioelements and 3-dimensional struc- tides and oxygen-deficient water masses are closely re- ture of vertical transport lated to vertical transport processes. As shown in Fig. 4, Interdisciplinary studies completed in the 1980s ad- stratified regions are adjacent to vertically mixed regions dressed two new fields of research: transport of in areas of the Seto Inland Sea. Takeoka (1993) deduced bioelements and vertical transport. The studies on hori- that vertical transport in stratified regions is significantly zontal transport processes and water exchange reviewed influenced by vertical transport in mixed regions, and in Subsection 3.2 essentially dealt with the movement of proposed the transport mechanism described below. water, and hence can only be applied to materials that are Figure 5 illustrates the density structure and the re- dissolved in and move with the water. However, sultant density currents in a vertical cross section of strati- bioelements such as nitrogen and phosphorus can trans- fied and mixed regions. Since the density of the water in form between dissolved and particulate forms and hence the mixed region is between that of the upper and lower their movement does not coincide with that of the water. layers in the stratified region, the mixed water tends to Yanagi and Takahashi (1988b) obtained the average resi- intrude into the middle layer of the stratified region, whilst dence times of river water and nitrogen flowing into Osaka water in the upper and lower layers flows into the mixed Bay, and stated that the average residence time of nitro- region. As a result of heat transport by these currents, the gen is 1.7 times longer than that of the river water. upper layer of the stratified region heats the mixed re- Takeoka and Hashimoto (1988) revealed that these dif- gion and the mixed layer heats the lower region. This ferences are caused by the following mechanism. Dis- means that there is a heat transport route from the upper solved inorganic nitrogen or phosphorus is transformed layer to the lower layer via the mixed region, as shown into particles by primary production in the upper euphotic by the thick solid line in Fig. 5(b). Horizontal mixing layer and settles to the lower layer as detritus. This detri- between mixed and stratified regions can also generate tus is then decomposed into a dissolved form whilst be- such a heat transport mechanism. Vertical heat transport

Progress in Seto Inland Sea Research 99 observed in many bays along the Japanese coast that face the open ocean, for example Sagami Bay (Matsuyama and Iwata, 1977), Uragami Bay (Tanaka et al., 1992) and Wakasa Bay (Yamagata et al., 1984). The first report of a kyucho in the Bungo Channel was by Takeoka and Yoshimura (1988). They observed intrusions of warm water into Uwajima Bay, a small bay on the eastern coast of the Bungo Channel, using a moored current meter system. The rise of water temperature reached between 4 and 5°C in a day in a typical kyucho. Thereafter, studies of this kyucho were continued by means of hydrographic observations (Takeoka et al., 1993b), analysis of the NOAA thermal imagery (Akiyama and Saitoh, 1993) and observations by high frequency ocean radar (Takeoka et al., 1995). These studies revealed the following: (1) the kyucho in the Bungo Channel is an intrusion of warm water from the Pacific Ocean into the eastern half of the channel, (2) it occurs mainly in sum- Fig. 5. (a) Density structure in the vertical section of mixed mer neap tidal periods, and (3) it is caused by the colli- and stratified regions and the resultant density induced cur- sion of the warm filament formed along the Kuroshio rents. (b) Transport routes of heat (solid line) and nutrients front, which is similar to the one formed in the Gulf (broken line). Thick lines denote bypasses via the mixed Stream region (Lee et al., 1981), to the southwestern coast region. of Shikoku Island. Takeoka et al. (2000) inferred that the cause of the spring-neap and seasonal periodicities of the kyucho is a spring-neap variation of vertical tidal mixing and seasonal variation of thermal convection. via the mixed region is called “heat bypass” (Takeoka, The kyucho in the Bungo Channel plays an impor- 1993). Dissolved oxygen produced by primary produc- tant role in determining the marine environment in the tion in the upper layer is also transported to the lower channel. The kyucho promotes water exchange in the bays layer by this mechanism (oxygen bypass). Moreover, rich along the eastern coast (Koizumi, 1991), suppressing nutrients in the lower layer can be transported to the up- eutrophication due to fish farming. Moreover, it plays an per layer, as illustrated by the thick dashed line in Fig. important role in maintaining biological production in the 5(b). This is called “nutrient bypass”. channel. Since the warm water of the kyucho originates An example of a nutrient bypass can be found in the from the Kuroshio which contains poor nutrients and as a primary production in a tidal front. Takeoka et al. (1993a) result is transparent, the water in the Bungo Channel turns found a prominent chlorophyll-a maximum in the sub- transparent due to the occurrence of the kyucho. There- surface of the tidal front formed in Iyo-Nada around the fore, the kyucho is called “sumishio” (transparent sea Hayasui Strait, following observations conducted in July water in Japanese) by local fishermen. However, blooms 1990. From analysis of the TS diagram, they inferred that of phytoplankton (mainly diatoms) usually occur after the the nutrients supporting the chlorophyll-a maximum were kyucho. Koizumi and Kohno (1994) and Koizumi et al. supplied not vertically from the lower layer but horizon- (1997) revealed that these blooms are caused by a com- tally from the mixed region around the Hayasui Strait. bination of the kyucho and bottom intrusions (see Sub- This nutrient supply route can be regarded as transport section 4.1, as for the bottom intrusion). through the nutrient bypass. In the Kii Channel, a weaker phenomenon similar to the kyucho sometimes occurs. Takeoka (1996) inferred 3.6 Influences from the Pacific Ocean that the difference between the kyucho in the Bungo and Although the Seto Inland Sea is very enclosed, phe- Kii Channels is due to the basic structure of the Seto In- nomena in the Pacific Ocean should strongly influence land Sea. Since the water depth is shallower and the river the marine environment in the boundary regions (such as runoff larger in the eastern section than in the western the Kii and Bungo Channels). A typical example of such one, the water density decreases in the eastern part dur- influences is a kyucho (in Japanese) in the Bungo Chan- ing the summer period. Therefore, the density contrast nel. A kyucho is a sudden stormy current that is usually between the Kuroshio and the coastal waters decreases in accompanied by a rise of water temperature and has been the Kii Channel, resulting in weaker and less frequent studied intensively since the mid 1980s. Kyucho have been kyucho in the Kii Channel.

100 H. Takeoka 3.7 Interdisciplinary studies in the 1990s The interdisciplinary studies in the 1980s focused on specific themes such as red tides, oxygen deficient water masses and frontal processes. In the 1990s, a study team with wider research objectives was organized under sponsorship of the Nippon Life Insurance Foundation. The team, which comprised natural science experts and social scientists including jurists and economists, aimed to clarify the basic natural, economic, social and legal as- pects regarding the preservation of both the fisheries industry and a desirable natural marine environment in the Seto Inland Sea. The study, entitled “Interdisciplinary study on the sustainable production of valuable fishes and preservation of environment in the Seto Inland Sea”, con- sisted of six core projects: (1) quantitative clarification of the primary production rate, (2) quantitative clarification of temporal variations in fish catches, (3) preserva- Fig. 6. Comparison of the enclosed or semi-enclosed seas in tion of existing and the creation of new fishing grounds, the world. (a) Rate of primary production in terms of nitrogen. (b) Nitrogen loading rate (Q in the North Sea is a sum (4) methods for reducing the nutrient load from the land s of the flux from the land and the flux from the Atlantic and an assessment of its effects, (5) local economic de- Ocean). (c) Average residence time of nitrogen. (d) Stock velopment and policy decisions, and (6) legal problems of nitrogen per unit area. (e) Efficiency of primary produc- related to fisheries and the marine environment. These tion (nitrogen cycling rate). (f) Fish catch per unit area (af- studies were implemented between 1992 and 1995 and ter Takeoka, 1997). summarized by Okaichi and Yanagi (1997). One of the significant results of this study was the identification of high productivity in the lower trophic levels of the pelagic food chain. The group carried out were proposed (Yanagi and Okaichi, 1997). field observations at 39 stations covering the entire Seto Takeoka (1997) discussed the reason of the high pro- Inland Sea on four occasions (October 1993, January, ductivity rates in the Seto Inland Sea and compared it April and June 1994). In addition to general hydrographic with the other semi-enclosed seas by analyzing the physi- observations at all stations, the concentrations of dissolved cal and biogeochemical parameters. Since these compari- nutrients and particulate matter, bacterial density, sons will help in understanding the Seto Inland Sea, the microzooplankton and net-zooplankton and primary pro- results are reproduced below in detail. However, the dis- duction rate were measured at selected stations. By cussions are restricted only to comparisons with analyzing both observed and historical data, Hashimoto Chesapeake Bay. et al. (1997) concluded that the average primary produc- Figure 6 shows some of the comparisons made by tion rate was as high as 731 mg C mÐ2dÐ1, and the sec- Takeoka (1997). Takeoka (1997) inferred that high pro- ondary production rate was 206 mg C mÐ2dÐ1. Hence the ductivity in the Seto Inland Sea is supported by the high transfer efficiency from primary to secondary production efficiency of primary production (EPC). The primary pro- was 28%, which is higher than the efficiency (<20%) com- duction efficiency given here is defined as EPC = PS/CS, monly accepted for the marine food chain. The tertiary where PS is the primary production rate per unit area and Ð2 Ð1 production rate was estimated to be 58 mg C m d , and CS is the standing stock of nitrogen per unit area. This the transfer efficiency from the secondary production was value states how many times the nitrogen stock in the 26%. These high transfer efficiencies suggest that water column is utilized for primary production per unit eutrophication is not significant over the entire Seto In- time and how many times the total stock of nitrogen in land Sea, because transfer efficiencies are usually lower the sea is utilized. Hence, this efficiency may also be in the eutrophic areas. called the nitrogen cycling rate. In Fig. 6, it can be seen Ð1 In comparison, benthic environments in some areas that EPC may be as high as 4 or 5 y in the Seto Inland such as Osaka Bay, Harima-Nada and Hiroshima Bay were Sea and Chesapeake Bay. Thus, from the efficiency EPC, found to deteriorate due to eutrophication. Guidelines for it can be stated that the primary production is highly effi- total nitrogen distribution were proposed in order to re- cient in these areas. In these two areas, high primary pro- store these environments and sustain local fisheries (Nagai ductivity is maintained by the regions ability to quickly and Ogawa, 1997), and the reduction rates of nitrogen and repeatedly utilize the reduced nitrogen content (see and phosphorus loads needed to realize these guidelines Fig. 6(e)) in the water column.

Progress in Seto Inland Sea Research 101 The water depth and the rate of vertical transport of Figure 6(f) shows the fish catch per unit area in each nutrients are supposed to be the main factors controlling of the seas. It can be seen that the fish catch in the Seto EPC. In Chesapeake Bay, strong stratification develops Inland Sea is much larger than in the other seas. The lower and the vertical transport of nutrients is restricted. How- fish catch in Chesapeake Bay may be caused by the in- ever, the bay is very shallow (average depth = 6.5 m, influence of oxygen deficiency on biological production in cluding tributaries) and hence the proportion of water in the higher trophic levels. the euphotic layer is large. This may be one of the causes In conclusion, biological production in the Seto In- of the large EPC in Chesapeake Bay. The average depth land Sea is extremely efficient due to the sea’s enclosed of the Seto Inland Sea (37 m) is larger than that of structure which maintains high nutrient concentrations Chesapeake Bay. Therefore, the main reason for the large and the many straits that bypass heat, nutrients and oxy- EPC in the Seto Inland Sea is believed to be the efficient gen, in consequence they contribute to the rapid and re- vertical transport mechanism supplying nutrients in the peated utilization of the nutrients. lower layer to the euphotic layer. It is inferred that the narrow channels and straits play an important role in the 3.8 Monitoring by ferry boats efficient transport of these nutrients. As described in Sub- As mentioned in Subsection 3.1, the prefectural fish- section 3.5, a narrow strait with a strong tidal current eries observatories have been conducting monthly obser- works as a bypass for the vertical transport of heat and vations at 320 fixed stations since 1964. Whilst these nutrients in the neighboring stratified regions. The nutri- observations are still important, the increased scope of ents rapidly return to the upper layer through this bypass, research in the area has meant that the requirement for and oxygen in the upper layer is supplied to the lower observations at higher temporal and spatial resolutions layer by a reverse bypass mechanism, promoting decom- has increased. Monitoring by ferry boats is one solution position of the organic matter. Moreover, heat transport to this problem. In 1991, the National Institute for Envi- through the bypass prevents the development of stratifi- ronmental Studies started a marine monitoring program cation. The many narrow straits and channels affect the in coastal and marginal seas (including the Seto Inland entire Seto Inland Sea, resulting in the large EPC. Sea) using commercial ferries. The ferry routes are shown The Seto Inland Sea and Chesapeake Bay thus main- in Fig. 1. Seawater was sampled continuously, and the tain a high productivity by means of different mecha- water temperature, salinity, pH and in vivo fluorescence nisms. However, there is another significant difference measured by in situ electrical sensors. The seawater was between the mechanisms that determine the nitrogen stock automatically filtered and dissolved inorganic nutrients per unit area (CS) in these seas. In Chesapeake Bay, even and phytoplankton pigments were analyzed in the labo- though the average residence time of the river water is ratory. only 0.3 years, the average residence time of nitrogen is Harashima et al. (1997) indicated based on results approximately one year due to the nutrient trap mecha- of the monitoring program that the temporal and spatial nism that results in a larger CS than in the case without variations of nutrients approve the “silica deficiency hy- the nutrient trap mechanism. However, this mechanism pothesis”, i.e., the overall human activities tend to en- requires strong stratification and is accompanied by a high hance the discharge of nitrogen and phosphorus and in- risk of oxygen depletion in the lower layer. On the other terrupt the natural supply of silica to the coastal seas. The hand, the highly enclosed geometry and the resultant weak increase of nitrogen and phosphorus will favor the non- water exchange rate in the Seto Inland Sea retain water diatom phytoplankton species than the diatom species and nutrients for approximately one year, also maintain- (Egge and Aksnes, 1992). Basically, three nutrients, DIN ing a larger CS than in the case if the sea were more open. (dissolved inorganic nitrogen), DIP (dissolved inorganic Such highly enclosed structures usually have a risk of phosphorus), and DSi (dissolved silicate or silica) de- oxygen depletion due to stagnation of water movement crease during spring bloom and one of these nutrients is in the interior and resultant strong stratification, especially exhausted at the end of spring bloom. Three nutrients are when there is a sill at the mouth of the bay. In the Seto recovered by the bio-decomposition of organic matter in Inland Sea, however, vertical transport of heat and oxy- the lower layer and the vertical transport by the mixing gen is maintained to some extent by the bypass mecha- caused by tide, wind and cooling after autumn. In the regu- nism in the many straits. Moreover, the straits in the Seto lar area remote to the river mouth, DIN primarily depletes Inland Sea are usually deeper than the neighboring ba- in summer. However, the ratio of DIN:DSi is large and sins, and such sills are rarely formed. Thus the straits in sometimes DSi is exhausted in Osaka Bay. the Seto Inland Sea play important roles in maintaining a The marine environmental monitoring using ferries high biological production and in preserving the marine as a platform has been proved to be feasible by the Seto environment. Inland Sea mission. The anticipated products are not lim-

102 H. Takeoka ited to the results of the temporal/spatial variation of nu- Several studies attempted to identify the route of trients but can also be applied to various environmental nutrient transport from the Pacific Ocean into the Seto indicators. Therefore, other marine research organizations Inland Sea. Hayashi et al. (2000) reported evidence sug- have initiated the similar monitoring programs, such as gesting the intrusion of nutrients from the Bungo Chan- the ALGALINE Program by the Finnish Institute of Ma- nel into Iyo-Nada. Further research is required to con- rine Research (1993Ðpresent) and “Monitoring using a firm this result. Another important issue for this region is ferry between Incheon and Cheju” by the Korea Ocean the maintenance of the nitrogen and phosphorous budg- Research & Development Institute (1998Ðpresent). Fur- ets. If nitrogen and phosphorous intrude into the inland thermore, an international consortium of marine research sea throughout the year, their stocks would increase in- organizations in Europe are planning to start the “Euro- definitely. One possible reason why this does not happen pean Ferry Box Program” in several coastal seas includ- is that there is an outflow of the nutrients in winter months, ing the Baltic, Adriatic and North Seas. but no studies have been attempted to verify this hypoth- esis. Related to this topic is the long term variation in the 4. Recent Research Topics and Activities nutrient budgets. Takeoka et al. (2000) analyzed historical data pertaining to the bottom water temperature in 4.1 Nutrients budgets and their long term variations the Bungo Channel as an index of bottom intrusion, and In enclosed coastal seas, the main sources of nutri- revealed that a decrease in the bottom intrusion and a re- ents are usually river inflows that transport terragenic sultant decrease in biological productivity occurred in the nutrients. It was previously believed that this was the case 1990s. The nutrient budgets and their long term variation in the highly enclosed Seto Inland Sea. However, nutri- are an important factor in determining the future for the ent fluxes from the Pacific Ocean may also contribute a marine environment of the Seto Inland Sea and require significant amount to the overall nutrient content in the intensive studies. sea. Fujiwara et al. (1997) observed concentrations of 4.2 Recently identified environmental issues nitrogen and current speeds in a transverse cross section Several studies relating to recently identified envi- in the Bungo Channel for 15 days in the summer of 1982, ronmental issues have been initiated. As mentioned in and concluded that a large quantity of nitrogen (ca. 70 Section 2, the dredging of sea sand and gravel is one such t dayÐ1) was supplied into the channel from the Pacific issue in the Seto Inland Sea. The sea-sand and gravel in Ocean. The fluxes of nitrogen and phosphorus from the the Seto Inland Sea were generated by continuous ero- Pacific Ocean into the Kii Channel were estimated to be sion at the bottom of the straits and channels for about 170 t dayÐ1 and 34 t dayÐ1 respectively, by similar obser- ten thousand years since the formation of the Seto Inland vations carried out on 23 and 24 August 1995. These re- Sea (Inouchi, 1990). Therefore, they are resources like sults are for one short period and do not indicate that the fossils, which cannot be recovered in a short time, and fluxes continue throughout the year. However, the flux the environmental impacts of dredging are distinctive. A values are sufficiently large to warrant further research, study of tidal current changes due to alterations to the as the fluxes of nitrogen and phosphorus from the land topography caused by dredging (Takahashi et al., 2001) into the Seto Inland Sea are approximately 470 t dayÐ1 and a study of the influences of turbid water generated and 30 t dayÐ1, respectively (Yuasa, 1994). by the dredging on sea forests (Montani and Hari, 2000) The phenomenon supplying nutrients to the Bungo have recently been initiated. Also, more basic studies of Channel was clearly observed by Koizumi (1999). Fol- the physical and biogeochemical conditions of offshore lowing repeated hydrographic observations in the Bungo sandy beds have been initiated, because little is known as Channel in the summer of 1994, an intrusion of cold and yet about these areas. These studies will contribute to a nutrient-rich water from the bottom of the shelf slope re- more comprehensive understanding of the Seto Inland gion south of the Bungo Channel into the lower layer of Sea. the channel was identified. Kaneda et al. (2002) carried Another issue that has recently been identified is jel- out long-term monitoring of water temperatures at the lyfish blooms. In recent years, blooms of jellyfish, par- bottom of the Bungo Channel, and concluded that the in- ticularly Aurelia aurita, were frequently observed in the trusion of cold water occurs intermittently, mainly in neap seas around Japan including the Seto Inland Sea (Uye, tidal periods in the summer. Since the intrusion pattern 1994). It is feared that jellyfish blooms disturb the ma- was similar to a bottom intrusion occurring in the Gulf rine food chain and decrease the production of valuable Stream region (Atkinson, 1977), Kaneda et al. (2002) also fish resources. Therefore, an interdisciplinary research called the intrusion in the Bungo Channel a bottom intru- group was set up in 2001 (headed by S. Uye of Hiroshima sion, although the generation mechanisms may not be the University) to study the spatial and temporal distributions same. of these blooms, the relationship between the bloom char-

Progress in Seto Inland Sea Research 103 acteristics, and changes in the environmental conditions, al. (1997) who analyzed seasonal variations in the water physiology and ecology of the jellyfish and technology surface temperature using NOAA/AVHRR data, but stud- to protect the blooms. ies using such data are limited due to the low resolution of the satellite data. However, studies are being under- 4.3 Monitoring using new technologies taken to evaluate chlorophyll concentrations (Tsukamoto New instruments for both academic and government and Yanagi, 2001) and to monitor occurrences of red tides monitoring are being introduced into the various Seto (Hashimoto et al., 2001) using SeaWiFS ocean color data. Inland Sea research programs. On the basis of studies on the kyucho and bottom intrusions in the Bungo Channel, 4.4 Numerical simulations the Center for Marine Environmental Studies (CMES) of Hitherto, numerical studies related to the Seto In- Ehime University developed a water temperature infor- land Sea focused on the individual processes such as tide- mation system in the Sea of Uwa (the eastern part of the induced residual current (e.g. Oonishi, 1977), water ex- Bungo Channel) in co-operation with the Ehime Prefec- change through a strait (e.g. Awaji et al., 1980), genera- tural Fisheries Observatory. In this system, water tem- tion of thermohaline fronts (e.g. Oonishi et al., 1978), perature data (measured at 2-hour intervals) are transmit- nutrient trap mechanism (Takeoka and Hashimoto, 1988) ted to the CMES via ORBCOM (Orbital Communication) and red tide (Yanagi et al., 1993). Local, specific areas of satellites and immediately posted on the CMES website the Seto Inland Sea were studied in most of these studies. for public use. As of May 2001, three stations are opera- With the progress of the studies on the Seto Inland Sea tional and the number of the stations will be gradually and the increasing social requirements, however, a more increased. Since this system is not costly to implement, comprehensive understanding of the Seto Inland Sea is further extension in other areas of the Seto Inland Sea is becoming definitely necessary, which requires a numeri- expected. In addition, the CMES started water quality cal model with an accurate geometry of the sea. A diag- monitoring using the autonomous monitoring system at nostic model seems not to be a good choice for this pur- Sada Point in March 2000. The main objective of this pose, because snapshot data are usually contaminated in monitoring is to detect long term variations in the coastal seas of wide spatial/temporal variabilities like the biogeochemical components in the sea, particularly those Seto Inland Sea, causing low accuracy of the diagnostic due to variations in the bottom intrusion in the Bungo calculation. It is therefore necessary to develop a fully Channel. Hourly water temperature, salinity, pH, dis- prognostic numerical model for the sea. The model grid solved oxygen, chlorophyll fluorescence, concentrations size should be 1 km or less. Moreover, higher resolution of ammonia, nitrate, phosphate and silicate readings are would be necessary in the areas of narrow straits and chan- obtained by the system (Takeoka et al., 2001). The Na- nels, which requires a nested model technique. Tidal and tional Institute for Environmental Studies is also plan- residual processes should be solved in one model, con- ning to implement real time monitoring using ferries by sidering their non-linear coupling which produce impor- sending the data via N-star satellite and to release this tant processes such as a heat bypass. information to collaborating institutions. By construct- In the future, the data assimilation technique will ing a network of time-series monitoring at fixed and mo- probably be needed to improve the accuracy of the model. bile (ferry) stations, it will become possible to obtain Moreover, a hindcast/forecast system would be necessary comprehensive real-time hydrographic and to meet the requirement of the sea management, particu- biogeochemical data. larly in the marine environmental prediction. The data Acoustic Doppler current profilers for current moni- from the above mentioned monitoring such as that by ferry toring have been fitted in the research vessels of the pre- boats and that using new technologies described in Sub- fectural fisheries observatories, and data obtained at section 4.3 will provide real-time initial and boundary monthly intervals have been accumulated. In addition, conditions for the forecast system. At present, most eco- acoustic tomographic techniques will be introduced in the system models of the Seto Inland Sea are based on the near future. This technology has already been applied to simple box model (Hayashi et al., 2001) and focus on current measurements in the deep ocean. A coastal appli- local problems at a basin scale. In the future, with the cation of this technology was developed recently (Zheng inclusion of biological processes in the hindcast/forecast et al., 1998), and experiments to verify its usefulness in system, simulation of the entire ecosystem and hence more coastal seas are being planned by a group headed by A. comprehensive understanding of the Seto Inland Sea will Kaneko of Hiroshima University. Continuous two- or be possible. three-dimensional current measurements using this system will provide valuable data for future studies. Acknowledgements Studies using satellite remote sensing data have al- The author expresses his sincere thanks to Dr. A. ready been completed, for example that by Tsukamoto et Harashima of the National Institute for Environmental

104 H. Takeoka Studies and Dr. X. Guo of the Center for Marine Envi- Kaneda, A., H. Takeoka, E. Nagaura and Y. Koizumi (2002): ronmental Studies, Ehime University, for the useful com- Periodic intrusion of cold water from the Pacific Ocean into ments on the manuscript. the bottom layer of the Bungo Channel, Japan. J. Oceanogr. (in press). References Kashiwai, M. (1984): The concept of tidal exchange and the Akiyama, H. and S. Saitoh (1993): The Kyucho in Sukumo Bay tidal exchange ratio. J. Oceanogr. Soc. Japan, 40, 135Ð147 induced by Kuroshio warm filament intrusion. J. Oceanogr., (in Japanese). 49, 667Ð682. Koizumi, Y. (1991): The process of water exchange in Shitaba Atkinson, L. P. (1977): Modes of Gulf Stream intrusion into Bay during the phenomenon of Kyucho. Bull. Coast. the South Atlantic Bight shelf waters. Geophys. Res. Lett., Oceanogr., 29, 82Ð89 (in Japanese). 4, 583Ð586. Koizumi, Y. (1999): Kyucho events and the mechanism of Awaji, T., N. Imasato and H. 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