93

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

Review

Keywords:⋅ Seto Inland Sea,⋅ anthropogenicimpacts,

⋅ heat bypass,⋅ nutrient trap,⋅ tide and tidalcurrent,

⋅ red tide,⋅ oxygen-deficientwater mass,

⋅ fronts.

* E-mail address: takeoka@dpc.ehime-u.ac.jp

Copyright © The Oceanographic Society of Japan.

Progress in Seto Inland Sea Research

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-etry and thus a variety of marine environments. This sea is, at the same time, one ofthe most industrialized areas in Japan, and its marine environment has been signifi-cantly affected by the anthropogenic impacts over the last four decades. The widerange of marine environments in this sea and the serious environmental issues result-ing from these impacts have attracted the attention of Japanese coastal oceanogra-phers. It is believed that the nature and scope of these studies might be an example ofthe progress of Japanese coastal oceanography. The historical changes in the SetoInland Sea environment in the last four decades are briefly summarized, and theprogress in the studies of the Seto Inland Sea is reviewed with reference to historicalchanges. Some recent research topics and activities are also mentioned.

studies might be a good example of the progress of Japa-nese coastal oceanography. This paper therefore presentsa review of these studies. In Section 2, historical changesin the marine environment of the Seto Inland Sea are dis-cussed as a background to the present paper. The variousindividual studies are then discussed in Section 3. Ad-vances in Seto Inland Sea studies are also reviewed inSection 3, classified according to subject. Recent researchtopics and activities are mentioned in Section 4.

2. Historical Changes in the Seto Inland Sea Envi-ronmentFigure 2 shows a chronological table of events af-

fecting the Seto Inland Sea over the last four decades,along with a description of completed studies concerningto the region and the annual number of red tide occur-rences in the region. The number of red tide incidentscan be used as an indicator of eutrophication. During the1970s, eutrophication advanced rapidly due to an increasein the volume of industrial and urban waste, resulting ina frequent occurrence of red tides and oxygen-deficientwater masses. These occurrences had a great impact onthe marine environment. In 1970, a mass mortality of fishoccurred in Hiuchi-Nada due to an oxygen-deficient wa-ter mass. Large-scale red tides of Chattonella often oc-curred in Harima-Nada. The red tides that occurred in1971 damaged the fish farming industry up to the valueof 7.1 billion yen.

1. IntroductionThe Seto Inland Sea in Japan is a semi-enclosed

coastal sea surrounded by Honshu (the main island ofJapan), Shikoku and Kyushu Islands (Fig. 1). It has alength of 500 km, an average depth of 30 m and containsapproximately 600 islands. The sea is divided by islandsand peninsulas into wide basins, some of which are called“nada” in Japanese, and these basins are connected bynarrow channels called “seto”. This complicated geom-etry results in wide variations in the marine environment.

The Seto Inland Sea region is also one of the mostindustrialized areas in Japan. After the New Industrial CityLaw was enacted in 1963, many facilities for heavy in-dustry were built in the coastal areas surrounding the sea.Urbanization of the coastal area also increased. At present,approximately 35 million people live within the Seto In-land Sea watershed. This industrialization and urbaniza-tion required substantial reclamation of land. The marineenvironment of the Seto Inland Sea has been significantlyaffected by these impacts over the last four decades.

The wide range of marine environments in the SetoInland Sea and the serious environmental issues result-ing from anthropogenic development in the region haveattracted the attentions of Japanese coastal oceanogra-phers. It is believed that the nature and scope of these

94 H. Takeoka

To solve these problems, the Environmental Agencyof Japan enacted “The Interim Law for Conservation ofthe Environment of the Seto Inland Sea” in 1973 and “TheLaw Concerning Special Measures for Conservation ofthe Environment of the Seto Inland Sea” in 1978. Theselaws resulted in a COD reduction to some extent. In ad-dition, the annual number of red tides gradually reducedbetween the mid 1970s and the mid 1980s, but has re-mained stable since then.

Prior to 1973, land reclamation was intensive. Inaverage, approximately 16 km2 was reclaimed annuallybetween 1965 and 1973. This was regulated by the In-terim Law for Conservation of the Environment of theSeto Inland Sea, but still took place to some extent afterthe law was introduced. The total area of land reclaimedsince 1965 is approximately 250 km2, which is approxi-mately 12% of the area with a depth of less than 10 m inthe Seto Inland Sea. More than half of the marine forestthat existed in the early 1960s has been lost by reclama-tion.

Another environmental issue that has recently be-come acute is the dredging of sand and gravel from theseabed. The great demand for building materials and thelack of suitable quarries in western Japan led to an in-crease in dredging activity. Sea-sand dredging increasedrapidly in the late 1960s, thereafter 2 × 107 m3 of sea-sand has been dredged annually. However, some prefec-tures surrounding the Seto Inland Sea prohibited dredg-ing due to public pressure.

3. Progress in Seto Inland Sea Research

3.1 Basic descriptive studiesThe Prefectural Fisheries Observatories around the

Seto Inland Sea started monthly observations of watertemperature, salinity and transparency at 320 fixed sta-tions in 1964 in a project supported by the FisheriesAgency. The Maritime Safety Agency accumulated tideand tidal current data. In the 1970s and 1980s, many de-scriptive studies summarizing these data were completed.Yanagi and Higuchi (1979) analyzed the historical tidalcurrent data measured by the Maritime Safety Agency,and produced a chart of residual current flow patterns.Yanagi and Higuchi (1981) also produced charts of theamplitude and phase lag of the M2 and K1 constituents oftide and tidal current. Summarizing the historicalhydrographic data obtained by fisheries observatories,Takeoka (1985) revealed the distribution of stratificationin the Seto Inland Sea. Takeoka (1987) also described thetransparency distribution and analyzed the seasonal andspatial differences in the distribution.

Among the results of these studies, distributions ofM2 tide and M2 tidal current are shown here, because thetide and tidal current are the most basic and importantfactors characterizing the Seto Inland Sea and the M2constituent is dominant over almost the whole Sea. Fig-ures 3(a) and (b) indicate the distributions of the tidalrange and the phase lag of the M2 tide, and Figs. 3(c) and(d) the distributions of the amplitude and the phase lag of

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

Progress in Seto Inland Sea Research 95

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

96 H. Takeoka

features can be seen from the tide and tidal current phasedistribution plots. The phase of the tide is almost equal inthe central region and differs spatially in the eastern andwestern regions. The phase difference between the tideand tidal current is approximately 90° in the central re-gion and is much smaller in the eastern and western re-gions. As a result of such tidal features, the flood tidalcurrents are directed to the area into which the tidal wavesassemble. Moreover, the transport volume of the tidalcurrent is larger in the outer regions, and almost vanishesin the assembling area. Therefore, except in the narrowstraits, the amplitude of the tidal current is generally largerin the outer regions and smaller in the eastern part ofHiuchi Nada (Fig. 3(c)). In the narrow straits the ampli-tudes of the tidal currents are much larger than those inthe basin interior. The amplitudes of the four major tidalcurrent constituents in the main straits are shown in Ta-ble 1. The maximum tidal speeds in some straits, such asNaruto and Kurushima Straits, approach 5 m s–1 in thespring tides. These strong tidal currents play a signifi-cant role in maintaining high biological productivity inthe Seto Inland Sea as described later.

Fig. 3. Distributions of (a) tidal range, (b) phase lag of tide, (c) current amplitude and (d) phase lag of tidal current of M2constituent in the Seto Inland Sea (after Yanagi and Higuchi, 1981). Areas where the current amplitude is larger than 70cm s–1 are hatched in (c).

the M2 tidal current (Yanagi and Higuchi, 1981). In thePacific Ocean, south of the Seto Inland Sea, tidal wavespropagate from east to west at significant speeds due tothe great water depth. Hence the phase of the tides at themouths of the Kii and Bungo Channels (see Fig. 1) areapproximately equal (Fig. 3(b)). The tidal waves propa-gate at a much lower speed in the Seto Inland Sea due tothe shallow water depth. The waves propagate from thetwo channels into the inland sea over a long period oftime and meet at the central part of the Seto Inland Seabetween the Bisan Strait and Hiuchi-Nada where the phaselag of M2 tide is the largest. They are delayed by about150° from the mouths of the channels (Fig. 3(b)). Bothtidal waves entering from the channels are dissipatedduring propagation. Therefore, the amplitudes of the tidalwaves propagating eastward and westward are approxi-mately equal in the central part of the inland sea, whilstin the eastern and western regions the amplitude of thetidal wave from the adjacent channel is greater than thatfrom the opposite channel. Thus the tidal wave in the cen-tral region becomes a stationary wave, whilst those in theeastern and western regions are progressive waves. These

Progress in Seto Inland Sea Research 97

3.2 Horizontal transport and water exchangeCoastal oceanographic studies in the 1970s and 1980s

focused on horizontal transport processes and their quan-titative evaluations, because the horizontal transport proc-esses expel pollutants from bays and were regarded as apurification mechanism against marine pollution.

Hayami and Unoki (1970) gave a significant impacton horizontal transport studies. On the basis of salt budgetanalysis, they concluded that the apparent one-dimen-sional diffusivity along the axis of the Seto Inland Sea is107 cm2s–1. Considering the scale dependence of horizon-tal diffusivity in marine environments, this value seemsto be too large. However, the value itself was acceptedbecause it was obtained from a very simple budget model.This large apparent diffusivity was due to dispersion pro-duced by the linked effect of eddy diffusion and currentshear, as proposed by Bowden (1965). On the basis ofthis assumption, Murakami et al. (1978, 1985) revealedthat 30 to 50% of this large apparent diffusivity can beattributed to density-induced vertical circulation. In ad-dition to these studies, the generation mechanisms of re-sidual currents, especially tide-induced residual currents,and their contribution to horizontal transport have beenstudied (e.g. Yanagi, 1976; Oonishi, 1977; Yasuda, 1980;Yanagi and Yoshikawa, 1983).

Since the Seto Inland Sea is divided by islands andpeninsulas into several basins, water exchange throughthe narrow channels and straits attracted particular inter-est. The studies of water exchange can be classified intotwo categories.

One group of studies examined tidal exchange proc-esses through the straits by means of field observationsand hydraulic and numerical experiments. Theoreticalstudies relating to the definition of the tidal exchange ratewere also carried out. Kashiwai (1984) summarized thetidal exchange rates at major straits in the Seto Inland

Sea, converting the values obtained by other researchersinto values under the same definition. Furthermore,Imasato et al. (1980) and Awaji et al. (1980) revealedthat the Stokes Drift due to large gradients of amplitudeand phase difference of tidal current around the straitsplays an important role in the tidal exchange.

The second group of studies examined the transportand renewal of water or materials filling the individualbasins or the whole Seto Inland Sea. Takeoka (1984a)published a theoretical study of the time scales that rep-resent the transport of materials and water renewal, andproposed some basic concepts such as residence, transitand turn-over times and the average age that can be ap-plied to problems in coastal seas. Takeoka (1984b) ap-plied this theory to the experimental results obtained witha 1/50000 scale hydraulic model of the Seto Inland Seaover some kinds of the various time scales including theaverage residence times of the river water, oceanic waterand total water. He concluded that the average residencetime of the total water was approximately 14 months.

3.3 Interdisciplinary studies in the 1980sUntil the early 1970s, the interest of oceanographers

was focused mainly within their own disciplines, althougha few collaborative studies with other disciplines weredone. Since the end of 1970s, with support of publicawareness of environmental conservation, several inter-disciplinary study groups were formed. These were mainlysupported by Grants-in-Aid for Scientific Research fromthe Ministry of Education, Science and Culture of Japan.

Two groups with special interests on red tides wereestablished in 1978 for basic biological studies on redtides (headed by T. Yanagida), and studies on the physi-cal, chemical and biological mechanisms of red tide gen-esis (headed by J. Ashida). These groups were the pio-neers of basic biological and interdisciplinary studies ofred tides, whilst previous studies were carried out fromthe viewpoints of fisheries biology. These interdiscipli-nary studies, with some changes to the research titles andthe constituent members, continued until 1984 with par-ticipation of more than 30 physical, chemical, biologicaland fisheries oceanographers. The surveyed areas werenot restricted to the Seto Inland Sea but covered the manycoastal areas around Japan where red tides occur. Amongthem, Harima-Nada was the major research area becauseof the frequent occurrence of the red tides and the seri-ous damage that affected the fisheries, as shown in Fig.2. The hydrographic structures related to red tides, therelationships between nutrient budget and red tides, thetransition of red tide species, the formation of cysts andtheir germination conditions, and the modeling of red tideformation and other related topics were mostly studied inthese projects. The results were summarized by Okaichi(1987).

M2 S2 K1 O1(cm s–1)

Tomogashima Strait 105 25 40 35Naruto Strait 330 90 50 50Akashi Strait 160 60 50 50Bisan Strait 95 35 15 10Kurusima Strait 250 100 40 30Tsurushima Strait 90 45 25 20Ohbatake Strait 250 75 25 20Hayasui Strait 155 70 35 25Kanmon Strait 225 80 65 45

Table 1. Amplitudes of four major tidal current constituents inthe main straits of the Seto Inland Sea (after Yanagi andHiguchi, 1981).

98 H. Takeoka

An interdisciplinary study group focusing on the SetoInland Sea was formed in 1981 with about 30 membersdivided into three subgroups. The title of the first sub-group was “Basic studies aiming at the comprehensiveevaluation of the Seto Inland Sea” (headed by K. Kosaka).This group collected historical data on environmental fac-tors in the Seto Inland Sea obtained by various institu-tions, and methods of data handling and environmentalindices suitable for the evaluation of the Seto Inland Seawere studied. The second subgroup studied “The biologi-cal processes and environmental dynamics in the estua-rine areas of the Seto Inland Sea—Focusing on the OhtaRiver and Hiroshima Bay—” (headed by T. Hayashi). Thedistributions of pelagic and benthic biota and their rela-tionship with the gradient of environmental factors wereinvestigated. The third subgroup focused on “Studies ofthe marine structure and the generation mechanisms ofthe oxygen-deficient water mass in Hiuchi-Nada” (headedby H. Higuchi, later by H. Takeoka). The distribution ofthe oxygen-deficient water mass and its seasonal change,its relationship with the hydrographic structure, its influ-ences on benthic communities and biogeochemical proc-esses in the benthic environments were studied by meansof frequent and vigorous field observations (Ochi andTakeoka, 1986; Takeoka et al., 1986; Imabayashi, 1986).In this study, STD was used probably for the first time inthe Seto Inland Sea region, and interesting thermal struc-tures were identified. In addition to the thermocline be-low the surface mixed layer, a thermocline that had notbeen previously recorded was often observed at approxi-mately 6 m above the sea-bottom, and oxygen-deficientwater was found below this level. This benthicthermocline was named “the second thermocline” by Ochiand Takeoka (1986). Thereafter, similar multi-layeredstructures were often found in various areas in the SetoInland Sea. The findings of the three subgroups were sum-marized by Kosaka (1985).

Other important interdisciplinary studies carried outin the 1980s were those focused on coastal fronts. A frontis a surface convergence zone between two different wa-ter masses, according to the definition by Yanagi (1987)which classified “siome” (in Japanese: tide rips), streaksand fronts. As described in the next section, there are manykinds of fronts in the Seto Inland Sea, and they are con-sidered to play important roles in material transport andbiological processes. A study group comprising 10 mem-bers (headed by T. Yanagi) was organized in 1984 to pro-mote interdisciplinary studies on the coastal fronts sup-ported by a Grant-in-Aid for Scientific Research from theMinistry of Education, Science and Culture of Japan (latersupported by the Nippon Life Insurance Foundation).They selected the tidal fronts in the Seto Inland Sea andthe thermohaline front in Tokyo Bay as the major sub-jects for their research. The generation and maintenance

mechanisms, the biological process in the frontal areasand the accumulation of pollutants in the frontal areaswere widely studied by means of field observations andnumerical experiments. The results were summarized byYanagi (1990).

3.4 Physical studies on frontsMany kinds of coastal fronts can be found in the Seto

Inland Sea due to large variations in marine environment.The generation mechanisms of these fronts attracted wideinterest from physical oceanographers, and as a result,several studies were completed.

The first front studied was a thermohaline front inthe Kii Channel. Yoshioka (1971, 1988) described in de-tail the structure of this front on the basis of field obser-vations. Another thermohaline front was discovered inIyo-Nada by Yanagi (1980). These thermohaline frontsare generated between cold, less saline coastal water andwarm, more saline oceanic water during the winter pe-riod. Detailed generation mechanisms of the thermohalinefront in the Kii Channel were studied by Oonishi et al.(1978) by means of numerical modeling procedures.

Another form of front in the Seto Inland Sea is atidal front formed mainly during the summer period. Atidal front is a transition zone between stratified andtidally mixed waters, the basic concept of which was in-troduced by Simpson and Hunter (1974). There are manynarrow straits with strong tidal currents in the Seto In-land Sea around which tidal fronts can be formed. Figure4 shows the distribution of log10(H/u

3) (H: water depth(m), u: amplitude of M2 tidal current (m s

–1)) after Yanagiand Okada (1993). According to the energetics given bySimpson and Hunter (1974), tidal fronts are aligned alongthe contour of a critical value of this parameter. The criti-cal value ranges from 2.5 to 3.0 in the Seto Inland Sea(Yanagi and Okada, 1993). This means that tidal frontsare expected at the margin of the hatched areas in Fig. 4.Some of them were studied by field observations; forexample, in the Bungo Channel (Yanagi and Ohba, 1985),in the western part of Hiuchi-Nada (Yanagi andYoshikawa, 1987; Takeoka, 1990), in Osaka Bay (Yanagiand Takahashi, 1988a; Yuasa, 1994) and in Iyo-Nada(Takeoka et al., 1993a). In the case of tidal fronts in Eu-ropean coastal regions, the shallower areas are verticallywell mixed. In contrast, the deeper areas are usually wellmixed in the Seto Inland Sea, because the straits in theSeto Inland Sea are deeply eroded by strong tidal cur-rents. Therefore, the tidal fronts in the Seto Inland Seaare formed by the effects of horizontal geometry.

Another kind of tidal front was found in the BungoChannel (Takeoka et al., 1997). As demonstrated in Fig.4, log10(H/u

3) in the Bungo Channel is larger in easternand western coastal areas than in offshore areas due to asmaller value of u (weaker tidal current) in the coastal

Progress in Seto Inland Sea Research 99

areas. This means that stratification is expected to be moredeveloped in coastal areas. However, Takeoka et al.(1997) found that stratification in coastal areas is muchweaker than in offshore areas, and tidal fronts were formedbetween the coastal and offshore areas. They showed thatthe strong vertical mixing in coastal areas is due to a highvertical mixing efficiency caused by the complicated hori-zontal current patterns. Therefore, it can be stated thatthese tidal fronts are induced by the horizontal contrastof vertical mixing efficiency.

Estuarine fronts and shelf fronts are also formed inthe Seto Inland Sea. Yanagi (1987) classified all the sur-face discontinuities including fronts and streaks, and sum-marized their generation mechanisms.

3.5 Transport of bioelements and 3-dimensional struc-ture of vertical transportInterdisciplinary studies completed in the 1980s ad-

dressed two new fields of research: transport ofbioelements and vertical transport. The studies on hori-zontal transport processes and water exchange reviewedin Subsection 3.2 essentially dealt with the movement ofwater, and hence can only be applied to materials that aredissolved in and move with the water. However,bioelements such as nitrogen and phosphorus can trans-form between dissolved and particulate forms and hencetheir movement does not coincide with that of the water.Yanagi and Takahashi (1988b) obtained the average resi-dence times of river water and nitrogen flowing into OsakaBay, and stated that the average residence time of nitro-gen is 1.7 times longer than that of the river water.Takeoka and Hashimoto (1988) revealed that these dif-ferences are caused by the following mechanism. Dis-solved inorganic nitrogen or phosphorus is transformedinto particles by primary production in the upper euphoticlayer and settles to the lower layer as detritus. This detri-tus is then decomposed into a dissolved form whilst be-

ing carried back to the inner bay by the current in thelower layer, and returns to the surface. Accordingly, thenitrogen or phosphorus remains in the bay longer thanthe river water, which stays mostly in the upper layer andflows rapidly out of the bay. The basic principles of thismechanism were provided in the earlier study of Redfield(1956). Takeoka and Hashimoto (1988) demonstrated bymeans of simple modeling techniques that differences inthe average residence times in Osaka Bay can be explainedby this nutrient trap mechanism.

Vertical transport processes were not the subject ofany significant research in earlier studies, because pol-lutants are not expelled by the processes from the con-cerned area. However, the need to study such processesincreased during the interdisciplinary studies, because redtides and oxygen-deficient water masses are closely re-lated to vertical transport processes. As shown in Fig. 4,stratified regions are adjacent to vertically mixed regionsin areas of the Seto Inland Sea. Takeoka (1993) deducedthat vertical transport in stratified regions is significantlyinfluenced by vertical transport in mixed regions, andproposed the transport mechanism described below.

Figure 5 illustrates the density structure and the re-sultant density currents in a vertical cross section of strati-fied and mixed regions. Since the density of the water inthe mixed region is between that of the upper and lowerlayers in the stratified region, the mixed water tends tointrude into the middle layer of the stratified region, whilstwater in the upper and lower layers flows into the mixedregion. As a result of heat transport by these currents, theupper layer of the stratified region heats the mixed re-gion and the mixed layer heats the lower region. Thismeans that there is a heat transport route from the upperlayer to the lower layer via the mixed region, as shownby the thick solid line in Fig. 5(b). Horizontal mixingbetween mixed and stratified regions can also generatesuch a heat transport mechanism. Vertical heat transport

Fig. 4. Distribution of log10(H/u3) in the Seto Inland Sea (after Yanagi and Okada, 1993). The value is smaller than 2.5 in the

shaded areas.

100 H. Takeoka

via the mixed region is called “heat bypass” (Takeoka,1993). Dissolved oxygen produced by primary produc-tion in the upper layer is also transported to the lowerlayer by this mechanism (oxygen bypass). Moreover, richnutrients in the lower layer can be transported to the up-per layer, as illustrated by the thick dashed line in Fig.5(b). This is called “nutrient bypass”.

An example of a nutrient bypass can be found in theprimary production in a tidal front. Takeoka et al. (1993a)found a prominent chlorophyll-a maximum in the sub-surface of the tidal front formed in Iyo-Nada around theHayasui Strait, following observations conducted in July1990. From analysis of the TS diagram, they inferred thatthe nutrients supporting the chlorophyll-a maximum weresupplied not vertically from the lower layer but horizon-tally from the mixed region around the Hayasui Strait.This nutrient supply route can be regarded as transportthrough the nutrient bypass.

3.6 Influences from the Pacific OceanAlthough the Seto Inland Sea is very enclosed, phe-

nomena in the Pacific Ocean should strongly influencethe marine environment in the boundary regions (such asthe Kii and Bungo Channels). A typical example of suchinfluences is a kyucho (in Japanese) in the Bungo Chan-nel. A kyucho is a sudden stormy current that is usuallyaccompanied by a rise of water temperature and has beenstudied intensively since the mid 1980s. Kyucho have been

observed in many bays along the Japanese coast that facethe open ocean, for example Sagami Bay (Matsuyama andIwata, 1977), Uragami Bay (Tanaka et al., 1992) andWakasa Bay (Yamagata et al., 1984).

The first report of a kyucho in the Bungo Channelwas by Takeoka and Yoshimura (1988). They observedintrusions of warm water into Uwajima Bay, a small bayon the eastern coast of the Bungo Channel, using a mooredcurrent meter system. The rise of water temperaturereached between 4 and 5°C in a day in a typical kyucho.Thereafter, studies of this kyucho were continued bymeans of hydrographic observations (Takeoka et al.,1993b), analysis of the NOAA thermal imagery (Akiyamaand Saitoh, 1993) and observations by high frequencyocean radar (Takeoka et al., 1995). These studies revealedthe following: (1) the kyucho in the Bungo Channel is anintrusion of warm water from the Pacific Ocean into theeastern half of the channel, (2) it occurs mainly in sum-mer neap tidal periods, and (3) it is caused by the colli-sion of the warm filament formed along the Kuroshiofront, which is similar to the one formed in the GulfStream region (Lee et al., 1981), to the southwestern coastof Shikoku Island. Takeoka et al. (2000) inferred that thecause of the spring-neap and seasonal periodicities of thekyucho is a spring-neap variation of vertical tidal mixingand seasonal variation of thermal convection.

The kyucho in the Bungo Channel plays an impor-tant role in determining the marine environment in thechannel. The kyucho promotes water exchange in the baysalong the eastern coast (Koizumi, 1991), suppressingeutrophication due to fish farming. Moreover, it plays animportant role in maintaining biological production in thechannel. Since the warm water of the kyucho originatesfrom the Kuroshio which contains poor nutrients and as aresult is transparent, the water in the Bungo Channel turnstransparent due to the occurrence of the kyucho. There-fore, the kyucho is called “sumishio” (transparent seawater in Japanese) by local fishermen. However, bloomsof phytoplankton (mainly diatoms) usually occur after thekyucho. Koizumi and Kohno (1994) and Koizumi et al.(1997) revealed that these blooms are caused by a com-bination of the kyucho and bottom intrusions (see Sub-section 4.1, as for the bottom intrusion).

In the Kii Channel, a weaker phenomenon similar tothe kyucho sometimes occurs. Takeoka (1996) inferredthat the difference between the kyucho in the Bungo andKii Channels is due to the basic structure of the Seto In-land Sea. Since the water depth is shallower and the riverrunoff larger in the eastern section than in the westernone, the water density decreases in the eastern part dur-ing the summer period. Therefore, the density contrastbetween the Kuroshio and the coastal waters decreases inthe Kii Channel, resulting in weaker and less frequentkyucho in the Kii Channel.

Fig. 5. (a) Density structure in the vertical section of mixedand stratified regions and the resultant density induced cur-rents. (b) Transport routes of heat (solid line) and nutrients(broken line). Thick lines denote bypasses via the mixedregion.

Progress in Seto Inland Sea Research 101

3.7 Interdisciplinary studies in the 1990sThe interdisciplinary studies in the 1980s focused

on specific themes such as red tides, oxygen deficientwater masses and frontal processes. In the 1990s, a studyteam with wider research objectives was organized undersponsorship of the Nippon Life Insurance Foundation. Theteam, which comprised natural science experts and so-cial scientists including jurists and economists, aimed toclarify the basic natural, economic, social and legal as-pects regarding the preservation of both the fisheries in-dustry and a desirable natural marine environment in theSeto Inland Sea. The study, entitled “Interdisciplinarystudy on the sustainable production of valuable fishes andpreservation of environment in the Seto Inland Sea”, con-sisted of six core projects: (1) quantitative clarificationof the primary production rate, (2) quantitative clarifica-tion of temporal variations in fish catches, (3) preserva-tion of existing and the creation of new fishing grounds,(4) methods for reducing the nutrient load from the landand an assessment of its effects, (5) local economic de-velopment and policy decisions, and (6) legal problemsrelated to fisheries and the marine environment. Thesestudies were implemented between 1992 and 1995 andsummarized by Okaichi and Yanagi (1997).

One of the significant results of this study was theidentification of high productivity in the lower trophiclevels of the pelagic food chain. The group carried outfield observations at 39 stations covering the entire SetoInland Sea on four occasions (October 1993, January,April and June 1994). In addition to general hydrographicobservations at all stations, the concentrations of dissolvednutrients and particulate matter, bacterial density,microzooplankton and net-zooplankton and primary pro-duction rate were measured at selected stations. Byanalyzing both observed and historical data, Hashimotoet al. (1997) concluded that the average primary produc-tion rate was as high as 731 mg C m–2d–1, and the sec-ondary production rate was 206 mg C m–2d–1. Hence thetransfer efficiency from primary to secondary productionwas 28%, which is higher than the efficiency (

102 H. Takeoka

The water depth and the rate of vertical transport ofnutrients are supposed to be the main factors controllingEPC. In Chesapeake Bay, strong stratification developsand the vertical transport of nutrients is restricted. How-ever, the bay is very shallow (average depth = 6.5 m, in-cluding tributaries) and hence the proportion of water inthe euphotic layer is large. This may be one of the causesof the large EPC in Chesapeake Bay. The average depthof the Seto Inland Sea (37 m) is larger than that ofChesapeake Bay. Therefore, the main reason for the largeEPC in the Seto Inland Sea is believed to be the efficientvertical transport mechanism supplying nutrients in thelower layer to the euphotic layer. It is inferred that thenarrow channels and straits play an important role in theefficient transport of these nutrients. As described in Sub-section 3.5, a narrow strait with a strong tidal currentworks as a bypass for the vertical transport of heat andnutrients in the neighboring stratified regions. The nutri-ents rapidly return to the upper layer through this bypass,and oxygen in the upper layer is supplied to the lowerlayer by a reverse bypass mechanism, promoting decom-position of the organic matter. Moreover, heat transportthrough the bypass prevents the development of stratifi-cation. The many narrow straits and channels affect theentire Seto Inland Sea, resulting in the large EPC.

The Seto Inland Sea and Chesapeake Bay thus main-tain a high productivity by means of different mecha-nisms. However, there is another significant differencebetween the mechanisms that determine the nitrogen stockper unit area (CS) in these seas. In Chesapeake Bay, eventhough the average residence time of the river water isonly 0.3 years, the average residence time of nitrogen isapproximately one year due to the nutrient trap mecha-nism that results in a larger CS than in the case withoutthe nutrient trap mechanism. However, this mechanismrequires strong stratification and is accompanied by a highrisk of oxygen depletion in the lower layer. On the otherhand, the highly enclosed geometry and the resultant weakwater exchange rate in the Seto Inland Sea retain waterand nutrients for approximately one year, also maintain-ing a larger CS than in the case if the sea were more open.Such highly enclosed structures usually have a risk ofoxygen depletion due to stagnation of water movementin the interior and resultant strong stratification, especiallywhen there is a sill at the mouth of the bay. In the SetoInland Sea, however, vertical transport of heat and oxy-gen is maintained to some extent by the bypass mecha-nism in the many straits. Moreover, the straits in the SetoInland Sea are usually deeper than the neighboring ba-sins, and such sills are rarely formed. Thus the straits inthe Seto Inland Sea play important roles in maintaining ahigh biological production and in preserving the marineenvironment.

Figure 6(f) shows the fish catch per unit area in eachof the seas. It can be seen that the fish catch in the SetoInland Sea is much larger than in the other seas. The lowerfish catch in Chesapeake Bay may be caused by the in-fluence of oxygen deficiency on biological production inthe higher trophic levels.

In conclusion, biological production in the Seto In-land Sea is extremely efficient due to the sea’s enclosedstructure which maintains high nutrient concentrationsand the many straits that bypass heat, nutrients and oxy-gen, in consequence they contribute to the rapid and re-peated utilization of the nutrients.

3.8 Monitoring by ferry boatsAs mentioned in Subsection 3.1, the prefectural fish-

eries observatories have been conducting monthly obser-vations at 320 fixed stations since 1964. Whilst theseobservations are still important, the increased scope ofresearch in the area has meant that the requirement forobservations at higher temporal and spatial resolutionshas increased. Monitoring by ferry boats is one solutionto this problem. In 1991, the National Institute for Envi-ronmental Studies started a marine monitoring programin coastal and marginal seas (including the Seto InlandSea) using commercial ferries. The ferry routes are shownin Fig. 1. Seawater was sampled continuously, and thewater temperature, salinity, pH and in vivo fluorescencemeasured by in situ electrical sensors. The seawater wasautomatically filtered and dissolved inorganic nutrientsand phytoplankton pigments were analyzed in the labo-ratory.

Harashima et al. (1997) indicated based on resultsof the monitoring program that the temporal and spatialvariations of nutrients approve the “silica deficiency hy-pothesis”, i.e., the overall human activities tend to en-hance the discharge of nitrogen and phosphorus and in-terrupt the natural supply of silica to the coastal seas. Theincrease of nitrogen and phosphorus will favor the non-diatom phytoplankton species than the diatom species(Egge and Aksnes, 1992). Basically, three nutrients, DIN(dissolved inorganic nitrogen), DIP (dissolved inorganicphosphorus), and DSi (dissolved silicate or silica) de-crease during spring bloom and one of these nutrients isexhausted at the end of spring bloom. Three nutrients arerecovered by the bio-decomposition of organic matter inthe lower layer and the vertical transport by the mixingcaused by tide, wind and cooling after autumn. In the regu-lar area remote to the river mouth, DIN primarily depletesin summer. However, the ratio of DIN:DSi is large andsometimes DSi is exhausted in Osaka Bay.

The marine environmental monitoring using ferriesas a platform has been proved to be feasible by the SetoInland Sea mission. The anticipated products are not lim-

Progress in Seto Inland Sea Research 103

ited to the results of the temporal/spatial variation of nu-trients but can also be applied to various environmentalindicators. Therefore, other marine research organizationshave initiated the similar monitoring programs, such asthe ALGALINE Program by the Finnish Institute of Ma-rine Research (1993–present) and “Monitoring using aferry between Incheon and Cheju” by the Korea OceanResearch & Development Institute (1998–present). Fur-thermore, an international consortium of marine researchorganizations in Europe are planning to start the “Euro-pean Ferry Box Program” in several coastal seas includ-ing the Baltic, Adriatic and North Seas.

4. Recent Research Topics and Activities

4.1 Nutrients budgets and their long term variationsIn enclosed coastal seas, the main sources of nutri-

ents are usually river inflows that transport terragenicnutrients. It was previously believed that this was the casein the highly enclosed Seto Inland Sea. However, nutri-ent fluxes from the Pacific Ocean may also contribute asignificant amount to the overall nutrient content in thesea.

Fujiwara et al. (1997) observed concentrations ofnitrogen and current speeds in a transverse cross sectionin the Bungo Channel for 15 days in the summer of 1982,and concluded that a large quantity of nitrogen (ca. 70t day–1) was supplied into the channel from the PacificOcean. The fluxes of nitrogen and phosphorus from thePacific Ocean into the Kii Channel were estimated to be170 t day–1 and 34 t day–1 respectively, by similar obser-vations carried out on 23 and 24 August 1995. These re-sults are for one short period and do not indicate that thefluxes continue throughout the year. However, the fluxvalues are sufficiently large to warrant further research,as the fluxes of nitrogen and phosphorus from the landinto the Seto Inland Sea are approximately 470 t day–1

and 30 t day–1, respectively (Yuasa, 1994).The phenomenon supplying nutrients to the Bungo

Channel was clearly observed by Koizumi (1999). Fol-lowing repeated hydrographic observations in the BungoChannel in the summer of 1994, an intrusion of cold andnutrient-rich water from the bottom of the shelf slope re-gion south of the Bungo Channel into the lower layer ofthe channel was identified. Kaneda et al. (2002) carriedout long-term monitoring of water temperatures at thebottom of the Bungo Channel, and concluded that the in-trusion of cold water occurs intermittently, mainly in neaptidal periods in the summer. Since the intrusion patternwas similar to a bottom intrusion occurring in the GulfStream region (Atkinson, 1977), Kaneda et al. (2002) alsocalled the intrusion in the Bungo Channel a bottom intru-sion, although the generation mechanisms may not be thesame.

Several studies attempted to identify the route ofnutrient transport from the Pacific Ocean into the SetoInland Sea. Hayashi et al. (2000) reported evidence sug-gesting the intrusion of nutrients from the Bungo Chan-nel into Iyo-Nada. Further research is required to con-firm this result. Another important issue for this region isthe maintenance of the nitrogen and phosphorous budg-ets. If nitrogen and phosphorous intrude into the inlandsea throughout the year, their stocks would increase in-definitely. One possible reason why this does not happenis that there is an outflow of the nutrients in winter months,but no studies have been attempted to verify this hypoth-esis. Related to this topic is the long term variation in thenutrient budgets. Takeoka et al. (2000) analyzed histori-cal data pertaining to the bottom water temperature inthe Bungo Channel as an index of bottom intrusion, andrevealed that a decrease in the bottom intrusion and a re-sultant decrease in biological productivity occurred in the1990s. The nutrient budgets and their long term variationare an important factor in determining the future for themarine environment of the Seto Inland Sea and requireintensive studies.

4.2 Recently identified environmental issuesSeveral studies relating to recently identified envi-

ronmental issues have been initiated. As mentioned inSection 2, the dredging of sea sand and gravel is one suchissue in the Seto Inland Sea. The sea-sand and gravel inthe Seto Inland Sea were generated by continuous ero-sion at the bottom of the straits and channels for aboutten thousand years since the formation of the Seto InlandSea (Inouchi, 1990). Therefore, they are resources likefossils, which cannot be recovered in a short time, andthe environmental impacts of dredging are distinctive. Astudy of tidal current changes due to alterations to thetopography caused by dredging (Takahashi et al., 2001)and a study of the influences of turbid water generatedby the dredging on sea forests (Montani and Hari, 2000)have recently been initiated. Also, more basic studies ofthe physical and biogeochemical conditions of offshoresandy beds have been initiated, because little is known asyet about these areas. These studies will contribute to amore comprehensive understanding of the Seto InlandSea.

Another issue that has recently been identified is jel-lyfish blooms. In recent years, blooms of jellyfish, par-ticularly Aurelia aurita, were frequently observed in theseas around Japan including the Seto Inland Sea (Uye,1994). It is feared that jellyfish blooms disturb the ma-rine food chain and decrease the production of valuablefish resources. Therefore, an interdisciplinary researchgroup was set up in 2001 (headed by S. Uye of HiroshimaUniversity) to study the spatial and temporal distributionsof these blooms, the relationship between the bloom char-

104 H. Takeoka

acteristics, and changes in the environmental conditions,physiology and ecology of the jellyfish and technologyto protect the blooms.

4.3 Monitoring using new technologiesNew instruments for both academic and government

monitoring are being introduced into the various SetoInland Sea research programs. On the basis of studies onthe kyucho and bottom intrusions in the Bungo Channel,the Center for Marine Environmental Studies (CMES) ofEhime University developed a water temperature infor-mation system in the Sea of Uwa (the eastern part of theBungo Channel) in co-operation with the Ehime Prefec-tural Fisheries Observatory. In this system, water tem-perature data (measured at 2-hour intervals) are transmit-ted to the CMES via ORBCOM (Orbital Communication)satellites and immediately posted on the CMES websitefor public use. As of May 2001, three stations are opera-tional and the number of the stations will be graduallyincreased. Since this system is not costly to implement,further extension in other areas of the Seto Inland Sea isexpected. In addition, the CMES started water qualitymonitoring using the autonomous monitoring system atSada Point in March 2000. The main objective of thismonitoring is to detect long term variations in thebiogeochemical components in the sea, particularly thosedue to variations in the bottom intrusion in the BungoChannel. Hourly water temperature, salinity, pH, dis-solved oxygen, chlorophyll fluorescence, concentrationsof ammonia, nitrate, phosphate and silicate readings areobtained by the system (Takeoka et al., 2001). The Na-tional Institute for Environmental Studies is also plan-ning to implement real time monitoring using ferries bysending the data via N-star satellite and to release thisinformation to collaborating institutions. By construct-ing a network of time-series monitoring at fixed and mo-bile (ferry) stations, it will become possible to obtaincomprehensive real-time hydrographic andbiogeochemical data.

Acoustic Doppler current profilers for current moni-toring have been fitted in the research vessels of the pre-fectural fisheries observatories, and data obtained atmonthly intervals have been accumulated. In addition,acoustic tomographic techniques will be introduced in thenear future. This technology has already been applied tocurrent measurements in the deep ocean. A coastal appli-cation of this technology was developed recently (Zhenget al., 1998), and experiments to verify its usefulness incoastal seas are being planned by a group headed by A.Kaneko of Hiroshima University. Continuous two- orthree-dimensional current measurements using this sys-tem will provide valuable data for future studies.

Studies using satellite remote sensing data have al-ready been completed, for example that by Tsukamoto et

al. (1997) who analyzed seasonal variations in the watersurface temperature using NOAA/AVHRR data, but stud-ies using such data are limited due to the low resolutionof the satellite data. However, studies are being under-taken to evaluate chlorophyll concentrations (Tsukamotoand Yanagi, 2001) and to monitor occurrences of red tides(Hashimoto et al., 2001) using SeaWiFS ocean color data.

4.4 Numerical simulationsHitherto, numerical studies related to the Seto In-

land Sea focused on the individual processes such as tide-induced residual current (e.g. Oonishi, 1977), water ex-change through a strait (e.g. Awaji et al., 1980), genera-tion of thermohaline fronts (e.g. Oonishi et al., 1978),nutrient trap mechanism (Takeoka and Hashimoto, 1988)and red tide (Yanagi et al., 1993). Local, specific areas ofthe Seto Inland Sea were studied in most of these studies.With the progress of the studies on the Seto Inland Seaand the increasing social requirements, however, a morecomprehensive understanding of the Seto Inland Sea isbecoming definitely necessary, which requires a numeri-cal model with an accurate geometry of the sea. A diag-nostic model seems not to be a good choice for this pur-pose, because snapshot data are usually contaminated incoastal seas of wide spatial/temporal variabilities like theSeto Inland Sea, causing low accuracy of the diagnosticcalculation. It is therefore necessary to develop a fullyprognostic numerical model for the sea. The model gridsize should be 1 km or less. Moreover, higher resolutionwould be necessary in the areas of narrow straits and chan-nels, which requires a nested model technique. Tidal andresidual processes should be solved in one model, con-sidering their non-linear coupling which produce impor-tant processes such as a heat bypass.

In the future, the data assimilation technique willprobably be needed to improve the accuracy of the model.Moreover, a hindcast/forecast system would be necessaryto meet the requirement of the sea management, particu-larly in the marine environmental prediction. The datafrom the above mentioned monitoring such as that by ferryboats and that using new technologies described in Sub-section 4.3 will provide real-time initial and boundaryconditions for the forecast system. At present, most eco-system models of the Seto Inland Sea are based on thesimple box model (Hayashi et al., 2001) and focus onlocal problems at a basin scale. In the future, with theinclusion of biological processes in the hindcast/forecastsystem, simulation of the entire ecosystem and hence morecomprehensive understanding of the Seto Inland Sea willbe possible.

AcknowledgementsThe author expresses his sincere thanks to Dr. A.

Harashima of the National Institute for Environmental

Progress in Seto Inland Sea Research 105

Studies and Dr. X. Guo of the Center for Marine Envi-ronmental Studies, Ehime University, for the useful com-ments on the manuscript.

ReferencesAkiyama, H. and S. Saitoh (1993): The Kyucho in Sukumo Bay

induced by Kuroshio warm filament intrusion. J. Oceanogr.,49, 667–682.

Atkinson, L. P. (1977): Modes of Gulf Stream intrusion intothe South Atlantic Bight shelf waters. Geophys. Res. Lett.,4, 583–586.

Awaji, T., N. Imasato and H. Kunishi (1980): Tidal exchangetrough a strait: A numerical experiment using a simple modelbasin. J. Phys. Oceanogr., 10, 1499–1508.

Bowden, K. F. (1965): Horizontal mixing in the sea due to shear-ing current. J. Fluid Mech., 21, 83–95.

Egge, J. K. and D. L. Aksnes (1992): Silicate as regulating nu-trient in phytoplanlton competition. Mar. Ecol. Prog. Ser.,83, 281–289.

Fujiwara, T., N. Uno, M. Tada, K. Nakatsuji, A. Kasai and W.Sakamoto (1997): Inflow of nitrogen and phosphorus fromthe ocean into Seto sea. Proc. Coast. Eng., JSCE, 44-2,1061–1065 (in Japanese).

Harashima, A., R. Tsuda, Y. Tanaka, T. Kimoto, H. Tatsuta andK. Furusawa (1997): Monitoring algal blooms and relatedbiogeochemical changes with a flow-through system de-ployed on ferries in the adjacent seas of Japan. p. 85–112.In Monitoring Algal Blooms: New Techniques for Detect-ing Large-Scale Environmental Change, ed. by M. Kahl andC. W. Brown, Springer.

Hashimoto, H., T. Hashimoto, O. Matsuda, K. Tada, K. Tamai,S. Uye and T. Yamamoto (1997): Biological productivityof lower trophic levels of the Seto Inland Sea. p. 17–58. InSustainable Development in the Seto Inland Sea, Japan—From the Viewpoint of Fisheries—, ed. by T. Okaichi and T.Yanagi, Terra Scientific Publishing Company, Tokyo.

Hashimoto, T., T. Yanagi, J. Ishizaka, K. Odawara and M.Matsuura (2001): Possibility of red tide monitoring usingsatellite ocean color. Bull. Coast. Oceanogr., 39, 15–19 (inJapanese).

Hayami, S. and S. Unoki (1970): Water exchange and materialtransport in the Seto Inland Sea. Proc. 28th Conf. Coast.Eng., JSCE, 385–393 (in Japanese).

Hayashi, M., T. Yanagi and T. Hashimoto (2000): Standing stockratios of nitrogen and phosphorus in the Seto Inland Sea.Oceanogr. Japan, 9, 83–89 (in Japanese).

Hayashi, M., T. Yanagi and T. Hashimoto (2001): Analysis ofphosphorus cycling in the inner part of Osaka Bay using anumerical ecosysytem model. Oceanogr. Japan, 10, 203–217 (in Japanese).

Imabayashi, H. (1986): Effect of oxygen-deficient water on thesettled abundance and size composition of the bivalveTheora lubrica. Bull. Japan Soc. Sci. Fish., 5, 391–397.

Imasato, N., T. Awaji and H. Kunishi (1980): Tidal exchangethrough Naruto, Akashi and Kitan Straits. J. Oceanogr. Soc.Japan, 36, 151–162.

Inouchi, Y. (1990): Origin of sand and its distribution patternin the Seto Inland Sea, southwest Japan. Bull. Geol. Surv.Japan, 41, 49–86.

Kaneda, A., H. Takeoka, E. Nagaura and Y. Koizumi (2002):Periodic intrusion of cold water from the Pacific Ocean intothe bottom layer of the Bungo Channel, Japan. J. Oceanogr.(in press).

Kashiwai, M. (1984): The concept of tidal exchange and thetidal exchange ratio. J. Oceanogr. Soc. Japan, 40, 135–147(in Japanese).

Koizumi, Y. (1991): The process of water exchange in ShitabaBay during the phenomenon of Kyucho. Bull. Coast.Oceanogr., 29, 82–89 (in Japanese).

Koizumi, Y. (1999): Kyucho events and the mechanism ofphytoplankton growth in the eastern coast of the BungoChannel. Doctoral Thesis, University of Tokyo, 145 pp. (inJapanese).

Koizumi, Y. and Y. Kohno (1994): An influence of the Kyuchoon a mechanism of diatom growth in Shitaba Bay in sum-mer. Bull. Coast. Oceanogr., 32, 81–89 (in Japanese).

Koizumi, Y., S. Nishikawa, F. Yakushiji and T. Uchida (1997):Germination of resting stage cells and growth of vegetativecells in diatoms caused by Kyucho events. Bull. JapaneseSoc. Fish. Oceanogr., 61, 275–287 (in Japanese).

Kosaka, K. (ed.) (1985): Setonaikai no Kankyo (Environmentsin the Seto Inland Sea). Koseisya Koseikaku, Tokyo, 342 pp.(in Japanese).

Lee, T. N., L. P. Atkinson and R. Legeckis (1981): Observa-tions of a Gulf Stream frontal eddy on the Georgia conti-nental shelf, April 1977. Deep-Sea Res., 28A, 347–378.

Matsuyama, M. and S. Iwata (1977): The Kyucho in SagamiBay. Bull. Fish. Oceanogr., 30, 1–7 (in Japanese).

Montani, S. and S. Hari (2000): Environmental impacts of tur-bid water due to dredging of sea sand and gravel in the SetoInland Sea. Scientific Forum of the Seto Inland Sea, No.22, 32–36 (in Japanese).

Murakami, M., Y. Oonishi, A. Harashima and H. Kunishi (1978):A numerical simulation on the distribution of water tem-perature and salinity in the Seto Inland Sea. Bull. Coast.Oceanogr., 15, 130–137 (in Japanese).

Murakami, M., Y. Oonishi and H. Kunishi (1985): A numericalsimulation of the distribution of water temperature and sa-linity in the Seto Inland Sea. J. Oceanogr. Soc. Japan, 41,213–244.

Nagai, T. and Y. Ogawa (1997): Fisheries production. p. 61–94. In Sustainable Development in the Seto Inland Sea, Ja-pan—From the Viewpoint of Fisheries—, ed. by T. Okaichiand T. Yanagi, Terra Scientific Publishing Company, To-kyo.

Ochi, T. and H. Takeoka (1986): The anoxic water mass inHiuchi Nada. Part 1. Distribution of the anoxic water mass.J. Oceanogr. Soc. Japan, 42, 1–11.

Okaichi, T. (ed.) (1987): Akashio no Kagaku (Sciences of RedTides). Koseisya Koseikaku, Tokyo, 294 pp. (in Japanese).

Okaichi, T. and T. Yanagi (eds.) (1997): Sustainable Develop-ment in the Seto Inland Sea, Japan—From the Viewpoint ofFisheries—. Terra Scientific Publishing Company, Tokyo,329 pp.

Oonishi, Y. (1977): A numerical study on the tidal residual flow.J. Oceanogr. Soc. Japan, 33, 207–218.

Oonishi, Y., A. Harashima and H. Kunishi (1978): Characteris-tics of a front formed by cooling of the sea surface and in-

106 H. Takeoka

flow of the fresh water. J. Oceanogr. Soc. Japan, 34, 17–23.

Redfield, A. C. (1956): The hydrography of the Gulf of Ven-ezuela. Deep-Sea Res., 3, 115–133.

Simpson, J. H. and J. R. Hunter (1974): Fronts in the Irish Sea.Nature, 250, 404–406.

Takahashi, S., K. Murakami, I. Yuasa, H. Tanabe and M.Yamasaki (2001): Effect of sand mining around the GeiyoIslands—Transition of flow field. Abstracts of 2001 SpringMeeting of the Oceanographical Society of Japan, p. 157(in Japanese).

Takeoka, H. (1984a): Fundamental concepts of exchange andtransport time scales in a coastal sea. Cont. Shelf Res., 3,311–326.

Takeoka, H. (1984b): Exchange and transport time scales inthe Seto Inland Sea. Cont. Shelf Res., 3, 327–341.

Takeoka, H. (1985): Density stratification in the Seto InlandSea. Umi to Sora (Sea and Sky), 60, 145–152 (in Japanese).

Takeoka, H. (1987): Distribution and seasonal variation of thetransparency in the Seto Inland Sea. Umi to Sora (Sea andSky), 63, 15–27 (in Japanese).

Takeoka, H. (1990): Criterion of tidal fronts around narrowstraits. Cont. Shelf Res., 10, 605–613.

Takeoka, H. (1993): Three-dimensional structure of a verticaltransport in coastal seas. Abstracts of 1993 Fall Meeting ofthe Oceanographical Society of Japan, 227–228 (in Japa-nese).

Takeoka, H. (1996): Interactions between coastal seas and outeroceans. Bull. Coast. Oceanogr., 34, 3–13 (in Japanese).

Takeoka, H. (1997): Comparison of the Seto Inland Sea withother enclosed seas from around the world. p. 223–247. InSustainable Development in the Seto Inland Sea, Japan—From the Viewpoint of Fisheries—, ed. by T. Okaichi and T.Yanagi, Terra Scientific Publishing Company, Tokyo.

Takeoka, H. and T. Hashimoto (1988): Average residence timeof matter in coastal waters in a transport system includingbiochemical processes. Cont. Shelf Res., 8, 1247–1256.

Takeoka, H. and T. Yoshimura (1988): The Kyucho in UwajimaBay. J. Oceanogr. Soc. Japan, 44, 6–16.

Takeoka, H., T. Ochi and K. Takatani (1986): The anoxic watermass in Hiuchi Nada. Part 2. The heat and oxygen budgetmodel. J. Oceanogr. Soc. Japan, 42, 12–21.

Takeoka, H., O. Matsuda and T. Yamamoto (1993a): Processescausing the chlorophyll a maximum in the tidal front in Iyo-Nada, Japan. J. Oceanogr., 49, 57–70.

Takeoka, H., H. Akiyama and T. Kikuchi (1993b): The Kyuchoin the Bungo Channel, Japan—Periodic intrusion of oce-anic warm water. J. Oceanogr., 49, 369–382.

Takeoka, H., Y. Tanaka, Y. Ohno, Y. Hisaki, A. Nadai and H.Kuroiwa (1995): Observation of the Kyucho in the BungoChannel by HF radar. J. Oceanogr., 51, 699–711.

Takeoka, H., A. Kaneda and H. Anami (1997): Tidal fronts in-duced by horizontal contrast of vertical mixing efficiency.J. Oceanogr., 53, 563–570.

Takeoka, H., Y. Koizumi and A. Kaneda (2000): Year-to-yearvariation of a kyucho and a bottom intrusion in the BungoChannel, Japan. p. 197–215. In Interactions between Estu-aries, Coastal Seas and Shelf Seas, ed. by T. Yanagi, TerraScientific Publishing Company, Tokyo.

Takeoka, H., Y. Hayami, A. Kaneda, T. Matsushiata, T. Kimoto,K. Watanabe and J. Fujikawa (2001): Long-term autono-mous nutrient monitoring in the Seto Inland Sea. Bull. Coast.Oceanogr., 38, 91–97 (in Japanese).

Tanaka, Y., J. Shinohara and R. Tsuda (1992): The Kyucho inUragami bay. Bull. Coast. Oceanogr., 30, 37–44 (in Japa-nese).

Tsukamoto, H. and T. Yanagi (2001): Infrared and ocean colorimages by remote sensing in the Seto Inland Sea. Bull.Coast. Oceanogr., 39, 9–13 (in Japanese).

Tsukamoto, H., T. Yanagi, F. Sakaida, H. Kawamura and A.Harashima (1997): Seasonal variation of sea surface tem-perature in the Seto Inland Sea by the NOAA/AVHRR. Umino Kenkyu, 6, 279–292 (in Japanese).

Uye, S. (1994): Do jellyfish become dominated in eutrophicinlets? Aquabiology, 16, 251 (in Japanese).

Yamagata, T., S. Umatani, N. Masunaga and T. Matsuura (1984):Observation of an intrusion of a warmer and less saline watermass into a bay. Cont. Shelf Res., 3, 475–488.

Yanagi, T. (1976): Fundamental study on the tidal residual cir-culation, I. J. Oceanogr. Soc. Japan, 32, 199–208.

Yanagi, T. (1980): A coastal front in the Sea of Iyo. J. Oceanogr.Soc. Japan, 35, 253–260.

Yanagi, T. (1987): Classification of “siome”, streaks and fronts.J. Oceanogr. Soc. Japan, 43, 149–158.

Yanagi, T. (ed.) (1990): Siome no Kagaku (Sciences of“Siome”). Koseisya Koseikaku, Tokyo, 169 pp. (in Japa-nese).

Yanagi, T. and H. Higuchi (1979): Constant flows in the SetoInland Sea. Bull. Coast. Oceanogr., 16, 123–127 (in Japa-nese).

Yanagi, T. and H. Higuchi (1981): Tide and tidal current in theSeto Inland Sea. Proc. 28th Conf. Coast. Eng., JSCE, 555–558 (in Japanese).

Yanagi, T. and T. Ohba (1985): Tidal front in the Bungo Chan-nel. Bull. Coast. Oceanogr., 23, 19–25 (in Japanese).

Yanagi, T. and S. Okada (1993): Tidal fronts in the Seto InlandSea. Mem. Fac. Eng. Ehime Univ., 12-4, 337–343.

Yanagi, T. and T. Okaichi (1997): Synthesis and proposal. p.217–219. In Sustainable Development in the Seto InlandSea, Japan—From the Viewpoint of Fisheries—, ed. by T.Okaichi and T. Yanagi, Terra Scientific Publishing Com-pany, Tokyo.

Yanagi, T. and S. Takahashi (1988a): A tidal front influencedby river discharge. Dyn. Atm. Oceans, 12, 191–206.

Yanagi, T. and S. Takahashi (1988b): Response to fresh waterdischarge in Osaka Bay. Umi to Sora (Sea and Sky), 64,63–70 (in Japanese).

Yanagi, T. and K. Yoshikawa (1983): Generation mechanismsof tidal residual circulation. J. Oceanogr. Soc. Japan, 39,156–165.

Yanagi, T. and K. Yoshikawa (1987): Tidal fronts in Hiuchi Nadaand Osaka Bay. Bull. Japanese Soc. Fish. Oceanogr., 51,115–119 (in Japanese).

Yanagi, T., T. Yamamoto, Y. Koizumi, T. Ikeda, M. Kamizonoand H. Tamori (1993): Numerical simulation on red tide ofGimnodinium in Suo-Nada and Iyo-Nada. Bull. JapaneseSoc. Fish. Oceanogr., 51, 319–331 (in Japanese).

Yasuda, H. (1980): Generation mechanism of the tidal residual

Progress in Seto Inland Sea Research 107

current due to the coastal boundary layer. J. Oceanogr. Soc.Japan, 35, 241–252.

Yoshioka, H. (1971): Oceanic front at Kii-Suido in winter (1).Umi to Sora (Sea and Sky), 46, 31–44 (in Japanese).

Yoshioka, H. (1988): The coastal front in the Kii Channel inwinter. Umi to Sora (Sea and Sky), 64, 79–111.

Yuasa, I. (1994): Residual circulation, coastal fronts and

behavior of nutrients in the inland sea. Reports of ChugokuNational Industrial Research Institute, 12, 184 pp. (in Japa-nese).

Zheng, H., H. Yamaoka, N. Gohda, H. Noguchi and A. Kaneko(1998): Design of the acoustic tomography system for ve-locity measurement with an application to the coastal sea.J. Acoust. Soc. Japan (E), 19, 199–210.