Download Progress in Seto Inland Sea Research

Journal of Oceanography, Vol. 58, pp. 93 to 107, 2002
Review
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 geometry and thus a variety of marine environments. This sea is, at the same time, one of
the most industrialized areas in Japan, and its marine environment has been significantly affected by the anthropogenic impacts over the last four decades. The wide
range of marine environments in this sea and the serious environmental issues resulting from these impacts have attracted the attention of Japanese coastal oceanographers. It is believed that the nature and scope of these studies might be an example of
the progress of Japanese coastal oceanography. The historical changes in the Seto
Inland Sea environment in the last four decades are briefly summarized, and the
progress in the studies of the Seto Inland Sea is reviewed with reference to historical
changes. Some recent research topics and activities are also mentioned.
Keywords:
⋅ Seto Inland Sea,
⋅ anthropogenic
impacts,
⋅ heat bypass,
⋅ nutrient trap,
⋅ tide and tidal
current,
⋅ red tide,
⋅ oxygen-deficient
water mass,
⋅ fronts.
studies might be a good example of the progress of Japanese coastal oceanography. This paper therefore presents
a review of these studies. In Section 2, historical changes
in the marine environment of the Seto Inland Sea are discussed as a background to the present paper. The various
individual studies are then discussed in Section 3. Advances in Seto Inland Sea studies are also reviewed in
Section 3, classified according to subject. Recent research
topics and activities are mentioned in Section 4.
1. Introduction
The Seto Inland Sea in Japan is a semi-enclosed
coastal sea surrounded by Honshu (the main island of
Japan), Shikoku and Kyushu Islands (Fig. 1). It has a
length of 500 km, an average depth of 30 m and contains
approximately 600 islands. The sea is divided by islands
and peninsulas into wide basins, some of which are called
“nada” in Japanese, and these basins are connected by
narrow channels called “seto”. This complicated geometry results in wide variations in the marine environment.
The Seto Inland Sea region is also one of the most
industrialized areas in Japan. After the New Industrial City
Law was enacted in 1963, many facilities for heavy industry 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 Inland Sea watershed. This industrialization and urbanization required substantial reclamation of land. The marine
environment of the Seto Inland Sea has been significantly
affected by these impacts over the last four decades.
The wide range of marine environments in the Seto
Inland Sea and the serious environmental issues resulting from anthropogenic development in the region have
attracted the attentions of Japanese coastal oceanographers. It is believed that the nature and scope of these
2.
Historical Changes in the Seto Inland Sea Environment
Figure 2 shows a chronological table of events affecting the Seto Inland Sea over the last four decades,
along with a description of completed studies concerning
to the region and the annual number of red tide occurrences in the region. The number of red tide incidents
can be used as an indicator of eutrophication. During the
1970s, eutrophication advanced rapidly due to an increase
in the volume of industrial and urban waste, resulting in
a frequent occurrence of red tides and oxygen-deficient
water masses. These occurrences had a great impact on
the marine environment. In 1970, a mass mortality of fish
occurred in Hiuchi-Nada due to an oxygen-deficient water mass. Large-scale red tides of Chattonella often occurred in Harima-Nada. The red tides that occurred in
1971 damaged the fish farming industry up to the value
of 7.1 billion yen.
* E-mail address: [email protected]
Copyright © The Oceanographic Society of Japan.
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
of Japan enacted “The Interim Law for Conservation of
the Environment of the Seto Inland Sea” in 1973 and “The
Law Concerning Special Measures for Conservation of
the Environment of the Seto Inland Sea” in 1978. These
laws resulted in a COD reduction to some extent. In addition, the annual number of red tides gradually reduced
between the mid 1970s and the mid 1980s, but has remained stable since then.
Prior to 1973, land reclamation was intensive. In
average, approximately 16 km2 was reclaimed annually
between 1965 and 1973. This was regulated by the Interim Law for Conservation of the Environment of the
Seto Inland Sea, but still took place to some extent after
the law was introduced. The total area of land reclaimed
since 1965 is approximately 250 km 2, which is approximately 12% of the area with a depth of less than 10 m in
the Seto Inland Sea. More than half of the marine forest
that existed in the early 1960s has been lost by reclamation.
Another environmental issue that has recently become acute is the dredging of sand and gravel from the
seabed. The great demand for building materials and the
lack of suitable quarries in western Japan led to an increase in dredging activity. Sea-sand dredging increased
rapidly in the late 1960s, thereafter 2 × 107 m 3 of seasand has been dredged annually. However, some prefectures surrounding the Seto Inland Sea prohibited dredging due to public pressure.
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H. Takeoka
3. Progress in Seto Inland Sea Research
3.1 Basic descriptive studies
The Prefectural Fisheries Observatories around the
Seto Inland Sea started monthly observations of water
temperature, salinity and transparency at 320 fixed stations in 1964 in a project supported by the Fisheries
Agency. The Maritime Safety Agency accumulated tide
and tidal current data. In the 1970s and 1980s, many descriptive studies summarizing these data were completed.
Yanagi and Higuchi (1979) analyzed the historical tidal
current data measured by the Maritime Safety Agency,
and produced a chart of residual current flow patterns.
Yanagi and Higuchi (1981) also produced charts of the
amplitude and phase lag of the M2 and K1 constituents of
tide and tidal current. Summarizing the historical
hydrographic data obtained by fisheries observatories,
Takeoka (1985) revealed the distribution of stratification
in the Seto Inland Sea. Takeoka (1987) also described the
transparency distribution and analyzed the seasonal and
spatial differences in the distribution.
Among the results of these studies, distributions of
M 2 tide and M2 tidal current are shown here, because the
tide and tidal current are the most basic and important
factors characterizing the Seto Inland Sea and the M 2
constituent is dominant over almost the whole Sea. Figures 3(a) and (b) indicate the distributions of the tidal
range 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. 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 s–1 are hatched in (c).
the M 2 tidal current (Yanagi and Higuchi, 1981). In the
Pacific Ocean, south of the Seto Inland Sea, tidal waves
propagate from east to west at significant speeds due to
the great water depth. Hence the phase of the tides at the
mouths of the Kii and Bungo Channels (see Fig. 1) are
approximately equal (Fig. 3(b)). The tidal waves propagate at a much lower speed in the Seto Inland Sea due to
the shallow water depth. The waves propagate from the
two channels into the inland sea over a long period of
time and meet at the central part of the Seto Inland Sea
between the Bisan Strait and Hiuchi-Nada where the phase
lag of M2 tide is the largest. They are delayed by about
150° from the mouths of the channels (Fig. 3(b)). Both
tidal waves entering from the channels are dissipated
during propagation. Therefore, the amplitudes of the tidal
waves propagating eastward and westward are approximately equal in the central part of the inland sea, whilst
in the eastern and western regions the amplitude of the
tidal wave from the adjacent channel is greater than that
from the opposite channel. Thus the tidal wave in the central region becomes a stationary wave, whilst those in the
eastern and western regions are progressive waves. These
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H. Takeoka
features can be seen from the tide and tidal current phase
distribution plots. The phase of the tide is almost equal in
the central region and differs spatially in the eastern and
western regions. The phase difference between the tide
and tidal current is approximately 90° in the central region and is much smaller in the eastern and western regions. As a result of such tidal features, the flood tidal
currents are directed to the area into which the tidal waves
assemble. Moreover, the transport volume of the tidal
current is larger in the outer regions, and almost vanishes
in the assembling area. Therefore, except in the narrow
straits, the amplitude of the tidal current is generally larger
in the outer regions and smaller in the eastern part of
Hiuchi Nada (Fig. 3(c)). In the narrow straits the amplitudes of the tidal currents are much larger than those in
the basin interior. The amplitudes of the four major tidal
current constituents in the main straits are shown in Table 1. The maximum tidal speeds in some straits, such as
Naruto and Kurushima Straits, approach 5 m s–1 in the
spring tides. These strong tidal currents play a significant role in maintaining high biological productivity in
the Seto Inland Sea as described later.
Table 1. Amplitudes of four major tidal current constituents in
the main straits of the Seto Inland Sea (after Yanagi and
Higuchi, 1981).
M2
S2
K1
O1
–1
(cm s )
Tomogashima Strait
Naruto Strait
Akashi Strait
Bisan Strait
Kurusima Strait
Tsurushima Strait
Ohbatake Strait
Hayasui Strait
Kanmon Strait
105
330
160
95
250
90
250
155
225
25
90
60
35
100
45
75
70
80
40
50
50
15
40
25
25
35
65
35
50
50
10
30
20
20
25
45
3.2 Horizontal transport and water exchange
Coastal oceanographic studies in the 1970s and 1980s
focused on horizontal transport processes and their quantitative evaluations, because the horizontal transport processes expel pollutants from bays and were regarded as a
purification mechanism against marine pollution.
Hayami and Unoki (1970) gave a significant impact
on horizontal transport studies. On the basis of salt budget
analysis, they concluded that the apparent one-dimensional diffusivity along the axis of the Seto Inland Sea is
107 cm 2s–1. Considering the scale dependence of horizontal diffusivity in marine environments, this value seems
to be too large. However, the value itself was accepted
because it was obtained from a very simple budget model.
This large apparent diffusivity was due to dispersion produced by the linked effect of eddy diffusion and current
shear, as proposed by Bowden (1965). On the basis of
this assumption, Murakami et al. (1978, 1985) revealed
that 30 to 50% of this large apparent diffusivity can be
attributed to density-induced vertical circulation. In addition to these studies, the generation mechanisms of residual currents, especially tide-induced residual currents,
and their contribution to horizontal transport have been
studied (e.g. Yanagi, 1976; Oonishi, 1977; Yasuda, 1980;
Yanagi and Yoshikawa, 1983).
Since the Seto Inland Sea is divided by islands and
peninsulas into several basins, water exchange through
the narrow channels and straits attracted particular interest. The studies of water exchange can be classified into
two categories.
One group of studies examined tidal exchange processes through the straits by means of field observations
and hydraulic and numerical experiments. Theoretical
studies relating to the definition of the tidal exchange rate
were also carried out. Kashiwai (1984) summarized the
tidal exchange rates at major straits in the Seto Inland
Sea, converting the values obtained by other researchers
into values under the same definition. Furthermore,
Imasato et al. (1980) and Awaji et al. (1980) revealed
that the Stokes Drift due to large gradients of amplitude
and phase difference of tidal current around the straits
plays an important role in the tidal exchange.
The second group of studies examined the transport
and renewal of water or materials filling the individual
basins or the whole Seto Inland Sea. Takeoka (1984a)
published a theoretical study of the time scales that represent the transport of materials and water renewal, and
proposed some basic concepts such as residence, transit
and turn-over times and the average age that can be applied to problems in coastal seas. Takeoka (1984b) applied 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
and total water. He concluded that the average residence
time of the total water was approximately 14 months.
3.3 Interdisciplinary studies in the 1980s
Until the early 1970s, the interest of oceanographers
was focused mainly within their own disciplines, although
a few collaborative studies with other disciplines were
done. Since the end of 1970s, with support of public
awareness of environmental conservation, several interdisciplinary study groups were formed. These were mainly
supported by Grants-in-Aid for Scientific Research from
the Ministry of Education, Science and Culture of Japan.
Two groups with special interests on red tides were
established in 1978 for basic biological studies on red
tides (headed by T. Yanagida), and studies on the physical, chemical and biological mechanisms of red tide genesis (headed by J. Ashida). These groups were the pioneers of basic biological and interdisciplinary studies of
red tides, whilst previous studies were carried out from
the viewpoints of fisheries biology. These interdisciplinary studies, with some changes to the research titles and
the constituent members, continued until 1984 with participation of more than 30 physical, chemical, biological
and fisheries oceanographers. The surveyed areas were
not restricted to the Seto Inland Sea but covered the many
coastal areas around Japan where red tides occur. Among
them, Harima-Nada was the major research area because
of the frequent occurrence of the red tides and the serious damage that affected the fisheries, as shown in Fig.
2. The hydrographic structures related to red tides, the
relationships between nutrient budget and red tides, the
transition of red tide species, the formation of cysts and
their germination conditions, and the modeling of red tide
formation and other related topics were mostly studied in
these projects. The results were summarized by Okaichi
(1987).
Progress in Seto Inland Sea Research
97
An interdisciplinary study group focusing on the Seto
Inland Sea was formed in 1981 with about 30 members
divided into three subgroups. The title of the first subgroup was “Basic studies aiming at the comprehensive
evaluation of the Seto Inland Sea” (headed by K. Kosaka).
This group collected historical data on environmental factors in the Seto Inland Sea obtained by various institutions, and methods of data handling and environmental
indices suitable for the evaluation of the Seto Inland Sea
were studied. The second subgroup studied “The biological processes and environmental dynamics in the estuarine areas of the Seto Inland Sea—Focusing on the Ohta
River and Hiroshima Bay—” (headed by T. Hayashi). The
distributions of pelagic and benthic biota and their relationship with the gradient of environmental factors were
investigated. The third subgroup focused on “Studies of
the marine structure and the generation mechanisms of
the oxygen-deficient water mass in Hiuchi-Nada” (headed
by H. Higuchi, later by H. Takeoka). The distribution of
the oxygen-deficient water mass and its seasonal change,
its relationship with the hydrographic structure, its influences on benthic communities and biogeochemical processes in the benthic environments were studied by means
of frequent and vigorous field observations (Ochi and
Takeoka, 1986; Takeoka et al., 1986; Imabayashi, 1986).
In this study, STD was used probably for the first time in
the Seto Inland Sea region, and interesting thermal structures were identified. In addition to the thermocline below the surface mixed layer, a thermocline that had not
been previously recorded was often observed at approximately 6 m above the sea-bottom, and oxygen-deficient
water was found below this level. This benthic
thermocline was named “the second thermocline” by Ochi
and Takeoka (1986). Thereafter, similar multi-layered
structures were often found in various areas in the Seto
Inland Sea. The findings of the three subgroups were summarized by Kosaka (1985).
Other important interdisciplinary studies carried out
in the 1980s were those focused on coastal fronts. A front
is a surface convergence zone between two different water masses, according to the definition by Yanagi (1987)
which classified “siome” (in Japanese: tide rips), streaks
and fronts. As described in the next section, there are many
kinds of fronts in the Seto Inland Sea, and they are considered to play important roles in material transport and
biological processes. A study group comprising 10 members (headed by T. Yanagi) was organized in 1984 to promote interdisciplinary studies on the coastal fronts supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Science and Culture of Japan (later
supported by the Nippon Life Insurance Foundation).
They selected the tidal fronts in the Seto Inland Sea and
the thermohaline front in Tokyo Bay as the major subjects for their research. The generation and maintenance
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H. Takeoka
mechanisms, the biological process in the frontal areas
and the accumulation of pollutants in the frontal areas
were widely studied by means of field observations and
numerical experiments. The results were summarized by
Yanagi (1990).
3.4 Physical studies on fronts
Many 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 wide
interest from physical oceanographers, and as a result,
several studies were completed.
The first front studied was a thermohaline front in
the Kii Channel. Yoshioka (1971, 1988) described in detail the structure of this front on the basis of field observations. Another thermohaline front was discovered in
Iyo-Nada by Yanagi (1980). These thermohaline fronts
are generated between cold, less saline coastal water and
warm, more saline oceanic water during the winter period. Detailed generation mechanisms of the thermohaline
front 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 a
tidal front formed mainly during the summer period. A
tidal front is a transition zone between stratified and
tidally mixed waters, the basic concept of which was introduced by Simpson and Hunter (1974). There are many
narrow straits with strong tidal currents in the Seto Inland Sea around which tidal fronts can be formed. Figure
4 shows the distribution of log 10(H/u3) (H: water depth
(m), u: amplitude of M2 tidal current (m s–1)) after Yanagi
and Okada (1993). According to the energetics given by
Simpson and Hunter (1974), tidal fronts are aligned along
the contour of a critical value of this parameter. The critical value ranges from 2.5 to 3.0 in the Seto Inland Sea
(Yanagi and Okada, 1993). This means that tidal fronts
are expected at the margin of the hatched areas in Fig. 4.
Some of them were studied by field observations; for
example, in the Bungo Channel (Yanagi and Ohba, 1985),
in the western part of Hiuchi-Nada (Yanagi and
Yoshikawa, 1987; Takeoka, 1990), in Osaka Bay (Yanagi
and Takahashi, 1988a; Yuasa, 1994) and in Iyo-Nada
(Takeoka et al., 1993a). In the case of tidal fronts in European coastal regions, the shallower areas are vertically
well mixed. In contrast, the deeper areas are usually well
mixed in the Seto Inland Sea, because the straits in the
Seto Inland Sea are deeply eroded by strong tidal currents. Therefore, the tidal fronts in the Seto Inland Sea
are formed by the effects of horizontal geometry.
Another kind of tidal front was found in the Bungo
Channel (Takeoka et al., 1997). As demonstrated in Fig.
4, log10(H/u3 ) in the Bungo Channel is larger in eastern
and western coastal areas than in offshore areas due to a
smaller value of u (weaker tidal current) in the coastal
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.
areas. This means that stratification is expected to be more
developed in coastal areas. However, Takeoka et al.
(1997) found that stratification in coastal areas is much
weaker than in offshore areas, and tidal fronts were formed
between the coastal and offshore areas. They showed that
the strong vertical mixing in coastal areas is due to a high
vertical mixing efficiency caused by the complicated horizontal current patterns. Therefore, it can be stated that
these tidal fronts are induced by the horizontal contrast
of vertical mixing efficiency.
Estuarine fronts and shelf fronts are also formed in
the Seto Inland Sea. Yanagi (1987) classified all the surface discontinuities including fronts and streaks, and summarized their generation mechanisms.
3.5 Transport of bioelements and 3-dimensional structure of vertical transport
Interdisciplinary studies completed in the 1980s addressed two new fields of research: transport of
bioelements and vertical transport. The studies on horizontal transport processes and water exchange reviewed
in Subsection 3.2 essentially dealt with the movement of
water, and hence can only be applied to materials that are
dissolved in and move with the water. However,
bioelements such as nitrogen and phosphorus can transform between dissolved and particulate forms and hence
their movement does not coincide with that of the water.
Yanagi and Takahashi (1988b) obtained the average residence times of river water and nitrogen flowing into Osaka
Bay, and stated that the average residence time of nitrogen is 1.7 times longer than that of the river water.
Takeoka and Hashimoto (1988) revealed that these differences are caused by the following mechanism. Dissolved inorganic nitrogen or phosphorus is transformed
into particles by primary production in the upper euphotic
layer and settles to the lower layer as detritus. This detritus is then decomposed into a dissolved form whilst be-
ing carried back to the inner bay by the current in the
lower layer, and returns to the surface. Accordingly, the
nitrogen or phosphorus remains in the bay longer than
the river water, which stays mostly in the upper layer and
flows rapidly out of the bay. The basic principles of this
mechanism were provided in the earlier study of Redfield
(1956). Takeoka and Hashimoto (1988) demonstrated by
means of simple modeling techniques that differences in
the average residence times in Osaka Bay can be explained
by this nutrient trap mechanism.
Vertical transport processes were not the subject of
any significant research in earlier studies, because pollutants are not expelled by the processes from the concerned area. However, the need to study such processes
increased during the interdisciplinary studies, because red
tides and oxygen-deficient water masses are closely related to vertical transport processes. As shown in Fig. 4,
stratified regions are adjacent to vertically mixed regions
in areas of the Seto Inland Sea. Takeoka (1993) deduced
that vertical transport in stratified regions is significantly
influenced by vertical transport in mixed regions, and
proposed the transport mechanism described below.
Figure 5 illustrates the density structure and the resultant density currents in a vertical cross section of stratified and mixed regions. Since the density of the water in
the mixed region is between that of the upper and lower
layers in the stratified region, the mixed water tends to
intrude into the middle layer of the stratified region, whilst
water in the upper and lower layers flows into the mixed
region. As a result of heat transport by these currents, the
upper layer of the stratified region heats the mixed region and the mixed layer heats the lower region. This
means that there is a heat transport route from the upper
layer to the lower layer via the mixed region, as shown
by the thick solid line in Fig. 5(b). Horizontal mixing
between mixed and stratified regions can also generate
such a heat transport mechanism. Vertical heat transport
Progress in Seto Inland Sea Research
99
Fig. 5. (a) Density structure in the vertical section of mixed
and stratified regions and the resultant density induced currents. (b) Transport routes of heat (solid line) and nutrients
(broken line). Thick lines denote bypasses via the mixed
region.
via the mixed region is called “heat bypass” (Takeoka,
1993). Dissolved oxygen produced by primary production in the upper layer is also transported to the lower
layer by this mechanism (oxygen bypass). Moreover, rich
nutrients in the lower layer can be transported to the upper 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 the
primary production in a tidal front. Takeoka et al. (1993a)
found a prominent chlorophyll-a maximum in the subsurface of the tidal front formed in Iyo-Nada around the
Hayasui Strait, following observations conducted in July
1990. From analysis of the TS diagram, they inferred that
the nutrients supporting the chlorophyll-a maximum were
supplied not vertically from the lower layer but horizontally from the mixed region around the Hayasui Strait.
This nutrient supply route can be regarded as transport
through the nutrient bypass.
3.6 Influences from the Pacific Ocean
Although the Seto Inland Sea is very enclosed, phenomena in the Pacific Ocean should strongly influence
the marine environment in the boundary regions (such as
the Kii and Bungo Channels). A typical example of such
influences is a kyucho (in Japanese) in the Bungo Channel. A kyucho is a sudden stormy current that is usually
accompanied by a rise of water temperature and has been
studied intensively since the mid 1980s. Kyucho have been
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H. Takeoka
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 summer neap tidal periods, and (3) it is caused by the collision of the warm filament formed along the Kuroshio
front, which is similar to the one formed in the Gulf
Stream region (Lee et al., 1981), to the southwestern coast
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.
The kyucho in the Bungo Channel plays an important role in determining the marine environment in the
channel. The kyucho promotes water exchange in the bays
along the eastern coast (Koizumi, 1991), suppressing
eutrophication due to fish farming. Moreover, it plays an
important role in maintaining biological production in the
channel. Since the warm water of the kyucho originates
from the Kuroshio which contains poor nutrients and as a
result is transparent, the water in the Bungo Channel turns
transparent due to the occurrence of the kyucho. Therefore, the kyucho is called “sumishio” (transparent sea
water in Japanese) by local fishermen. However, blooms
of phytoplankton (mainly diatoms) usually occur after the
kyucho. Koizumi and Kohno (1994) and Koizumi et al.
(1997) revealed that these blooms are caused by a combination of the kyucho and bottom intrusions (see Subsection 4.1, as for the bottom intrusion).
In the Kii Channel, a weaker phenomenon similar to
the kyucho sometimes occurs. Takeoka (1996) inferred
that the difference between the kyucho in the Bungo and
Kii Channels is due to the basic structure of the Seto Inland Sea. Since the water depth is shallower and the river
runoff larger in the eastern section than in the western
one, the water density decreases in the eastern part during the summer period. Therefore, the density contrast
between the Kuroshio and the coastal waters decreases in
the Kii Channel, resulting in weaker and less frequent
kyucho in the Kii Channel.
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 aspects 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”, consisted of six core projects: (1) quantitative clarification
of the primary production rate, (2) quantitative clarification of temporal variations in fish catches, (3) preservation of existing and the creation of new fishing grounds,
(4) methods for reducing the nutrient load from the land
and an assessment of its effects, (5) local economic development and policy decisions, and (6) legal problems
related to fisheries and the marine environment. These
studies were implemented between 1992 and 1995 and
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
field observations at 39 stations covering the entire Seto
Inland Sea on four occasions (October 1993, January,
April and June 1994). In addition to general hydrographic
observations at all stations, the concentrations of dissolved
nutrients and particulate matter, bacterial density,
microzooplankton and net-zooplankton and primary production rate were measured at selected stations. By
analyzing both observed and historical data, Hashimoto
et al. (1997) concluded that the average primary production rate was as high as 731 mg C m–2d–1, and the secondary production rate was 206 mg C m–2d–1. Hence the
transfer efficiency from primary to secondary production
was 28%, which is higher than the efficiency (<20%) commonly accepted for the marine food chain. The tertiary
production rate was estimated to be 58 mg C m–2d–1, and
the transfer efficiency from the secondary production was
26%. These high transfer efficiencies suggest that
eutrophication is not significant over the entire Seto Inland Sea, because transfer efficiencies are usually lower
in the eutrophic areas.
In comparison, benthic environments in some areas
such as Osaka Bay, Harima-Nada and Hiroshima Bay were
found to deteriorate due to eutrophication. Guidelines for
total nitrogen distribution were proposed in order to restore these environments and sustain local fisheries (Nagai
and Ogawa, 1997), and the reduction rates of nitrogen
and phosphorus loads needed to realize these guidelines
Fig. 6. Comparison of the enclosed or semi-enclosed seas in
the world. (a) Rate of primary production in terms of nitrogen. (b) Nitrogen loading rate (Qs in the North Sea is a sum
of the flux from the land and the flux from the Atlantic
Ocean). (c) Average residence time of nitrogen. (d) Stock
of nitrogen per unit area. (e) Efficiency of primary production (nitrogen cycling rate). (f) Fish catch per unit area (after Takeoka, 1997).
were proposed (Yanagi and Okaichi, 1997).
Takeoka (1997) discussed the reason of the high productivity rates in the Seto Inland Sea and compared it
with the other semi-enclosed seas by analyzing the physical and biogeochemical parameters. Since these comparisons will help in understanding the Seto Inland Sea, the
results are reproduced below in detail. However, the discussions are restricted only to comparisons with
Chesapeake Bay.
Figure 6 shows some of the comparisons made by
Takeoka (1997). Takeoka (1997) inferred that high productivity in the Seto Inland Sea is supported by the high
efficiency of primary production (E PC). The primary production efficiency given here is defined as E PC = PS /CS,
where PS is the primary production rate per unit area and
CS is the standing stock of nitrogen per unit area. This
value states how many times the nitrogen stock in the
water column is utilized for primary production per unit
time and how many times the total stock of nitrogen in
the sea is utilized. Hence, this efficiency may also be
called the nitrogen cycling rate. In Fig. 6, it can be seen
that E PC may be as high as 4 or 5 y–1 in the Seto Inland
Sea and Chesapeake Bay. Thus, from the efficiency E PC,
it can be stated that the primary production is highly efficient in these areas. In these two areas, high primary productivity is maintained by the regions ability to quickly
and repeatedly utilize the reduced nitrogen content (see
Fig. 6(e)) in the water column.
Progress in Seto Inland Sea Research
101
The water depth and the rate of vertical transport of
nutrients are supposed to be the main factors controlling
E PC. In Chesapeake Bay, strong stratification develops
and the vertical transport of nutrients is restricted. However, the bay is very shallow (average depth = 6.5 m, including tributaries) and hence the proportion of water in
the euphotic layer is large. This may be one of the causes
of the large E PC in Chesapeake Bay. The average depth
of the Seto Inland Sea (37 m) is larger than that of
Chesapeake Bay. Therefore, the main reason for the large
E PC in the Seto Inland Sea is believed to be the efficient
vertical transport mechanism supplying nutrients in the
lower layer to the euphotic layer. It is inferred that the
narrow channels and straits play an important role in the
efficient transport of these nutrients. As described in Subsection 3.5, a narrow strait with a strong tidal current
works as a bypass for the vertical transport of heat and
nutrients in the neighboring stratified regions. The nutrients rapidly return to the upper layer through this bypass,
and oxygen in the upper layer is supplied to the lower
layer by a reverse bypass mechanism, promoting decomposition of the organic matter. Moreover, heat transport
through the bypass prevents the development of stratification. The many narrow straits and channels affect the
entire Seto Inland Sea, resulting in the large EPC.
The Seto Inland Sea and Chesapeake Bay thus maintain a high productivity by means of different mechanisms. However, there is another significant difference
between the mechanisms that determine the nitrogen stock
per unit area (CS ) in these seas. In Chesapeake Bay, even
though the average residence time of the river water is
only 0.3 years, the average residence time of nitrogen is
approximately one year due to the nutrient trap mechanism that results in a larger CS than in the case without
the nutrient trap mechanism. However, this mechanism
requires strong stratification and is accompanied by a high
risk of oxygen depletion in the lower layer. On the other
hand, the highly enclosed geometry and the resultant weak
water exchange rate in the Seto Inland Sea retain water
and nutrients for approximately one year, also maintaining a larger CS than in the case if the sea were more open.
Such highly enclosed structures usually have a risk of
oxygen depletion due to stagnation of water movement
in the interior and resultant strong stratification, especially
when there is a sill at the mouth of the bay. In the Seto
Inland Sea, however, vertical transport of heat and oxygen is maintained to some extent by the bypass mechanism in the many straits. Moreover, the straits in the Seto
Inland Sea are usually deeper than the neighboring basins, and such sills are rarely formed. Thus the straits in
the Seto Inland Sea play important roles in maintaining a
high biological production and in preserving the marine
environment.
102
H. Takeoka
Figure 6(f) shows the fish catch per unit area in each
of the seas. It can be seen that the fish catch in the Seto
Inland Sea is much larger than in the other seas. The lower
fish catch in Chesapeake Bay may be caused by the influence of oxygen deficiency on biological production in
the higher trophic levels.
In conclusion, biological production in the Seto Inland Sea is extremely efficient due to the sea’s enclosed
structure which maintains high nutrient concentrations
and the many straits that bypass heat, nutrients and oxygen, in consequence they contribute to the rapid and repeated utilization of the nutrients.
3.8 Monitoring by ferry boats
As mentioned in Subsection 3.1, the prefectural fisheries observatories have been conducting monthly observations at 320 fixed stations since 1964. Whilst these
observations are still important, the increased scope of
research in the area has meant that the requirement for
observations at higher temporal and spatial resolutions
has increased. Monitoring by ferry boats is one solution
to this problem. In 1991, the National Institute for Environmental Studies started a marine monitoring program
in coastal and marginal seas (including the Seto Inland
Sea) using commercial ferries. The ferry routes are shown
in Fig. 1. Seawater was sampled continuously, and the
water temperature, salinity, pH and in vivo fluorescence
measured by in situ electrical sensors. The seawater was
automatically filtered and dissolved inorganic nutrients
and phytoplankton pigments were analyzed in the laboratory.
Harashima et al. (1997) indicated based on results
of the monitoring program that the temporal and spatial
variations of nutrients approve the “silica deficiency hypothesis”, i.e., the overall human activities tend to enhance the discharge of nitrogen and phosphorus and interrupt the natural supply of silica to the coastal seas. The
increase of nitrogen and phosphorus will favor the nondiatom phytoplankton species than the diatom species
(Egge and Aksnes, 1992). Basically, three nutrients, DIN
(dissolved inorganic nitrogen), DIP (dissolved inorganic
phosphorus), and DSi (dissolved silicate or silica) decrease during spring bloom and one of these nutrients is
exhausted at the end of spring bloom. Three nutrients are
recovered by the bio-decomposition of organic matter in
the lower layer and the vertical transport by the mixing
caused by tide, wind and cooling after autumn. In the regular area remote to the river mouth, DIN primarily depletes
in summer. However, the ratio of DIN:DSi is large and
sometimes DSi is exhausted in Osaka Bay.
The marine environmental monitoring using ferries
as a platform has been proved to be feasible by the Seto
Inland Sea mission. The anticipated products are not lim-
ited to the results of the temporal/spatial variation of nutrients but can also be applied to various environmental
indicators. Therefore, other marine research organizations
have initiated the similar monitoring programs, such as
the ALGALINE Program by the Finnish Institute of Marine Research (1993–present) and “Monitoring using a
ferry between Incheon and Cheju” by the Korea Ocean
Research & Development Institute (1998–present). Furthermore, an international consortium of marine research
organizations in Europe are planning to start the “European Ferry Box Program” in several coastal seas including the Baltic, Adriatic and North Seas.
4. Recent Research Topics and Activities
4.1 Nutrients budgets and their long term variations
In enclosed coastal seas, the main sources of nutrients are usually river inflows that transport terragenic
nutrients. It was previously believed that this was the case
in the highly enclosed Seto Inland Sea. However, nutrient fluxes from the Pacific Ocean may also contribute a
significant amount to the overall nutrient content in the
sea.
Fujiwara et al. (1997) observed concentrations of
nitrogen and current speeds in a transverse cross section
in the Bungo Channel for 15 days in the summer of 1982,
and concluded that a large quantity of nitrogen (ca. 70
t day–1) was supplied into the channel from the Pacific
Ocean. The fluxes of nitrogen and phosphorus from the
Pacific Ocean into the Kii Channel were estimated to be
170 t day–1 and 34 t day –1 respectively, by similar observations carried out on 23 and 24 August 1995. These results are for one short period and do not indicate that the
fluxes continue throughout the year. However, the flux
values are sufficiently large to warrant further research,
as the fluxes of nitrogen and phosphorus from the land
into 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). Following repeated hydrographic observations in the Bungo
Channel in the summer of 1994, an intrusion of cold and
nutrient-rich water from the bottom of the shelf slope region south of the Bungo Channel into the lower layer of
the channel was identified. Kaneda et al. (2002) carried
out long-term monitoring of water temperatures at the
bottom of the Bungo Channel, and concluded that the intrusion of cold water occurs intermittently, mainly in neap
tidal periods in the summer. Since the intrusion pattern
was similar to a bottom intrusion occurring in the Gulf
Stream region (Atkinson, 1977), Kaneda et al. (2002) also
called the intrusion in the Bungo Channel a bottom intrusion, although the generation mechanisms may not be the
same.
Several studies attempted to identify the route of
nutrient transport from the Pacific Ocean into the Seto
Inland Sea. Hayashi et al. (2000) reported evidence suggesting the intrusion of nutrients from the Bungo Channel into Iyo-Nada. Further research is required to confirm this result. Another important issue for this region is
the maintenance of the nitrogen and phosphorous budgets. If nitrogen and phosphorous intrude into the inland
sea throughout the year, their stocks would increase indefinitely. One possible reason why this does not happen
is that there is an outflow of the nutrients in winter months,
but no studies have been attempted to verify this hypothesis. Related to this topic is the long term variation in the
nutrient budgets. Takeoka et al. (2000) analyzed historical data pertaining to the bottom water temperature in
the Bungo Channel as an index of bottom intrusion, and
revealed that a decrease in the bottom intrusion and a resultant decrease in biological productivity occurred in the
1990s. The nutrient budgets and their long term variation
are an important factor in determining the future for the
marine environment of the Seto Inland Sea and require
intensive studies.
4.2 Recently identified environmental issues
Several studies relating to recently identified environmental issues have been initiated. As mentioned in
Section 2, the dredging of sea sand and gravel is one such
issue in the Seto Inland Sea. The sea-sand and gravel in
the Seto Inland Sea were generated by continuous erosion at the bottom of the straits and channels for about
ten thousand years since the formation of the Seto Inland
Sea (Inouchi, 1990). Therefore, they are resources like
fossils, which cannot be recovered in a short time, and
the environmental impacts of dredging are distinctive. A
study of tidal current changes due to alterations to the
topography caused by dredging (Takahashi et al., 2001)
and a study of the influences of turbid water generated
by the dredging on sea forests (Montani and Hari, 2000)
have recently been initiated. Also, more basic studies of
the physical and biogeochemical conditions of offshore
sandy beds have been initiated, because little is known as
yet about these areas. These studies will contribute to a
more comprehensive understanding of the Seto Inland
Sea.
Another issue that has recently been identified is jellyfish blooms. In recent years, blooms of jellyfish, particularly Aurelia aurita, were frequently observed in the
seas around Japan including the Seto Inland Sea (Uye,
1994). It is feared that jellyfish blooms disturb the marine food chain and decrease the production of valuable
fish resources. Therefore, an interdisciplinary research
group was set up in 2001 (headed by S. Uye of Hiroshima
University) to study the spatial and temporal distributions
of these blooms, the relationship between the bloom char-
Progress in Seto Inland Sea Research
103
acteristics, and changes in the environmental conditions,
physiology and ecology of the jellyfish and technology
to protect the blooms.
4.3 Monitoring using new technologies
New instruments for both academic and government
monitoring are being introduced into the various Seto
Inland Sea research programs. On the basis of studies on
the kyucho and bottom intrusions in the Bungo Channel,
the Center for Marine Environmental Studies (CMES) of
Ehime University developed a water temperature information system in the Sea of Uwa (the eastern part of the
Bungo Channel) in co-operation with the Ehime Prefectural Fisheries Observatory. In this system, water temperature data (measured at 2-hour intervals) are transmitted to the CMES via ORBCOM (Orbital Communication)
satellites and immediately posted on the CMES website
for public use. As of May 2001, three stations are operational and the number of the stations will be gradually
increased. Since this system is not costly to implement,
further extension in other areas of the Seto Inland Sea is
expected. In addition, the CMES started water quality
monitoring using the autonomous monitoring system at
Sada Point in March 2000. The main objective of this
monitoring is to detect long term variations in the
biogeochemical components in the sea, particularly those
due to variations in the bottom intrusion in the Bungo
Channel. Hourly water temperature, salinity, pH, dissolved oxygen, chlorophyll fluorescence, concentrations
of ammonia, nitrate, phosphate and silicate readings are
obtained by the system (Takeoka et al., 2001). The National Institute for Environmental Studies is also planning to implement real time monitoring using ferries by
sending the data via N-star satellite and to release this
information to collaborating institutions. By constructing a network of time-series monitoring at fixed and mobile (ferry) stations, it will become possible to obtain
comprehensive real-time hydrographic and
biogeochemical data.
Acoustic Doppler current profilers for current monitoring have been fitted in the research vessels of the prefectural fisheries observatories, and data obtained at
monthly intervals have been accumulated. In addition,
acoustic tomographic techniques will be introduced in the
near future. This technology has already been applied to
current measurements in the deep ocean. A coastal application of this technology was developed recently (Zheng
et al., 1998), and experiments to verify its usefulness in
coastal seas are being planned by a group headed by A.
Kaneko of Hiroshima University. Continuous two- or
three-dimensional current measurements using this system will provide valuable data for future studies.
Studies using satellite remote sensing data have already been completed, for example that by Tsukamoto et
104
H. Takeoka
al. (1997) who analyzed seasonal variations in the water
surface temperature using NOAA/AVHRR data, but studies using such data are limited due to the low resolution
of the satellite data. However, studies are being undertaken to evaluate chlorophyll concentrations (Tsukamoto
and Yanagi, 2001) and to monitor occurrences of red tides
(Hashimoto et al., 2001) using SeaWiFS ocean color data.
4.4 Numerical simulations
Hitherto, numerical studies related to the Seto Inland Sea focused on the individual processes such as tideinduced residual current (e.g. Oonishi, 1977), water exchange through a strait (e.g. Awaji et al., 1980), generation 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 of
the Seto Inland Sea were studied in most of these studies.
With the progress of the studies on the Seto Inland Sea
and the increasing social requirements, however, a more
comprehensive understanding of the Seto Inland Sea is
becoming definitely necessary, which requires a numerical model with an accurate geometry of the sea. A diagnostic model seems not to be a good choice for this purpose, because snapshot data are usually contaminated in
coastal seas of wide spatial/temporal variabilities like the
Seto Inland Sea, causing low accuracy of the diagnostic
calculation. It is therefore necessary to develop a fully
prognostic numerical model for the sea. The model grid
size should be 1 km or less. Moreover, higher resolution
would be necessary in the areas of narrow straits and channels, which requires a nested model technique. Tidal and
residual processes should be solved in one model, considering their non-linear coupling which produce important processes such as a heat bypass.
In the future, the data assimilation technique will
probably be needed to improve the accuracy of the model.
Moreover, a hindcast/forecast system would be necessary
to meet the requirement of the sea management, particularly in the marine environmental prediction. The data
from the above mentioned monitoring such as that by ferry
boats and that using new technologies described in Subsection 4.3 will provide real-time initial and boundary
conditions for the forecast system. At present, most ecosystem models of the Seto Inland Sea are based on the
simple box model (Hayashi et al., 2001) and focus on
local problems at a basin scale. In the future, with the
inclusion of biological processes in the hindcast/forecast
system, simulation of the entire ecosystem and hence more
comprehensive understanding of the Seto Inland Sea will
be possible.
Acknowledgements
The author expresses his sincere thanks to Dr. A.
Harashima of the National Institute for Environmental
Studies and Dr. X. Guo of the Center for Marine Environmental Studies, Ehime University, for the useful comments on the manuscript.
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