Journal of Oceanography, Vol. 57, pp. 15 to 27, 2001
Current and Turbidity Variations in the Western Part of
Suo-Nada, the Seto Inland Sea, Japan: A Hypothesis on
the Oxygen-Deficient Water Mass Formation
TOMOHARU SENJYU1*, HIDEKAZU YASUDA2, SHIGEHIKO SUGIHARA 1 and MASATO KAMIZONO3
1
Department of Fishery Science and Technology, National Fisheries University,
2-7-1, Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan
2
Department of Fisheries Information and Management, National Fisheries University,
2-7-1, Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan
3
Fukuoka Fisheries and Marine Technology Research Center, Buzen-kai Laboratory,
76-30, Unoshima, Buzen 828-0022, Japan
(Received 20 April 2000; in revised form 25 July 2000; accepted 29 August 2000)
A hydrographic survey and a 25-hour stationary observation were carried out in the
western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated
the upper layer water from the bottom water over the observational area except for
near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with
semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M 2 tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer
water is controlled by the advection due to the M2 tidal current. On the other hand,
the quarter-diurnal variation in the bottom water turbidity is caused by the
resuspension of bottom sediments due to the M2 tidal current. The steep thermocline
and halocline were maintained throughout the observation period in spite of the rather
strong tidal currents. This implies an active intrusion of the low temperature and
high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the
oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a
course of circulation.
1. Introduction
Suo-Nada (the sea of Suo) is the westernmost sound
in the Seto Inland Sea surrounded by Yamaguchi,
Fukuoka, and Oita prefectures (Fig. 1). The area is 3,100
km2, the third largest among sounds in the Seto Inland
Sea, and the average depth is 23.7 m. The eastern part of
Suo-Nada opens to the North Pacific through the Hoyo
Strait (the Hayasui Strait) and the Bungo Channel, but
the western part is in a semi-enclosed environment, though
there is the Kanmon Strait connecting with the Japan Sea.
This is because that the water exchange through the
Keywords:
⋅ Turbidity,
⋅ resuspension,
⋅ advection,
⋅ M 2 tidal current,
⋅ oxygen-deficient
water mass,
⋅ stratification,
⋅ halocline,
⋅ quarter-diurnal
variation,
⋅ oxygen consumption,
⋅ suspended matter.
Kanmon Strait is small (Kamizono et al., 1991).
It is reported that a steep thermocline (pycnocline)
is formed in the western part of Suo-Nada in summer
(Takasugi et al., 1996a) and this strong stratification develops oxygen-deficient water mass near the bottom along
with the semi-enclosed condition (Kamizono et al., 1991,
1995; Isobe et al., 1993). The fisheries in particular oyster aqua-culture industry in this area has been largely
damaged by the oxygen-deficient water mass, for example, about 90% of oyster died in the mass mortality occurred in the summer 1988 (Tokuda and Kamizono, 1989).
For the generation of oxygen-deficient water mass
in Suo-Nada, Kamizono et al. (1995, 1996) reported that
the oxygen consumption by the suspended organic matters is essential in the bottom water. In addition, Kamizono
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
15
Fig. 1. Location and bottom topography of Suo-Nada. Solid circles with number denote observation points. Sections along the
solid line connecting Sts. 3, 2, 4, 5, 7, and 8 are shown in Fig. 3.
et al. (1995) found that the organic matters in the bottom
water are mostly originated from resuspended bottom
sediments. This suggests that the tidal currents play an
important role for the generation of oxygen-deficient
water mass, because strong tidal currents can resuspend
the bottom sediments and form a high turbidity layer near
the bottom (Kawana and Tanimoto, 1981, 1984; Yasuda
et al., 1997). In fact, Kamizono et al. (1995) pointed out
the possibility of resuspension caused by the tidal currents on the basis of weekly data from oxygen and moored
current meter. However, because of few concurrent measurements of turbidity and current, the relationship between
current and turbidity variations in this area is still obscure.
In order to elucidate the formation mechanism of the
oxygen-deficient water mass, a hydrographic survey and
a mooring observation in the western part of Suo-Nada
were carried out in the late summer 1998. The results indicated a clear relationship in their variations not only
near the bottom but also near the sea surface. Further,
these observational results introduce a new hypothesis
about the formation of the oxygen-deficient water mass
in Suo-Nada.
16
T. Senjyu et al.
Fig. 2. Vertical profiles of temperature, salinity, density (σt),
dissolved oxygen (DO), turbidity, and chlorophyll-a at
St. 5 observed in leg 1.
Fig. 3. Vertical sections of temperature (a), salinity (b), dissolved oxygen (c), turbidity (d), and chlorophyll-a (e) along the solid
line in Fig. 1.
Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan
17
Fig. 4. Lateral distributions of temperature (a), salinity (b), dissolved oxygen (c), turbidity (d), and chlorophyll-a (e) at a depth
of 2.0 m below the sea surface (in the upper layer water).
2. Observation
The observation consisted of two legs: the first leg
for hydrographic survey and the second for 25-hour mooring and stationary observation. Observation points and
bottom topography around the observational area are
shown in Fig. 1.
The first leg (hydrographic survey) was carried out
from 08:35 to 13:00 on August 25, 1998 using R/V Buzen
of the Fukuoka Fisheries and Marine Technology Re18
T. Senjyu et al.
search Center. Vertical distribution of temperature, salinity, turbidity, and chlorophyll a (strictly speaking, concentration of chlorophyll a and pheo-pigments) at 0.1 m
interval and dissolved oxygen concentration at about 2 m
interval were observed at 15 stations (Fig. 1) with the
multi-property profiler (CHLOROTECH model
ACL1182-PDK, ALEC Electronics Co. Ltd.). Surface
water was sampled at stations 2, 4, 6, 7, 8 and 12 for the
calibration of chlorophyll a sensor.
Fig. 4. (continued).
The second leg (stationary observation) was made
at St. 5 located in the center of observational area in the
first leg from 15:30 on August 25 to 16:30 on August 26,
1998. The yo-yo observation using the multi-property
profiler was carried out from an anchored fishing boat;
distributions of temperature, salinity, turbidity, and chlorophyll a were measured every 1 hour at 0.1 m interval
and dissolved oxygen at about 2 m interval.
In the second leg, the current-shear meter was installed to observe current and turbidity variations near
the bottom. This instrument consisted of four sets of current and turbidity sensors installed at every 50 cm and
temperature, salinity, and pressure sensors on the top
(Yasuda et al., 1997). It can be stood up on the floor using buoys at the top and anchor weights at the bottom.
Thus, not only currents and turbidity at 0.5, 1.0, 1.5, and
2.0 m above the bottom but also temperature, salinity,
and pressure at 2.0 m from the bottom were obtained every
2 seconds.
In addition, an electro-magnetic current meter (model
ACM8M, ALEC Electronics Co. Ltd.) was moored from
the anchored boat to measure the current near the sea surface. The current at 2.0 m below the sea surface was recorded every 1 second.
The turbidity sensors mounted on the multi-property
profiler and the current-shear meter detect the infrared
back-scattering intensity from mainly particulate matter.
Therefore, turbidity defined by the beam attenuation coefficient was not measured but infrared (940 nm) volume
scattering function at 165° was obtained. In this study,
however, the scattering intensity was converted into the
concentration of kaolin in ppm using the experimental
formula given by a producer, and this converted value is
called “turbidity”.
3. Results
3.1 Distribution of water characteristics
Firstly, spatial distribution of water properties is described on the basis of the data obtained in the
hydrographic survey (leg 1).
As a typical example, vertical profiles of temperature, salinity, density (σt), dissolved oxygen, turbidity, and
chlorophyll a at St. 5 are shown in Fig. 2. A very steep
thermocline and halocline (pycnocline) is observed around
8 m deep; vertical gradients of temperature, salinity, and
σt are 1.69°C, 0.12 psu, and 0.61 kg m–3 per 1.0 m, respectively. This thermocline coincides with a steep gradient of dissolved oxygen, turbidity, and chlorophyll a.
On the contrary, vertical gradient of water properties both
above and below the thermocline is very weak except for
turbidity near the bottom. Since this configuration can be
approximated to the two-layer system, waters above and
below the thermocline are named the upper layer water
and the bottom water, respectively. The bottom water
shows lower temperature, higher salinity, higher density,
lower dissolved oxygen, higher turbidity, and higher chlorophyll a compared to the upper layer water.
Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan
19
Fig. 5. Same as Fig. 4 but for a depth of 1.0 m above the bottom (in the bottom water).
Vertical sections along the solid line in Fig. 1 are
shown in Fig. 3 for temperature, salinity, dissolved oxygen, turbidity, and chlorophyll a. The steep thermocline
and halocline are found south of St. 7 (Figs. 3(a) and (b));
this layer coincides with the oxycline (Fig. 3(c)). An oxygen-deficient water mass of less than 4.0 mg l–1 exists
under the oxycline in the southern part of this section.
On the other hand, a weak stratification is seen in the
temperature and salinity sections at St. 8 forming benthic
front between Sts. 8 and 7. The benthic front is also found
20
T. Senjyu et al.
in the turbidity section (Fig. 3(d)), but the vertical gradient is intensified toward the north especially below 10 m
deep. As for the chlorophyll a section (Fig. 3(e)), two
patches of high concentration appear at the depth of
thermocline at Sts. 7 and 4. Another high patch is found
near the sea surface at St. 2, which coincides with the
patch of high turbidity.
Since the vertical gradients of water properties in
both waters are very weak as mentioned above, the depths
of 2.0 m below the sea surface (surface –2 m) and 1.0 m
Fig. 5. (continued).
above the bottom (bottom +1 m) can be regarded as the
representative depth of the upper layer water and the bottom water, respectively. Distributions of temperature,
salinity, dissolved oxygen, turbidity, and chlorophyll a at
each depth are shown in Figs. 4 and 5.
Temperature shows high values near the coast and
decreases toward the offshore in both layers (Figs. 4(a)
and 5(a)). Isotherms run parallel with the isobath; this is
because the shallow area is subject to heat up in summer
due to its small heat capacity (Unoki, 1993). Similar but
opposite distribution to the temperature is seen in salinity of the bottom water (Fig. 5(b)), which indicates higher
values in the offshore and lower near the coast. On the
other hand, upper layer salinity exhibits relatively high
values at coastal stations showing small influence of river
discharge (Fig. 4(b)). Low salinity areas appear in the
northern and southern part of the observational area in
both layers.
An oxygen-deficient water mass of less than 4.0
mg l–1 exists in the bottom layer from the central through
the southern part of the observational area (Fig. 5(c)); in
particular, low oxygen of less than 2.0 mg l–1 appears sporadically in the coastal zone of shallower than 10 m. This
pattern agrees with the typical distribution of oxygendeficient water mass in summer reported by Kamizono et
al. (1991). In contrast to the bottom water, the dissolved
oxygen concentration in the upper layer water shows almost saturated values of more than 7.0 mg l–1 (Fig. 4(c)).
The distribution of turbidity is similar to that of chlorophyll a in the upper layer water (Figs. 4(d) and (e));
there are two high value areas in the northwestern and
the southern part of the observational area. This indicates
that the turbid matter in the upper layer water is mainly
composed by phytoplanktons. Extremely high turbidity
(22.10 ppm) is recorded in the bottom water at the northernmost station (St. 8) forming a strong benthic front (Fig.
5(d)). It must be noted that St. 8 is located in the deep
channel connecting with the Kanmon Strait, and the
benthic front exists along the deep channel. It is well
known that the tidal flow in the Kanmon Strait is one of
the strongest currents around Japan (up to 8 knots at the
maximum). The observation time at St. 8 (11:12 on August 25) corresponds to the expected time of the maximum current in the Kanmon Strait (westward 4.8 knots
at 11:04; Maritime Safety Agency, 1997). Thus, the extremely high turbidity at St. 8 is probably suspended bottom sediments stirred up by the strong tidal current along
the deep channel. This interpretation is supported by the
facts that observation at St. 8 shows a weak vertical stratification (Figs. 3(a) and (b)) and a high dissolved oxygen
of more than 6.0 mg l–1 in the bottom layer (Fig. 5(c)).
These facts suggest a vertical mixing along the deep channel. The distribution of chlorophyll a in the bottom water
(Fig. 5(e)) shows a high concentration in the western part
of the observational area instead of the northern part.
3.2 Temporal variation
Vertical distributions of water properties are observed
at St. 5 every one hour from 15:30 on August 25 to 16:30
on August 26, 1998 (leg 2). Figure 6 shows the temporal
Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan
21
Fig. 6. Temporal variation of vertical distribution of water properties: temperature (a), salinity (b), dissolved oxygen (c), turbidity (d), and chlorophyll-a (e). The vertical axis denotes height from the bottom.
variation of temperature, salinity, dissolved oxygen, turbidity, and chlorophyll a distributions. The vertical axis
of Fig. 6 indicates height from the bottom. A curved thick
solid line denotes the sea surface, which changes its level
with semi-diurnal period without diurnal inequality.
22
T. Senjyu et al.
The strong stratification exists throughout the observation period; the steep thermocline and halocline are
found about 3 m above the bottom and they are undulated with the amplitude of about 1.5 m (Figs. 6(a) and
(b)). The bottom water tends to be low temperature and
Fig. 7. Time series of turbidity at the depths of 2.0, 1.5, 1.0,
and 0.5 m above the bottom obtained with the current-shear
meter. Mean values for 1 minute are plotted.
high salinity in the period of high tide.
The bottom layer is occupied by the oxygen-deficient water mass of less than 2.0 mg l–1 throughout the
observation period except for 15:30 on August 25 and
11:30–12:30 on August 26 (Fig. 6(c)). To the contrary,
the upper layer water shows almost saturated concentration of more than 7.0 mg l–1 throughout the observation
period. It is remarkable that the vertical gradient of dissolved oxygen in the upper layer water is very small. The
depth of oxycline is in good accordance with the
thermocline level and changes with the same phase and
amplitude as those of thermocline displacement.
Turbidity in the bottom water shows a distinct variation from that in the upper layer water (Fig. 6(d)). The
upper layer turbidity changes with semi-diurnal period
and decreases (increases) to the high tide (low tide). Similar variation to the upper layer turbidity is found in chlorophyll a in the upper layer water (Fig. 6(e)). On the other
hand, turbidity in the bottom water changes with quarterdiurnal period and seems to indicate high values at the
flood and ebb tide periods, though chlorophyll a in the
bottom water does not indicate the quarter-diurnal variation.
Fig. 8. Stick diagrams of current at 2.0 m below the sea surface and at the depths of 2.0, 1.5, 1.0, and 0.5 m above the
bottom. Each stick indicates averaged current vector for 10
minutes.
The quarter-diurnal variation in the bottom water
turbidity is also found in the turbidity records obtained
with the current-shear meter. Figure 7 shows time series
of turbidity at 2.0, 1.5, 1.0, and 0.5 m above the bottom.
These turbidities are averaged value for 1 minute. The
turbidity at 2.0 m above the bottom is lack of data in the
period from 23:35 on August 25 to 00:28 on August 26.
The turbidity increase occurs about 6-hour interval at all
the depths; four large peaks are found around 18:00 and
23:30 on August 25 and around 07:00 and 12:30 on August 26. The level of turbidity and the amplitude of the
quarter-diurnal variation become larger approaching to
the bottom (see Table 2 for the average values).
Time series of current vectors at 2.0 m below the sea
surface and at the depths of 2.0, 1.5, 1.0, and 0.5 m above
the bottom are displayed in Fig. 8. These currents are
averaged value for 10 minutes. The east-west currents
prevail and rather strong flows of more than 20 cm s–1
are observed at all the depths. The currents vary with semidiurnal period throughout the depths in accordance with
the sea level. However, the current in the upper layer de-
Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan
23
lays by 2–3 hours to that in the bottom water except for
the period from 12:00 on August 26. As a result, flows in
the bottom water show the maximum speed when the
upper layer current is in the slack water, and vice versa.
4. Discussions
4.1 Relationship between current and turbidity variations
The observation revealed that turbidity and chlorophyll a changed with semi-diurnal period in the upper
layer water, but the bottom water turbidity showed quarter-diurnal variation, though rather strong east-west currents (more than 20 cm s–1) varying with semi-diurnal
period were dominant in both the upper layer and the
bottom waters.
The harmonic analysis is carried out in order to
clarify the phase relation between currents and turbidity
variations. Harmonic constants for the currents and the
sea level measured by the pressure sensor on the currentshear meter are listed on Table 1. The amplitude of residual currents in the bottom water shows 4.4–6.6
cm s –1; this is comparable to that in the upper layer water
of 7.2 cm s–1. However, the direction of residual currents
in the bottom water is in the north-northwest to northnortheast, though that in the upper layer water is in the
south-southeast; this fact suggests a meridional vertical
circulation. As for the oscillating components, M2 constituent with semi-diurnal period is dominant in both currents and sea level. The amplitude of M2 currents decreases toward the bottom. The major axis of M2 current
in the upper layer water is in the west-east direction
(269.4°T); the major axis in the bottom water is rotated
clockwise by about 30° to that in the upper layer. The
phase in the bottom water leads about 50° to that in the
upper layer M2 current.
Harmonic constants of turbidity are tabulated in Table 2 on the basis of the current-shear meter data shown
in Fig. 7. Harmonic constants at 2.0 m deep are also calculated from the hourly turbidity data shown in Fig. 6(d).
The semi-diurnal variation (M2) is dominant in the upper
layer water, whereas the M4 constituent with quarter-diurnal period prevails in the bottom water, as found in Fig.
6(d). It is noteworthy that the amplitude of M4 constituent increases and the phase lag decreases toward the bottom in the bottom water. This implies that the turbid matter in the bottom water is supplied from the bottom.
Table 1. Harmonic constants for the current and sea level.
Figures in upper and lower rows of M1, M 2, and M4 currents at each level are for the major and minor axes, respectively.
Table 2. Harmonic constants for turbidity.
24
T. Senjyu et al.
Comparing Table 1 with Table 2, the current of M2
constituent leads by about 80° to the M2 turbidity in the
upper layer. This means that turbidity shows the maximum when the eastward current ceases and reaches to
the minimum at the end of the westward current in the
upper layer water. (Note that the westward (strictly speaking, the direction of 269.4°T) is positive for the M2 constituent at 2 m deep in Table 1.) Considering the turbidity
distribution in the upper layer water of which the high
turbidity water places west of the observation site (Fig.
4(d)), the turbidity variation in the upper layer is controlled by the advection due to the M2 tidal current. The chlorophyll a variation in the upper layer water is in the same
situation as turbidity (Figs. 6(e) and 4(e)).
The turbidity in the bottom water changes with quarter-diurnal period, though the M2 tidal current is dominant in the bottom water. Thus, the harmonic analysis is
applied to the current speed (scalar of current) in the bottom water. Table 3 lists harmonic constants of M4 constituent for the current speed at the four levels in the bottom water. The average speed, amplitude, and phase lag
decrease toward the bottom. It is notable that the phase
lag of current speeds almost coincides with that of M4
constituent of turbidity at each level (Table 2). This fact
indicates that the turbidity variation in the bottom water
is caused by the resuspension of sediments from the bottom due to the strong tidal current. Similar phenomena
are reported in other sounds of the Seto Inland Sea such
as Hiuchi-Nada (Kawana and Tanimoto, 1981; Yasuda et
al., 1997) and Harima-Nada (Kawana, 1982; Kawana and
Tanimoto, 1984). The reason why chlorophyll a does not
show the quarter-diurnal variation in the bottom water
(Fig. 6(e)) is maybe that the resuspended matter includes
either little or inactive chlorophyll a.
4.2 Formation of the oxygen-deficient water mass in SuoNada—a hypothesis
One of the characteristic features revealed by the
observation is that the bottom water showed lower temperature, higher salinity, lower dissolved oxygen, higher
turbidity, and higher chlorophyll a compared to the upper layer water. This strong stratification with the steep
Table 3. Harmonic constants of M4 constituent for the current
speed.
Level
Average
(cm/s)
Amplitude
(cm/s)
Phase lag
(deg.)
bottom +2.0 m
+1.5 m
+1.0 m
+0.5 m
15.8
13.6
12.8
12.0
4.1
4.1
3.7
2.7
225.9
212.8
202.5
192.6
halocline was maintained throughout the observation period in spite of the rather strong tidal currents. This is the
distinct point from other sounds such as Hiuchi-Nada and
Harima-Nada in the Seto Inland Sea; in these sounds salinity stratification is weak (Kawana, 1982; Ochi and
Takeoka, 1986).
The source of high salinity in the bottom water must
be to the east of the observational area, because the bottom water salinity increases toward the east (Fig. 6(b)).
Many observational results have suggested the intrusion
of the high salinity water into the bottom layer of SuoNada from the Bungo Channel (ex. Uda and Watanabe,
1933; Unoki, 1972; Oceanographical Division of Kobe
Marine Observatory, 1985; Kamizono et al., 1988). Recently, Takeoka et al. (1993) and Yamamoto et al. (2000)
pointed out that the open ocean water in the Bungo Channel is mixed with the coastal water by the strong tidal
currents around the Hoyo Strait, and then the resultant
high salinity water intrudes into the subsurface layer of
Iyo-Nada. Though the volume of the high salinity water
intruding into Suo-Nada is unknown, the existence of the
steep halocline in the strong currents suggests a fairly
active intrusion of the high salinity water from the east.
The high salinity water around the Hoyo Strait shows
almost saturated concentration of dissolved oxygen
(Yamamoto et al., 2000) because this water is formed by
the active tidal mixing accompanied by the vertical mixing (Takeoka et al., 1993). However, the bottom water in
Suo-Nada is oxygen-deficient. This discrepancy may be
explained by the high turbidity in the bottom water as
found in the observation. Generally, sediments below the
bottom surface are in a deoxidized condition. If these
deoxidized sediments are stirred up by strong currents
and resuspended in the bottom water, then effective oxygen consumption occurs. Moreover, Hata (1981) reported
that the particulate organic matter is most effectively decomposed by the microbial activities under the suspended
condition; this suggests active oxygen consumption in the
high turbidity water. Indeed, Sato (1979) experimentally
demonstrated that the concentration of dissolved oxygen
under the mud-suspended condition obeys exponential
decrease with time, though the oxygen decreases linearly
with time under the no-stirring conditions. Similar experiments using the mud from Suo-Nada exhibited about
twelve times larger oxygen consumption in the case of
the mud-suspended condition than that in the no-stirring
condition (Kamizono et al., 1994).
According to our observational results, the bottom
water undergoes four times high turbidity in a day, which
is caused by the M2 tidal current. This phenomenon probably occurs in an extensive area of Suo-Nada because the
fine sediment and high concentration of suspended matter near the bottom are observed in the wide area of SuoNada (Kawana and Tanimoto, 1984). Moreover, the M2
Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan
25
tidal current (up to 0.5–2.0 knots) is dominant in whole
of Suo-Nada (Miita and Kamizono, 1989). This frequent
high turbidity arisen in the wide area accelerates the oxygen consumption in the bottom water, and is likely to
modify quickly the oxygen-rich water in the east to the
oxygen-deficient bottom water in Suo-Nada in a course
of circulation.
Though the high turbidity water consumes dissolved
oxygen effectively in the above discussion, the variation
of dissolved oxygen in the bottom water does not synchronize with that of turbidity (Figs. 6(c) and (d)). This
is because that the time scale for oxygen depletion is considered to be longer than that of turbidity due to the M2
tidal current. Takeoka et al. (1986) introduced the time
scale for anoxia that refers to the time necessary for oxygen depletion down to e–1 times of the initial concentration. For the two layer system, the time scale for anoxia
(t*) is given by
−1
 K
R 
t =
+ L ,
 hHL C0 
∗
where h is the thickness of the thermocline, HL is the lower
layer thickness, C0 the dissolved oxygen concentration
in the upper layer, K the vertical diffusivity, and RL the
mean oxygen consumption rate in the lower layer. From
our observation, h, H L, and C0 are estimated to be 2 m,
4 m, and 7.0 mg l–1, respectively. As for the vertical diffusivity, Takasugi et al. (1996a) reported small values of
about 5 × 10 –2 cm2s –1 in the thermocline on the basis of
the Micro-Scale Profiler (MSP) observation. The net consumption rate of oxygen in the bottom water is estimated
by Kamizono et al. (1996) to be 1.45 g m–2day–1 for the
water column of 4 m height; this value corresponds to
the oxygen consumption of 4.2 × 10–6 mg l–1s–1. Substituting these values into the above formula, t* is estimated
as 9.4 days; this time scale is very long compared to that
of turbidity variation of 6 hours. The laboratory experiments by Sato (1979) and Kamizono et al. (1994) support the longer time scale for anoxia; their experiments
spent much longer time than 6 hours for the decrease of
dissolved oxygen by 50% saturation.
In addition to the above reason, there are some other
factors to vary the oxygen concentration in the bottom
water. For example, the photosynthesis by phytoplankton
(Kamizono et al., 1995) and the vertical mixing induced
by the strong vertical shear, as observed at St. 8 near the
Kanmon Strait, can suppress the oxygen-deficient condition. The oxygen transport by the bottom current and horizontal mixing is also important for the variation of dissolved oxygen (Takasugi et al., 1996b). The observed oxygen variation is the result from all the factors, and the
photosynthesis and vertical mixing effects seem to act
26
T. Senjyu et al.
more seriously on the dissolved oxygen variation in Fig.
6(c).
5. Concluding Remark
The hydrographic survey and 25-hour stationary observation were carried out in the western part of Suo-Nada
in the summer 1998. The steep thermocline and halocline
separated the upper layer water from the bottom water
over the observational area except for near the Kanmon
Strait. The bottom water indicated lower temperature,
higher salinity, lower dissolved oxygen, higher turbidity,
and higher chlorophyll a compared to the upper layer
water. The strong stratification with the steep thermocline
and halocline was maintained throughout the observation
period in spite of the rather strong tidal currents.
Turbidity and chlorophyll a in the upper layer water
varied with semi-diurnal period while the bottom water
turbidity showed a quarter-diurnal variation, though rather
strong east-west currents with semi-diurnal period were
dominant in both waters. The turbidity and chlorophyll a
variations in the upper layer water are controlled by the
advection due to the M2 tidal current. On the other hand,
the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due
to the M2 tidal current.
On the basis of the observational results, the new
conceptual model about the generation process of the
oxygen-deficient water mass in Suo-Nada is proposed:
the periodical turbidity variation in the bottom water
modifies quickly the oxygen-rich water in the east to the
oxygen-deficient bottom water in Suo-Nada in the course
of circulation. This hypothesis is based on the only two
days observation in the summer 1998. Thus, of course,
more extensive observations are necessary for verification and improvement of the hypothesis. In particular, the
chemical and biochemical approaches are essential for
the quantitative evaluation of the in situ oxygen consumption in the moving bottom water parcel. Besides, little is
known about the behavior of the suspended matters in
the bottom water, though the resuspended sediments play
the important role in the present hypothesis. The
resuspension of bottom sediments occurs in the benthic
boundary layer where the frictional force is important. In
order to consider the detail process of resuspension from
the bottom, it is essential to investigate the structure of
benthic boundary layer.
Acknowledgements
We would like to thank crew of R/V Buzen (Captain
S. Hirowatari), Messrs. K. Takiguchi and T. Etoh of the
Fukuoka Fisheries and Marine Technology Research
Center, and Messrs. T. Hirata, Y. Sameshima, and M. Araki
of National Fisheries University for their cooperation in
the observation field. Professor Y. Hayakawa of National
Fisheries University and Dr. Y. Takasugi of Chugoku
National Industrial Research Institute are acknowledged
for their contribution of the chlorophyll a determination
and the part of data processing, respectively. Thanks are
also due to Mr. H. Kajiya of National Fisheries University for assistance in the data analysis.
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