Estuarine, Coastal and Shelf Science 71 (2007) 210e217www.elsevier.com/locate/ecss

Flow structure and hypoxia in Hiuchi-nada, Seto Inland Sea, Japan

Akihide Kasai a,*, Tatsuo Yamada b, Hiroshi Takeda a,1

a Kyoto University, Graduate School of Agriculture, Oiwake, Kitashirakawa, Sakyo, Kyoto 606-8502, Japanb Kagawa Prefectural Fisheries Experimental Station, 75-5 Higashicho, Yashima, Takamatu, Kagawa 761-0111, Japan

Received 21 April 2006; accepted 1 August 2006

Available online 11 September 2006

Abstract

Oxygen in the bottom water of the eastern basin of Hiuchi-nada, which is located in the central part of the Seto Inland Sea, Japan, is oftendepleted in summer. To clarify the formation mechanism of the hypoxic water mass, oxygen consumption rate experiments and detailed hydro-graphic observations were conducted. The experimental results showed that the oxygen consumption is unremarkable at the bottom in Hiuchi-nada, compared with other waters that show higher oxygen concentrations. From the hydrographic observations, the water is stratified and a colddome, which corresponds to oxygen depleted water, is detected under the thermocline in the eastern basin of Hiuchi-nada. By contrast, outside ofthe dome, the water column is well mixed by strong tidal currents and the oxygen concentration is high. A prominent bottom front separatesthese two different water masses. There is a strong correlation between temperature and oxygen concentration under the thermocline (r2> 0.7).Residual currents from ADCP records show cyclonic circulation above the cold dome, while the water is nearly stationary inside the dome.Estimates of geostrophic currents from the density distribution are consistent with the observed flow pattern. These results indicate that thecold dome is isolated from the surrounding water such that water exchange is insufficient, and thus the oxygen concentration reduces in summer.Therefore, in Hiuchi-nada, the contribution of physical processes outweighs that of biochemical processes in the formation of the hypoxia.� 2006 Elsevier Ltd. All rights reserved.

Keywords: circulation; hypoxia; mixing; oxygen consumption; stratification; water exchange; Japan; Seto Inland Sea; Hiuchi-nada

1. Introduction

Oxygen depletion exerts a serious impact on marine eco-systems. For example, oxygen deficiency lower than4 mg l�1 exerts a baneful influence upon cultured fish (Inoue,1998) and the number of benthic animals seriously decreasesin the dissolved oxygen (DO) concentration under 3 mg l�1

(Suzuki, 1998). The water pool where DO concentrations areseriously reduced can have harmful effects on marine animalsand is called hypoxia or hypoxic water. It has been reportedthat in many eutrophic estuaries and coastal areas, hypoxia of-ten occurs in the lower layer in the stratified season and causeswater quality problem (e.g., Borsuk et al., 2001; Hagy et al.,

* Corresponding author.

E-mail address: kasai@kais.kyoto-u.ac.jp (A. Kasai).1 Present address: Okinawa Branch, Metocean Environment Inc., 2-6-19

Aja, Naha, Okinawa 900-0003, Japan.

0272-7714/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2006.08.001

2004; Gilbert et al., 2005). It is therefore crucially importantto clarify the formation mechanism of hypoxia and to developa scheme to reduce it.

Hiuchi-nada is situated in the central part of the Seto InlandSea, Japan (Fig. 1). It is well known that in the eastern part ofthe Hiuchi-nada the oxygen is depleted every summer in the bot-tom layer. The serious hypoxia has often damaged fisheries since1960s in this area (Ochi and Takeoka, 1986). The historical viewon the formation mechanism of the hypoxia is described as fol-lows. Several pulp factories were constructed in 1960se1970sin Kawanoe and Iyomishima, which are on the southern coastof Hiuchi-nada (Fig. 1). The pulp factories loaded a large amountof organic and inorganic matter into the southern area of Hiuchi-nada. In the eastern part of Hiuchi-nada, there is a cyclonic circu-lation (Sixth Regional Coast Guard Headquarters, 1973), whichtransports the allochthonous and autochthonous organic matterto the northeast. En route this organic matter sinks and thereforeaccumulates at the bottom. Oxygen is thus consumed during thedecomposition of this organic matter at the bottom (Ochi, 1992).

211A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

Fig. 1. Bathymetry of the study area. Hydrographic observations were carried out at the solid circles. Velocities were measured along the solid line with an ADCP.

Water samples for the oxygen consumption rate experiments were taken from Hiuchi-nada (Stations H7, H12 and H15), Harima-nada (Station H) and Bisan-seto

(Station B). BS and SP denote Bisan-seto and the Shonai Peninsula, respectively.

For this reason, various measures have been taken to reducethe organic matter load from the factories since the early 1970s.These antipollution measures had an effect on water quality suchthat chemical oxygen demand (COD) in the region rapidly de-creased in the late 1970s (Ukita, 1998). However, the DO condi-tion has unsatisfactorily remained. Fig. 2 shows year-to-yearvariation in the minimum DO concentration in the eastern partof Hiuchi-nada. Unlike the other chemical components, DOconcentration in the bottom layer has stayed at a low level. Ithas increased gradually in recent years, but is still lower than4 mg l�1. This situation indicates that another mechanism couldaffect the oxygen depletion in Hiuchi-nada.

In general, both physical and biochemical processes controlthe generation of hypoxia. When the oxygen consumption (OC)exceeds the supply (OS), the oxygen concentration reduces andthe water becomes hypoxia. OS is mainly determined by thephysical processes such as water exchange between the watermass and the surrounding water. OS by biological processescan be neglected in the lower layer, because poor light conditionoften limits primary production under the thermocline. On theother hand, OC is mainly determined by the biochemical

0

1

2

3

4

5

1975 1985 1995 2005Year

DO

con

cent

ratio

n (m

g/L)

Fig. 2. Time sequence of minimum DO concentration in the east part of

Hiuchi-nada.

processes concerning bacterial activity. No clear picture of therelative contribution of OS and OC has been given on the hyp-oxia in Hiuchi-nada. Therefore, in this study, we first measuredoxygen consumption rates, which are related to OC, at the bot-tom in the eastern part of Hiuchi-nada. Comparing the resultswith those in the other regions, where hypoxia has never beenobserved, allows the importance of biochemical processes tobe evaluated. Direct observation of current structure is alsosparse in Hiuchi-nada (Guo et al., 2004). Therefore, to estimatethe effect of OS by the physical processes on the hypoxia in theregion, extensive CTD and ADCP observations were also con-ducted. Water exchange by horizontal advection in the bottomlayer was evaluated based on the observational results. Combin-ing the results of oxygen consumption experiments and field ob-servations, it will be discussed whether biochemical or physicalprocesses contribute more to the formation of the hypoxia inHiuchi-nada.

2. Materials and methods

2.1. Description of the study area

The study area, Hiuchi-nada, is located in the center of theSeto Inland Sea, Japan (Fig. 1). Hiuchi-nada is w50 km long(eastewest) and w30 km wide (northesouth), and has a meandepth of w18 m. In the western part of Hiuchi-nada the geog-raphy is very varied and includes many small islands, while inthe eastern part the coastline is smooth. The bottom topogra-phy in the eastern part is relatively flat, although there is a gen-tle depression (w25 m) running along the coastline. Typicaltidal currents are less than 0.1 m s�1 in the eastern part, butthe complicated topography leads to stronger tidal currentsover 1 m s�1 in the western part of the Hiuchi-nada and onthe northern side of the Shonai Peninsula (Bisan-seto). Thestrong tidal currents result in relatively mixed water, whereas

212 A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

the weaker current waters in the east strongly stratified in sum-mer. Hypoxia has often been observed in the eastern lowerlayer, but never in the west. No large rivers empty intoHiuchi-nada, so that temperature predominantly controls den-sity. The fragmentary data set by observations is suggestive ofcyclonic and anti-cyclonic circulation in the surface easternand western part of Hiuchi-nada, respectively (Sixth RegionalCoast Guard Headquarters, 1973).

2.2. Oxygen consumption rate experiments

Sediment cores were taken from the surface to a depth of20 cm using acrylic core tubes (6.7 cm internal diame-ter� 30 cm long) from three stations in Hiuchi-nada (Fig. 1c).After taking the cores to the laboratory, the water in the coreswas replaced with the filtered oxygen-saturated water, takingcare not to disturb the surface of the sediment. The top of eachcore was completely capped to exclude air bubbles, and theywere incubated in a 20 �C water bath in a dark room. The con-centration of dissolved oxygen in the water was measuredwith an oxygen electrode (YSI-58, Yellow Spring Inst.) insertedin each core tube. The water in the cores was gently stirred by thestirrer equipped with the electrode. DO data were continuouslymonitored for 14e24 h. As the oxygen-saturated water was usedin the experiment, the DO consumption rate R(t) was large at firstand converged to a certain level gradually within 10 h. There-fore, R(t) was estimated by the following equation with the con-centration of DO after 10 h.

RðtÞ ¼ dC

dt

V

S; ð1Þ

where C is the concentration of DO, V is volume of the waterand S is internal area of the core tube.

To compare the results from Hiuchi-nada with the other ox-ygen rich areas, the same experiments were conducted usingthe waters from Harima-nada and Bisan-seto (Fig. 1b). De-tailed information on the sampling is listed in Table 1.

2.3. Field survey

Field surveys consist of wide range hydrographic observa-tions in 2002 and line current observations in 2003. The

hydrographic observations were carried out in the easternpart of Hiuchi-nada on 21 June, 19 July, 16 August, and 19September 2002. Observational stations are shown inFig. 1c. Vertical profiles of temperature and conductivity wereobtained using a CTD profiler (CLOROTEC ACL1180-DK,Alec Electronics). DO was measured using an oxygen meter(YSI-58, Yellow Spring Inst.). DO data were collected at 5 mdepth intervals, and at every 1 m depth intervals around oxy-clines (usually near the bottom).

Currents were measured by a shuttling operation with anacoustic Doppler Current Profiler (ADCP, RD Instruments,600 kHz) on 3 August 2003. An ADCP-equipped vesselsteamed back and forth along the eastewest transect(Fig. 1c) for about 12 h measuring current velocities at 1 mdepth intervals starting at 1.7 m below the surface. Velocitieswere measured for 120 s at nine stations separated by2.8 km on the observational line. Each station was occupiedseven times in total. The raw data were analyzed by a harmonicmethod to obtain tidal residual currents (Simpson, 1990). Onlythe M2 component was calculated in the analysis because di-urnal inequality was low. In order to separate M2 componentfrom the observed velocity, data over 12.5 h are required. Ourdata are unfortunately limited within 12 h. Therefore, a virtualvelocity, which was estimated from sinusoidal function, wasextrapolated at 13 h after the beginning of the first observationat each station to complete the data set. Changing the extrap-olated data within 10%, M2 component was estimated severaltimes, but the obtained results are slightly different. The ana-lyzed amplitudes of the M2 component were 0.07e0.28 m s�1.

Temperature and conductivity were measured once at eachstation using the CTD profiler. The same CTD observation wasconducted at a point on the northern side of the Shonai Penin-sula, as a representative of the mixed region.

3. Results and discussion

3.1. Oxygen consumption rate experiments

Measured DO concentration decreased monotonically inthe three samples from Hiuchi-nada. The results are listed inTable 1 with those from the other sampling stations. The ratesin Hiuchi-nada were from 0.032 to 0.040 g m�2 h�1. Ochi and

Table 1

Details of the oxygen consumption experiments. DO concentration does not indicate the concentration in the laboratory, but indicates the in situ concentration

Station Latitude (north) Longitude (east) Depth

(m)

Date DO consumption

rate (g m�2 h�1)

DO concentration

(mg l�1)

Hiuchi-H7 34�11.60 0 133�33.60 0 26 22 Aug. 2002 0.036� 0.013 0.85

Hiuchi-H12 34�07.23 0 133�36.75 0 20 22 Aug. 2002 0.032� 0.007 3.50

Hiuchi-H15 34�04.00 0 133�34.50 0 23 22 Aug. 2002 0.040� 0.012 1.72

Harima-A 34�15.05 0 134�26.16 0 25 7 Oct. 2002 0.034� 0.012 6.34

Harima-B 34�13.94 0 134�25.46 0 20 7 Oct. 2002 0.024� 0.014 6.25

Harima-C 34�15.05 0 134�26.16 0 25 7 Oct. 2002 0.023� 0.010 6.30

Bisan-A 34�23.30 0 134�07.20 0 5 6 Sep. 1999 0.061� 0.011 5.39

Bisan-B 34�22.95 0 134�06.95 0 5 6 Sep. 1999 0.070� 0.048 5.59

Bisan-C 34�23.20 0 134�06.60 0 5 6 Sep. 1999 0.118� 0.028 5.36

213A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

Takeoka (1986) and Hoshika et al. (1989) measured in situDO consumption rates using a bell-jar type chamber in theeastern part of Hiuchi-nada. Their results were 0.02 and0.03 g m�2 h�1 on average, respectively, which are slightlylower but still comparable to ours.

On the other hand, the Bisan-seto samples showed the con-siderably high consumption rates (>0.05 g m�2 h�1), althoughthe in situ DO concentration is high (Table 1). Unfinishedfeeds from adjoining fish farms accumulate at the bottom inBisan-seto, where aquaculture is well developed. This largeamount of organic matter would result in the highest DO

0

2

4

6

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

DO Consumption rate (g/m2/h)

DO

Con

cent

ratio

n (m

g/L)

Hiuchi-nada

Harima-nada

Bisan-seto

Fig. 3. Relation between the DO consumption rates and in situ DO concentra-

tions at the bottom.

consumption. The rates measured in Harima-nada were from0.023 to 0.034 g m�2 h�1, which are comparable to those inHiuchi-nada. In both Bisan-seto and Harima-nada, hypoxiahas never been observed.

The DO consumption rates should be correlative to the insitu DO concentration, if the strength of DO consumption con-trols generation of hypoxia at the bottom. However, the rela-tionship demonstrates that there is no significant correlationbetween them (Fig. 3, r2< 0.02). This indicates that themain reason for the generation of hypoxia in Hiuchi-nada isnot the high DO consumption rate (not biochemical pro-cesses), but other processes.

3.2. Hydrographic observations

The strong hypoxia was detected at the bottom in the east-ern part of Hiuchi-nada in 2002. Fig. 4 shows the observedhorizontal distributions of temperature and DO concentrationat the bottom. From June to August, cold (and dense) wateris distributed along the depression. Strong temperature fronts(w0.2 �C km�1) lie along the western edge of the cold water.The shape of the cold water is similar in each observation, al-though the temperature increases 4e6 �C from June to August.Sea surface cooling destroys the stratification in September,and thus the cold water disappears and temperature becamenearly uniform.

Distributions of DO concentration resemble those of tem-perature; the minimum DO area is observed from the center

Fig. 4. Horizontal distributions of temperature and DO observed at the bottom in 2002.

214 A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

Fig. 5. Vertical distributions of (a) temperature, (b) salinity, (c) density, and (d) DO along the eastewest section on 21 June 2002. The panels show views looking

northward with east being to the right. Triangles indicate observational points.

to east of the observational area and hypoxia corresponds rea-sonably well to cold water from June to August. Strong DOfronts (w0.5 mg l�1 km�1) are created along the temperaturefronts. The DO concentration decreases until August, andthen recovers in September by the mixing, in the same wayas the cold water mass.

Vertical sections along the eastewest line (Fig. 5) show thewater is strongly stratified in the center and eastern area (5e18 km from the western end of the observational line). Tem-perature and density resemble each other in structure, indicat-ing that temperature predominantly controls density. Salinitydifference in the observational section is moderate comparingwith temperature. A strong thermocline is observed at around10 m depth and the temperature difference between the surfaceand the bottom reaches 9 �C in June. However, the water atthe most western point (H5) is relatively well mixed and thetemperature difference is less than 3 �C. The bottom front(T¼ 19e20 �C) separates the mixed water from the easterncold water with a temperature 2e3 �C lower than the mixedwater. The distribution of DO deeper than 15 m depth is sim-ilar to that of temperature in the vertical section, as shown inthe horizontal distributions. DO concentration is over 6 mg l�1

in the upper layer, and lower than 5 mg l�1 in the cold dome.On the other hand, the DO concentration is relatively high(>5.5 mg l�1) in the western mixed area even at the bottom.Similar characteristics of temperature and DO are also ob-served in the northesouth section (not shown here).

A scatter diagram of observed temperature and DO (Fig. 6)shows that the oxygen is saturated and nearly uniform in the

surface layer (between 0 m and 5 m). However, under the ther-mocline (deeper than 10 m), DO concentration decreases withtemperature. There are significant correlations between temper-ature and DO, since the coefficient of determination (r2) is over0.7 in each month except for September. The colder water is con-sidered to be older water (relict spring water) in the heating sea-son (Hill et al., 1994; Kasai et al., 2002). Therefore, the olderwater contains lower DO in Hiuchi-nada, indicating that thephysical processes are important for the formation of hypoxia.

2

4

6

8

16 20 24 28Temperature (ºC)

DO

Con

cent

ratio

n (m

g/L)

5m10m

Fig. 6. Relation between temperature and DO concentration observed on 21

June 2002.

215A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

3.3. ADCP observations

Temperature, DO and residual currents observed on 3August 2003 are shown in Fig. 7. The pattern of salinity anddensity (not shown here) is similar to that of temperature.The thermocline is 4e7 m deep in the center, and deeperand gentler in the eastern and western part of the section. Adome-like cold (T< 22 �C) and saline (S> 32.3) water existedunder the thermocline around the center of the observationalsection. This dome corresponds to low-oxygen water mass(DO< 6 mg l�1). These characteristics are same as those ob-served in the hydrographic survey in 2002.

Residual currents from ADCP records show cyclonic circu-lation over the bottom cold dome, with maxima of 14 cm s�1

southward and 10 cm s�1 northward in the western and easternpart, respectively. This result is consistent with the flow pat-tern estimated by Sixth Regional Coast Guard Headquarters(1973). However, the speed is considerably small within thecold dome with a maximum <5 cm s�1. The vertical profilesof DO and speed at the center of the observational line corre-spond to each other (Fig. 8). In the surface layer, DO andspeed are high, while in the bottom water, they are signifi-cantly lower. Both properties reduce rapidly in 12e19 mdepths under the thermocline. Oxygen is depleted in the

(a) Temperature (ºC)

0

10

20

30

Dep

th (m

)

22

23

24

(c) Residual Current V-comp. (cm/s)

0

10

20

30

Dep

th (m

)

5

0-10

-55

(b) D.O. (mg/L)

0

10

20

30

8 8

6 4

10 cm/s(d) Residual Current Vector

0 10 20Distance (km)

0

10

20

30

(e) Geostrophic current (cm/s)

0 10 20Distance (km)

0

10

20

30

Dep

th (m

)

0

-55

-10

10

Fig. 7. Vertical distributions of (a) temperature, (b) DO, (c) northward components of residual currents, and (d) vectors of residual currents observed on 3 August

2003. (e) Vertical distribution of estimated geostrophic current (northward component). The panels show views looking northward, with east being to the right.

Triangles indicate observational points. Positive and negative values indicate northward and southward flow, respectively, in (c). Arrows directed upward represent

northward currents, and arrows directed to the right represent eastward currents in (d). Shaded area indicates the area of the temperature from 23.5 to 26.5 �C,

which is same as that in the northern side of the Shonai Peninsula (H1).

216 A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

stagnant bottom water, suggesting that horizontal advectionwould play an important role for generation of hypoxia. It isconsidered that little oxygen-sufficient water would be sup-plied to the bottom layer.

Since the horizontal scale of the target phenomena is about20 km, which is considerably larger than the Rossby internalradius of deformation (usually several kilometers in Hiuchi-nada), the Earth’s rotation is expected to be important forthe dynamics of water circulation. Therefore the first-order dy-namical balance ought to be geostrophic. The vertical shear inthe along front current is related to the cross-front density gra-dient through the thermal wind relation:

vv

vz¼� g

rf

vr

vx; ð2Þ

where v is the along frontal velocity (northward velocity forFig. 7), z is taken to be vertically upwards, g is gravitationalacceleration, r is density, f is Coriolis parameter and x is thecross frontal co-ordinate (eastward for Fig. 7). Using thisequation, geostrophic velocities were calculated from the ob-served density profiles relative to an assumed level of no mo-tion at the deepest common depth between adjacent stations.The result (Fig. 7e) demonstrates the existence of geostrophiccirculation in the upper layer of the stratified eastern Hiuchi-nada. Associated with the thermocline slope, northward andsouthward geostrophic velocities of 10 cm s�1 are detectedon the eastern and western side of the dome over the bottomfront, respectively. In the cold dome, however, the velocityis appreciably weaker (v w 1 cm s�1), implying insufficientwater exchange. Estimates of geostrophic currents (Fig. 7e)are consistent with the observed flow pattern (Fig. 7c), indicat-ing that the residual currents are in approximate geostrophicbalance. The estimated velocity is inconsistent with the ob-served velocity at the eastern end of the observational line.

Fig. 8. Vertical profiles of speed of residual currents and DO at a position

14 km from the west end of the ADCP line.

The geostrophic estimates are biased there, because the bottomtopography is abruptly changed. Therefore, this minor incon-sistency is unessential.

Using a diagnostic model, Guo et al. (2004) demonstratedthat the water in Bisan-seto intrudes into Hiuchi-nada and cir-culates cyclonically in the surface layer. The water in Hiuchi-nada is well mixed even in summer, because of the strong tidalcurrents. In addition, freshwater supply from rivers reduces itsdensity. In Fig. 7, the area where the density ranges from 20.3to 21.5 kg m�3, which is same as that in Bisan-seto (H1), isshaded. This area is limited to the upper layer along theADCP line, and is compatible with the cyclonic circulation.At the most western point of the eastewest section in Fig. 7(H5), the water shallower than 15 m has the same characteras that at H1, indicating that comparatively mixed water atH1 flows into the western side of the cold water. The mixedwater contains sufficient DO, so that the DO concentrationis relatively high at H5. On the other hand, the cold dome wa-ter is explicitly different in character; the temperature is lowerthan 19 �C, which is colder than the water in Bisan-Seto.These features correspond to the flow pattern as shown inFigs. 7c, d and 8. The cold bottom water is stagnant and iso-lated from the surrounding water. The prominent bottom frontand the thermocline separate the western southward flow fromthe bottom cold dome.

Baroclinic circulation in coastal areas shares many aspectsof its dynamics with tidal mixing fronts. The cold domes havebeen observed under the thermocline in a weak tidal area ad-jacent to the mixed area (LeFavre, 1986; Svendsen et al.,1991; Hill et al., 1994, 1997; Kasai et al., 2002). The forma-tion mechanism of the dome was clarified by Hill (1993) usinga simple dynamical model. In a region of weak tidal currentsthe water stratifies during the heating season, even though thesurrounding area is well mixed by strong tides. The water un-der the thermocline is warmed very slowly by the weak diffu-sion of heat. This relict winter water is, therefore, isolatedfrom surrounding mixed waters by horizontal bottom fronts,and develops into a cold dome. The water over the dome cir-culates anticlockwise above the bottom front, based on thethermal wind relation as shown in the aforementioned esti-mates (Hill et al., 1994, 1997). Isolation of the bottom waterfrom the surrounding water is essential: in addition to heat, ox-ygen is not transported into the cold dome from the surround-ings, and thus the cold dome becomes hypoxic.

4. Conclusions

In order to clarify the formation mechanism of the hypoxicwater mass in Hiuchi-nada, oxygen consumption rate experi-ments and detailed hydrographic observations were conducted.The oxygen consumption rate experiments showed the rates inHiuchi-nada are less than or comparable to other oxygen richareas, indicating that high oxygen consumption is not the mainreason for the formation of hypoxia. On the other hand, the de-tailed hydrographic observations demonstrated that an isolateddome is created under the thermocline in the central and east-ern part of the observational area. The water in the dome is

217A. Kasai et al. / Estuarine, Coastal and Shelf Science 71 (2007) 210e217

significantly colder than the surrounding water and the currentspeed is weak, indicating it is the relict spring water and stag-nant. Therefore, the main reason for the formation of the hyp-oxia in the eastern part of Hiuchi-nada is not the high oxygenconsumption, but rather the insufficient water exchange in thebottom water.

Acknowledgments

The authors would like to express their thanks to the cap-tain and crew of R/V Yakuri of Kagawa Prefectural FisheriesStation for their helpful assistance in the field observation, andto Dr. Jonathan Malarkey of University of Wales, Bangor forfruitful discussion. This study was supported in part by a Grantin Aid for Scientific Research from the Ministry of Education,Science, Sports and Culture of Japan.

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