Journal of Oceanography, Vol. 56, pp. 319 to 329, 2000
Estimation of Oxygen Consumption Rate Using T-DO Diagram in the Benthic Layer of Ohmura Bay, Kyushu, Japan MASAKO N OGAMI1*, TAKESHI MATSUNO 2, T AKEHIRO N AKAMURA3 and TADASHI FUKUMOTO4 1
Graduate School of Marine Science and Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki, Nagasaki 852-8521, Japan 2 Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 3 Faculty of Environmental Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki, Nagasaki 852-8521, Japan 4 Nishimatsu Construction Co., Ltd., 2570-4 Shimotsuruma, Yamato, Kanagawa 242-0001, Japan (Received 15 April 1999; in revised form 12 October 1999; accepted 18 October 1999)
We have observed the temporal variation of oxygen deficient water with short time scale (less than a few days) in the central area of Ohmura Bay, Kyushu, Japan, in summer, 1995 and 1996. The vertical profiles of temperature were similar to those of dissolved oxygen. We noticed a linear relation between temperature and dissolved oxygen in the bottom layer, and applied the T-DO relation to estimate the net oxygen consumption rate, rather than direct evaluation of the advection and diffusion. Oxygen consumption rate just above the bottom was estimated to be about 0.21 g O2 m –3day–1 in July 1995, and about 0.28 g O 2 m–3day–1 in August 1996. The net oxygen consumption rate estimated for the bottom layer below the second thermocline was about 0.61 g O2 m–2day–1 with variability from 0.55 to 0.66 g O2 m –2day–1 during July 25 to 29, 1995. This is was about 0.64 g O2 m –2day–1 with variability from 0.18 to 1.4 g O 2 m–2day–1 during August 22 to 30, 1996. The net oxygen consumption rates are about half of those measured with a closed system in the Seto Inland Sea.
1. Introduction Oxygen deficiency in aquatic environment has a serious influence on living organisms in the area. A number of studies on oxygen-deficient or anoxic water have been carried out and reported for various bays and estuaries. Among those studies, some report calculations of the rate of change of dissolved oxygen change rate with observation data, and others give the measurements of oxygen consumption rate using some experimental systems in situ and/or in a laboratory. They refer to various time scales and use various ways. For example, some in situ experiments to determine the oxygen consumption rate have adopted the light-dark bottle method (e.g. Ochi and Takeoka, 1986) and some experiments have used the benthic chamber method (e.g. Edberg and Hofsten, 1973; James, 1974; Boynton and Kemp, 1985). In the other cases, laboratory experiments using bottom sediment core
Keywords: ⋅ Ohmura Bay, ⋅ net oxygen consumption rate, ⋅ T-DO diagram.
have been carried out (e.g. Edberg and Hofsten, 1973; Bowman and Delhino, 1980; Seiki et al., 1994; Kamizono et al., 1996). These experiments evaluate the oxygen consumption rate in a closed system and estimate it in the absence of any ambient current. James (1974) compared two in situ methods with a laboratory method or oxygen budget in the bottom water, and reported that oxygen consumption rate increases depending on current velocity. Hosoi et al. (1992) also reported that the oxygen consumption rate would be influenced by current just above the surface of the sediment. Hence, the oxygen consumption rate estimated with a closed system would give a different value from that in situ. Several investigators have tried to examine the behavior of oxygen-deficient water through field studies. For example, Codispoti et al. (1991) measured the long term variation of a vertical profile of nutrients, dissolved oxygen and H 2S in the Black Sea, and reported that the oxic/anoxic interface rose during the years of their study. In general, oxygen deficient water formed in the bottom layer of coastal waters shows a distinct seasonal variation. Recently some investigations have noticed short
* Corresponding author. E-mail:
[email protected] Copyright © The Oceanographic Society of Japan.
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time scale variations. Takeoka et al. (1986) calculated vertical diffusivities and oxygen consumption rate from observation data using the heat and oxygen budget model. They pointed out that the time scale of anoxia is about a week in the case where serious anoxia occurs in HiuchiNada, the Seto Inland Sea. Isobe et al. (1993) applied a two-layer box model for the analysis of oxygen decrease using field data obtained thirteen times with a one week interval in the Suo-Nada, western end of the Seto Inland Sea. Their results showed that oxygen deficiency would progress rapidly with a time scale of less than one week. Nomura et al. (1996) measured dissolved oxygen in the benthic layer continuously in Shizukawa Bay, northern part of Japan, and captured the migration of oxygen-deficient water with a time scale of several days. Our study area, Ohmura Bay, is a mostly enclosed inner bay (Fig. 1), connected with Sasebo Bay by two narrow channels. Hence the tidal range in the bay is sig-
nificantly small, that is, mean tidal range at spring tides is 0.74 m (Iizuka and Bin, 1989). Oxygen-deficient water is formed in the bottom layer of this bay in summer every year, though the scale of oxygen deficient water depends on the observed years. The seasonal variations of dissolved oxygen distributions are described in the literature (e.g. Todoroki, 1977, 1978, Iizuka and Bin, 1989). Todoroki (1977, 1978) showed a considerable decrease of dissolved oxygen in the bottom layer in Ohmura Bay in summer. Iizuka and Bin (1989) calculated the oxygendecrease rate with data about twice a month, and proposed an oxygen consumption rate in summer. The value is an average value through the summer season. The detailed processes governing the formation of an oxygen-deficient water mass are not yet well known. In this paper we focus our effort on clarifying the oxygen consumption rate with a short time scale of less than a few days. Temperature and dissolved oxygen
Fig. 1. Observation stations in Ohmura Bay. The stations F05N, F05E, F05S and F05W are located at 900 m north, east, south and west of F05, respectively. Distance between the neighboring points is about 3 km for the stations F04, F45, F05 and F09. 320
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change remarkably in the vertical rather than the horizontal. If they are homogenous in the horizontal, we need to consider only the vertical diffusion as a physical process. However, although flow, particularly just above the sea floor, is weak in the central area of the bay, temperature and dissolved oxygen in the bottom water have a horizontal gradient. Therefore, it would be necessary to consider the effect of horizontal advection (Tamakawa, 1980; Fukumoto, 1997). Instead of direct evaluation of the advection as well as diffusion, we used a relation between temperature (T) and dissolved oxygen (DO), T-DO diagram, to estimate the oxygen consumption rate. Both heat and oxygen are supplied to seawater through the sea surface. In addition there are significant sources and sinks for dissolved oxygen below the surface through biological production and consumption processes around the sediment, which is quite different from temperature, which would act mostly as a conservative material except through the sea surface. We took notice of the difference between temperature and dissolved oxygen, viz., there could be a sink in the bottom for dissolved oxygen, but not for temperature. Therefore, it is possible to estimate the decreasing rate of dissolved oxygen for isothermal water by means of a T-DO diagram. The T-DO diagram could not be applied to long term variation during which independent surface conditions of temperature and dissolved oxygen could affect the variations in the bottom layer. However, for the short time scale variations without any predominant changes, such as overturning by strong wind, it is possible to estimate the oxygen consumption rate independent of physical processes. Using the procedure, not only vertical diffusion but also horizontal advection could be excluded from the change of dissolved oxygen observed under a certain condition, where water masses have a same linear T-DO diagram over a given area.
2. Observations Intensive observations were carried out in summer 1995 and 1996, in the central area of Ohmura Bay (Fig. 1), where anoxic water is usually formed (Todoroki, 1977). In 1995, vertical profiles of temperature, salinity, and dissolved oxygen were obtained from July 25 to August 4 with a two day interval using a multi factor profiler, ACL (Alec Co.) at five stations (Fig. 1). According to Iizuka and Bin (1989), it is expected that oxygen-deficient water would be developing in the bottom layer in this season. The stations F05N, F05E, F05S and F05W are located at 900 m north, east, south and west of F05, respectively. The values of dissolved oxygen obtained with ACL were calibrated by the Winkler method based on the guide for marine observations (Japan Meteorological Agency, 1985). In 1996, vertical profiles of temperature, salinity and dissolved oxygen were obtained at four observation points (F04, F45, F05 and F09; Fig. 1), with a distance of about 3 km between the neighboring stations on August 22, 24, 26, 28 and 30, with the same instrument as in 1995. 3. Results 3.1 Vertical profiles of temperature, salinity and dissolved oxygen in summer 1995 No significant horizontal variability was detected over the observation period, probably because of the short distance between each station. Vertical profiles of temperature, salinity and dissolved oxygen obtained at F05 during the observation period are shown in Fig. 2. The range of variation in temperature at the upper and bottom were 26.3–29.8°C and 22.7–24.2°C, respectively, and their difference was 3.6–6.5°C. The thermocline can be seen around 5 m and beneath 17m depth. The lower one is called the second thermocline. Ochi and Takeoka (1986)
Fig. 2. Vertical profiles of water temperature, salinity and dissolved oxygen at Stn. F05 on July 25 to August 4 1995. Oxygen Consumption Rate Using T-DO Diagram
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Fig. 3. Vertical profiles of water temperature, salinity, dissolved oxygen at Stn. F05 on August 22 to 30, 1996.
from July 25 to 29. On July 31 temperature and salinity in the intermediate layer increased significantly. Dissolved oxygen in the surface layer was about 8 to 9 g O2 m–3, that is about 110 to 140% in saturation percentage, and in the bottom layer it was about 2 to 3 g O2 m–3 and about 20 to 50%, respectively. A large vertical gradient can be seen in dissolved oxygen deeper than 15 m. The vertical profiles of dissolved oxygen appear to be similar to those of temperature. The tendency of coincidence between low temperature and low dissolved oxygen concentration can be seen even in the horizontal distribution, and similar phenomena can be also seen in the data obtained by Todoroki (1978).
Fig. 4. Vertical profiles of water temperature and salinity observed at Stns. F04, F45, F05 and F09 on August 28, 1996.
pointed out that the second thermocline play an important role in the formation of anoxic water in the study of Hiuchi-Nada. The profiles of temperature and salinity from July 31 to August 4 are quite different from those 322
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3.2 Vertical profiles of temperature, salinity and dissolved oxygen in summer 1996 Vertical profiles of temperature, salinity and dissolved oxygen obtained at Stn. F05 in summer 1996, are shown in Fig. 3. The range of variation in temperature at the upper layer and the bottom were 28.2–28.7°C and 25.8–26.7°C, respectively. Generally, temperature and dissolved oxygen decrease near the bottom. It should be noted that the thickness of the bottom layer with low temperature and low dissolved oxygen was quite large on August 28. It is suggested that vertical mixing would be enhanced in the bottom layer. This feature was observed just on August 28, and the vertical profiles were completely recovered on August 30. During the observation period, temperature difference between the surface and bottom was about 1.6– 2.7°C, and the second thermocline was located around 2 m above the bottom except for August 22 and 28. There was no significant second thermocline on August 22 and the thickness of the bottom mixed layer was larger on August 28, as mentioned above. The abrupt increase in the thickness of the bottom mixed layer was clearly observed at F04, F45 and F05, too, while it was not clear at
Fig. 5. T-S and T-DO diagrams for the observation from July 25 to August 4 in 1995. These plots include data observed at Stns. F05N, F05E, F05S and F05W.
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F09 in the northern part of the domain (Fig. 4). Salinity over the observed area was between 32.0 and 32.5 and a low salinity layer was seen just above the bottom at F45, F05 and F09. Dissolved oxygen in the surface layer at Stn. F45 was about 6 to 7 g O2 m–3, that is about 92 to 102% in saturation percentage, and in the bottom layer it
was about 1 to 2 g O2 m–3 and about 11 to 30%, respectively. The saturation percentage at the bottom layer had a large variation both in time and in space from 5 to 70% over the observation area. The vertical profiles of dissolved oxygen appear to be similar to those of temperature, as in 1995.
Fig. 6. T-S and T-DO diagrams for the observation from August 22 to 30 in 1996. These plots include data observed at Stns. F04, F45, F05 and F09. 324
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4.
T-DO Diagram and Estimation of Oxygen Consumption Rate Just above the Bottom As mentioned in the previous section, vertical profiles of temperature and the dissolved oxygen have a correlation. To study this in detail, T-DO relations as well as T-S diagrams for the observations in 1995 and 1996 are shown Figs. 5 and 6, respectively. The diagrams at all stations are superimposed on the same panel and represent the fact that the T-S relations are almost the same over all stations for each day. The T-DO diagrams are the same. In 1995, the T-DO diagrams for July 25, 27 and 29 have similar shapes (Fig. 5). The T-DO diagrams show a linear relation in the low temperature range (less than about 25.0°C), which corresponds to the temperature at the uppermost level of the second thermocline. The inclination changed gradually, becoming steeper day by day, namely, the ratios of DO/T were 1.71, 1.97 and 2.21 on July 25, 27 and 29, respectively. The shapes of T-S and T-DO diagrams change significantly on July 31, compared with those on July 25, 27 and 29 (Fig. 5). The T-DO diagrams for all observations in 1996 have a linear relation in the temperature range lower than 27.0°C, and the in-
clination of the T-DO relation gradually increases as in 1995 (Fig. 6). The T-S and T-DO diagrams magnified for the temperature range less than 25.0°C in 1995 and less than 27.0°C in 1996 are shown in Fig. 7. Whereas the shapes of T-S diagram for all stations on July 25, 27 and 29 in 1995 are almost the same, that on July 31 is different. This means that a water mass having a different origin was observed at the stations on July 31. On the other hand, the shapes of the T-DO diagram change significantly during July 25–29 in 1995. Similarly to the period in 1995, the T-S diagrams in the low temperature range have the same shape through the observation period in 1996, while the inclination of the T-DO relation gradually increases, as mentioned above. This means that dissolved oxygen was decreasing in the water mass. The dissolved oxygen budget in the benthic layer is composed of advection, diffusion, and biological production and consumption. It is assumed that the eddy-diffusion acts equally both in terms of heat and dissolved oxygen. Since the T-DO diagram in the lower temperature represents a linear relation, vertical diffusion for a certain patch of water can just move the point of T-DO along
Fig. 7. Magnified T-S and T-DO diagrams for the observations in 1995 and 1996. These plots include data observed at all stations in each observation period. Oxygen Consumption Rate Using T-DO Diagram
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the line of the pre-existing T-DO diagram. That is, the inclination of the T-DO diagram does not change. Even if horizontal advection occurred between water masses with the same T-DO relation, the T-DO value should be plotted on the same T-DO diagram. Therefore, the change of T-DO inclination should come from another effect rather than diffusion and advection. Under the situation with a linear T-DO relation, change of dissolved oxygen caused by diffusion and advection must be associated with change of temperature along the linear T-DO diagram. In other words, the decrease of dissolved oxygen for the same temperature implies net oxygen consumption in the water mass. We estimated the net oxygen consumption rates from
Fig. 8. Temporal variations of inclination of T-DO diagram in the lower layer. Marks (except for triangle) denote the lowest temperature at all stations on each day. Linear lines are best fit ones for T-DO diagrams below the second thermocline. Vertical diffusion and horizontal advection within the same water mass can move the point of T-DO along the specified linear line of T-DO diagram for each day. Namely, the decrease in dissolved oxygen for the same temperature would give net oxygen consumption rate in the water mass. Triangle indicates plot of the T-DO line on July 25, corresponding to the lowest temperature on July 27 (see text for details). 326
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the decreasing rate of dissolved oxygen for specified temperatures in the T-DO diagram as follows. In 1995, the lowest temperature observed on July 25, was 22.6°C (square) in Fig. 8, which was detected just above the bottom. Two days later, on July 27, the lowest temperature changed to 22.5°C (diamond). This temperature change involves effects of advection and diffusion. The change of dissolved oxygen by advection and diffusion is estimated from the difference of temperature between two days, that is, the value of dissolved oxygen corresponding to 22.5°C on the linear line of the T-DO diagram for July 25 (open triangle) is assumed to result from advection and diffusion. The change of dissolved oxygen by advection and diffusion, corresponding to the temperature change from 22.6°C to 22.5°C, is 0.17 g O 2 m–3 for the data on July 25. Dissolved oxygen at the lowest temperature on July 27 was less than that on July 25 by 0.72 g O 2 m–3. Therefore, the difference of dissolved oxygen, excluding advection and diffusion effects, is about 0.55 g O 2 m –3 for two days, and net oxygen consumption rate is 0.28 g O 2 m –3day–1. That from July 27 to 29 is 0.14 g O2 m–3day–1, which is estimated in the same way. The average value is 0.21 g O 2 m–3day–1 in July, 1995. The lowest temperature observed on August 22, 1996 was 25.8°C (square) and 25.9°C on August 24 (diamond). The decrease in dissolved oxygen by advection and diffusion is estimated –0.4 g O 2 m–3. Dissolved oxygen at the lowest temperature on August 24 was less than that on August 22 by 0.25 g O2 m –3. Therefore, the difference of dissolved oxygen excluding advection and diffusion effects is about 0.65 g O2 m–3 for two days, and net oxygen consumption rate is 0.33 g O 2 m –3day–1. The lowest temperature on August 26 was 25.5°C, then the DO calculated from T-DO line is negative. So, DO is reset to 0 g O 2 m –3, and oxygen consumption rate is also 0 g O 2 m –3day–1 at this time. That from August 26 to 28 is 0.31 g O2 m–3day–1, and August 28 to 30 is 0.21 g O 2 m–3day–1, which is estimated in the same way. The average value is 0.28 g O 2 m –3day–1 in August 1996, excluding the rate from August 24 to 26. Comparing the results for July 1995 and August 1996, the net oxygen consumption rate has mostly the same value at the water mass within the benthic layer, though there is a difference of bottom temperature by 3.0°C. Iizuka and Bin (1989) estimated the net oxygen consumption rate in the bottom layer at a fixed point of Ohmura Bay from observed data. Their value, 0.03–0.04 g O2 m–3day–1, is one order smaller than our value. The discrepancy would come from the time scale, that is, the time scale of their data, 2 months, was quite a lot longer than that of our data, a few days. In other words, they estimated the net oxygen consumption rate with low-pass variations and omitted the variability of the short time scale.
5.
Oxygen Consumption Rate in the Benthic Layer below the Second Thermocline The net oxygen consumption rate was estimated for every 1 m below the second thermocline with the method
explained in the previous section, and integrated over the layer below the thermocline to evaluate the net oxygen consumption rate in the benthic layer. It could then be compared with the rates presented in some of the previ-
Fig. 9. Vertical profiles of change in observed and calculated dissolved oxygen at Stn. F05 during two days in July, 1995. The opened circle marks denote the change of dissolved oxygen calculated with the effect of the advection and diffusion, and the closed circle denote that of observed data. Square marks in the right column of each panel denote difference between the observed and calculated profiles, indicating biological change of dissolved oxygen.
Fig. 10. Same as Fig. 9 except for the data in August, 1996. Oxygen Consumption Rate Using T-DO Diagram
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ous studies, where oxygen consumption rates integrated over the benthic layer were given. The second thermocline was located at 17 m, which indicates 4–5 m above the bottom, during July 25 to 29, 1995 (Fig. 2). Vertical profiles of two days of change in observed and calculated dissolved oxygen in Stn. F05 are shown in Fig. 9, along with difference between the observed and calculated ones below the second thermocline. The calculated value indicates the change of dissolved oxygen caused by advection and diffusion, that is by physical processes only. The difference between the observed and calculated ones indicates the net consumption rate by biological processes. The net oxygen consumption rate in the bottom layer was estimated integrating the difference between the observed and calculated profiles below the second thermocline. The integration resulted in about 0.61 g O2 m–2day–1 with variability from 0.55 to 0.66 g O2 m–2day–1. On the other hand, vertical profile of considerably temperature changed during the observation period in August 1996 (Fig. 3). That is, the height of the second thermocline changed from 1 to 11 m above the bottom. It is considered that horizontal advection should have a significant effect on the change of vertical profile of temperature and dissolved oxygen. That is distinct evidence that we cannot estimate oxygen consumption rate with a vertical one-dimensional analysis. However, the procedure using the T-DO diagram can estimate net oxygen consumption rate independently of horizontal advection, if the T-DO diagram over the area concerned has a similar shape with a linear relation. Figure 10 shows vertical profiles of change of dissolved oxygen in Stn. F05. Similarly to the case in Fig. 9, integration of the difference of both profiles gives net oxygen consumption rate within the benthic layer below the second thermocline. The calculated net oxygen consumption rate is 0.64 g O2 m–2 day –1 with variability from 0.18 to 1.4 g O2 m–2day–1. The large variability is caused by the considerable change of the benthic layer thickness. Kamizono et al. (1996) calculated the average dissolved oxygen budget at the bottom layer in Suo-Nada. Their data of production rate within the benthic layer below the second thermocline was 1.40 g O2 m –2day–1 and consumption rate was 2.85 g O2 m–2day–1 and net consumption rate was 1.45 g O2 m–2day–1. Ochi and Takeoka (1986) estimated the net oxygen consumption rate in Hiuchi-Nada as 1.2 g O 2 m–2day–1. These values, estimated with a closed system, are about twice the average value in our estimation, though our values have large variability. As mentioned by James (1974), the closed system might give underestimated values, where there is neither advection nor diffusion. If it is evaluated with an open system including the effect of ambient flows in the Seto Inland Sea, the consumption rate might be larger. There-
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fore, the net consumption rate in the Seto Inland Sea might be more than twice that in Ohmura Bay. Although the oxygen consumption rate in our estimation has large variability, the average value is about one half those in the reports of the Seto Inland Sea (e.g. Ochi and Takeoka, 1986; Kamizono et al., 1996). Since the tidal motions are quite large in the Seto Inland Sea, mean tidal range at spring tides is about 3 m (Takeoka, 1985), vertical diffusion should be enhanced compared with that in Ohmura Bay. Considering that the oxygen deficient water is formed even under such conditions, it is reasonable that the net oxygen consumption rate is much larger in the Seto Inland Sea than Ohmura Bay. Acknowledgements We would like to thank Dr. T. Yamamoto, Hiroshima University, and Dr. I. Kaneko, Meteorological Research Institute, for their helpful comments and advice on the first and revised manuscripts. We also thank the staff of Ohmura Bay Fisheries Cooperative Association and Mr. S. Oosaki, student of Nagasaki University, for assistance in the observations. References Bowman, G. T. and J. J. Delhino (1980): Sediment oxygen demand techniques: a review and comparison of laboratory and in situ systems. Wat. Res., 14, 491–499. Boynton, R. W. and W. M. Kemp (1985): Nutrient regeneration and oxygen consumption by sediments along an estuarine salinity gradient. Mar. Ecol. Prog. Ser., 23, 45–55. Codispoti, L. A., G. E. Frienderich, J. W. Murray and C. M. Sakamoto (1991): Chemical variability in the Black Sea: implications of continuous vertical profiles that penetrated the oxic/anoxic interface. Deep-Sea Res., 38, 691–710. Edberg, N. and B. V. Hofsten (1973): Oxygen uptake of bottom sediments studied in situ and in the laboratory. Wat. Res., 7, 1285–1294. Fukumoto, T. (1997): Study on characteristic of current and estimation of water quality in Ohmura Bay. Doctoral Thesis, Nagasaki University (in Japanese). Hosoi, Y., H. Murakami and Y. Kouzuki (1992): Oxygen consumption by bottom sediment. Journals of the Japan Society of Civil Engineers, No. 456/II-21, 83–92 (in Japanese with English abstract). Iizuka, S. and H. M. Bin (1989): Formation of anoxic bottom waters in Omura Bay. Bull. Coast. Oceanogr., 26, 75–86 (in Japanese with English abstract). Isobe, A., M. Kamizono and S. Tawara (1993): An oxygen-deficient water mass in the southwestern part of the Suo Sea. Bull. Coast. Oceanogr., 31, 109–119 (in Japanese with English abstract). James, A. (1974): The measurement of benthal respiration. Wat. Res., 8, 955–959. Japan Meteorological Agency (1985): Manuals for oceanographic observation (in Japanese). Kamizono, M., T. Eto and H. Sato (1996): Oxygen budget of the bottom layer in the southwestern part of Suo-Nada. Umi
no Kenkyu, 5, 87–95 (in Japanese with English abstract). Nomura, M., N. Chiba, K. Q. Xu and R. Sudo (1996): The formation of anoxic water mass in Shizukawa Bay. Bull. Coast. Oceanogr., 33, 203–210 (in Japanese with English abstract). Ochi, T. and H. Takeoka (1986): The anoxic water mass in Hiuchi-Nada Part 1. Distribution of the anoxic water mass. J. Oceanogr. Soc. Japan, 42, 1–11. Seiki, T., H. Izawa, E. Date and H. Sunahara (1994): Sediment oxygen demand in Hiroshima Bay. Wat. Res., 28, 385–393. Takeoka, H. (1985): The Seto Inland Sea II (B. Hiuchi-Nada), Coastal Oceanography of Japanese Islands. Coastal Oceanography Research Committee, The Oceanographic Society of Japan, 694–698 (in Japanese). Takeoka, H., T. Ochi and K. Takatani (1986): The anoxic water
mass in Hiuchi-Nada Part 2. The heat and oxygen budget model. J. Oceanogr. Soc. Japan, 42, 12–21. Tamakawa, M. (1980): Influence of wind on the distribution of low oxygen bottom water-mass in Ohmura Bay, in summer 1972. Bull. Nagasaki Pref. Inst. Fish., 6, 29–35 (in Japanese). Todoroki, S. (1977): Distribution of low dissolved oxygen water near the bottom in the Ohmura Bay, in summer from 1972 to 1976. Bull. Nagasaki Pref. Inst. Fish., 3, 95–100 (in Japanese). Todoroki, S. (1978): Correlation of dissolved oxygen and water temperature near the bottom in the Ohmura Bay, in summer from 1973 to 1977. Bull. Nagasaki Pref. Inst. Fish., 4, 1–6 (in Japanese).
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