CHARACTERISTICS OF HYPOXIC WATER MASS IN ISAHAYA BAY

Annual Journal of Hydraulic Engineering, JSCE, Vol.5, 1, February CHARACTERISTICS OF HYPOXIC WATER MASS IN ISAHAYA BAY Ami ZAIZEN 1, Hongyuan LI, Yuji SUGIHARA 3 and Nobuhiro MATSUNAGA 1 Student Member of JSCE, Graduate Student, Dept. of Earth System Science and Technology, Kyushu University (-1 Kasuga-Kouen, Kasuga, Fukuoka 1-5, Japan) Member of JSCE, Dr. of Eng., Assistant Professor, Dept. of Earth System Science and Technology, Kyushu University (-1 Kasuga-Kouen, Kasuga, Fukuoka 1-5, Japan) 3 Member of JSCE, Dr. of Eng., Associate Professor, Dept. of Earth System Science and Technology, Kyushu University (-1 Kasuga-Kouen, Kasuga, Fukuoka 1-5, Japan) Fellow of JSCE, Dr. of Eng., Professor, Dept. of Earth System Science and Technology, Kyushu University (-1 Kasuga-Kouen, Kasuga, Fukuoka 1-5, Japan) In Isahaya Bay, the secular variation of hypoxic water mass and the correlation of concentration of dissolved oxygen (DO) near the seabed were investigated using the data obtained by the Kyushu Regional Agricultural Administration Office. The factors controlling the DO concentration and the characteristics of the spatial variation were also investigated by means of multiple regression analysis and principal component analysis, respectively. The rate at which Isahaya Bay was covered with the hypoxic water mass (DO mg/l) was largest in. The hypoxic water mass occurred most frequently near the center of the bay, and the probability decreases toward the inner area and the Shimabara Peninsula. The multiple regression analysis indicated that the density stratification decreases the DO concentration near the seabed, and the Chl.a concentration increases the DO concentration. The three main modes in the spatial variation of DO in Isahaya Bay are shown using the principal component analysis. Key Words: hypoxic water mass, multiple regression analysis, principal component analysis, Isahaya Bay 1. INTRODUCTION In 1997, the inner area of Isahaya Bay was closed by the reclamation project for the purposes of farm land reclamation and disaster prevention countermeasure. As a result, the shallow water area of 35.5 km including the tidal flat of about 15km disappeared. In, the issue of the poor harvest of seaweed occurred in the whole region of Ariake Sea. The relation between the environmental degradation of Ariake Sea and the reclamation project became a social severe problem. Since the reclamation dike was constructed, the hypoxic water mass appears frequently in Isahaya Bay. There are growing concerns about the impact on ecosystem and marine resources in the bay 1). Yamaguchi and Kyozuka ) investigated the formation mechanism of hypoxic water mass in Isahaya Bay based on box model analysis and field observations in the summer of. They found that hypoxic water mass develops with the time scale of 3 or days at the neap tide. They also revealed that the horizontal advection accounted for 3% of the oxygen supply. Li et al. 3) observed simultaneously flow velocity and water quality along Konagai-Mizuho line in August, and showed that the south wind uplifted a hypoxic water mass off Mizuho. Matsunaga and Li ) investigated the wind response of hypoxic water mass in Isahaya Bay by analyzing the data of the Kyushu Regional Agricultural Administration Office. They found that the hypoxic water mass near the seabed was transported to the inner area by the southerly wind and to the entrance area by the northerly wind. Tada et al. 5) observed the surface current using a drifting buoy in order to investigate the behavior of fresh water drained from the south gate in August 3, 9. They showed that keeping the water depth about.5 m, the fresh water drained from the reservoir was coming up along the reclamation dike during flood tide. Tanaka and Odagiri ) made a numerical model to simulate the formation

processes of hypoxic water mass in the inner area of Ariake Sea. Moreover, they found that the organic matter was supplied through the primary production by red tide and induced the hypoxic water mass. Li et al. 7) investigated the formation processes of low salinity water mass in Isahaya Bay and its wind response using a 3-D flow simulation model, in which the time-varying wind stress model was incorporated on the water surface. When the southerly wind blows, the low salinity water mass near the water surface is swept out Isahaya Bay and the salinity stratification disappeared with the wind. On the other hand, the inflow of low salinity water mass to Isahaya Bay is accelerated by the northerly wind and a strong salinity stratification is formed. Many field observations have been performed for Ariake Sea and Isahaya Bay, but few studies were made on secular variation and formation factors of hypoxic water mass in Isahaya Bay. The objective of this study is to investigate statistically the characteristics of DO concentration near the seabed in Isahaya Bay by analyzing the data obtained by the Kyushu Regional Agricultural Administration Office.. OUTLINE OF OBSERVATION In Fig. 1, the sites of S1, S,, B, and B are the observation sites of water quality in Isahaya Bay, which were constructed by the Kyushu Regional Agricultural Administration Office. Water temperature, salinity, turbidity, chlorophyll a (Chl.a), DO, and ph have been measured at a vertical interval of.5 m every hour on the hour since June by using a multiple water quality measurement system. In addition, the 1-minute average values of the wind velocity are recorded at every hour on the hour. In this paper, the average of the two values observed at the positions of.1m and.5m above the seabed was defined as the value in the bottom layer. The averaging was employed to increase the reliability of the data. The water density (σ t ) was calculated from water temperature and salinity. The density difference of seawater (Δσ t ) was calculated from the difference of water densities in the bottom layer and surface layer. The value in the surface layer is the average of the densities at the position.5m below the water surface and on the water surface. Here, the hypoxic water mass is defined as the water mass of DO mg/l. 3. DATA ANALYSIS (1) Multiple regression analysis The multiple regression equation for DO in the bottom layer is given by DO (t) = a Δσ t (t) + b T(t) + c Chl.a (t) + d W NNE-SSW (t) + e Tur (t) + f Depth (t) (1) Here, Δσ t (t) is the density difference, T (t) is the water temperature in bottom layer, Chl.a (t) is the Chl.a concentration in the bottom layer, W NNE-SSW (t) is the NNE-SSW component of the wind velocity, Tur (t) is the turbidity in bottom layer and Depth (t) is the water depth. The coefficients of a, b, c, d, e and f are the multiple regression coefficients of Δσ t (t), T (t), Chl.a (t), W NNE-SSW (t), Tur (t) and Depth (t), respectively. Here, it should be noted that the north-northeasterly wind and south-southwesterly wind are predominant as local wind in Isahaya Bay during the summer season. The positive value of W NNE-SSW (t) represents the north-northeasterly wind, and the negative value represents the south-southwesterly wind. Except the water depth, the tidal fluctuations included in all the variables of Eq. (1) were removed by taking the moving average with 5 hour span. All the variables of Eq. (1) were standardized by dividing the deviation from the averaged value by the standard deviation. The data obtained from June to September between and 1 were used in the data analysis. () Principal component analysis The principal component analysis of DO was conducted using the DO concentration in the bottom layer. It was the data which were measured at the same time at sites from June to September between and 1. The number of the data set was 1,95.. RESULTS AND DISCUSSION (1) Secular variation of hypoxic water mass Fig. shows the rates at which the DO concentration in the bottom layer at the sites became equal to mg/l or less in June to September between and 1. The bars show the rates of hypoxic occurrence at each site (see the left axis for the scale). The dashed line shows the rate at which the sites were covered with hypoxic water mass at 3 57 3 N 3 5 N North gate 13 9 3 E Oura Takezaki Konagai S1 13 1 E Reclamation dike S Mizuho Kunimi South gate Fig. 1 Observation sites of water quality in Isahaya Bay. B B

DO mg/l [%] DO mg/l [%] ( sites) the same time (see the right axis for the scale). This variation represents the occurrence rate of large-scale hypoxic water mass formed throughout the whole region of Isahaya Bay. The value of 7 shows the rate at which the hypoxic water mass covered the 5 sites at the same time because the data at B site was missing. The bar graph shows that the rate of hypoxic occurrence at was the largest in and 1, and took the value of about 5%. Generally speaking, the hypoxic occurrence rate at, B and B was large through to 1. From the monthly variations (not shown), we can see the tendency that the rates of hypoxic occurrence at each site were large in July and August, and were small in June and September. The secular variation of hypoxic occurrence at each site was nearly constant between and 1, though the rates of occurrence at the sites are different each other. The dashed line shows that the occurrence rate of the large-scale hypoxic water mass was largest in and took the value of.%. In 9, the rate took the minimum value of.39%. This means that the large-scale hypoxic water mass which covered the whole region of Isahaya Bay hardly occurred in 9. The averaged value through to 1 was about.7%. () The correlation between the bottom DO concentrations at observation sites The data obtained at the same time at sites was used to investigate the correlation between two sites. The number of data set was 1,95. Figs. 3 (a) to (f) show the correlations between DO concentrations in bottom layer at two sites selected from S1, S,, B, and B. Here, R is the correlation coefficient. The occurrence probabilities of the four regions divided by two dashed lines are given in these figures. The solid lines are ones with the slope of the ratio1:1. Fig. 3 (a) shows a relation between DO (B) and DO (). The data are plotted around the solid line and a high correlation is seen. The probability that and B are covered with high oxygen water at the same time, i.e., P{DO () > mg/l, DO (B) > mg/l}, is 9.5 %. In reverse, P{DO () mg/l, DO (B) mg/l} is equal to 37.%. P{DO () mg/l} is 5.%. P{DO (B) mg/l} takes.7%. Thus, the probabilities that and B are 7 5 3 1 covered with the hypoxic water mass are approximately equal. The probability that B is covered with high oxygen water when is in high oxygen state, i.e., P{DO (B) > mg/l DO () > mg/l}, is 91.%. Fig. 3 (b) shows a relation between DO (B) and DO (). The correlation coefficient between the data is.7 and is smaller than that between and B. P{DO (B) > mg/l, DO () > mg/l} is.7%. In reverse, P{DO (B) mg/l, DO () mg/l} is 35.%. P{DO (B) mg/l} is.9% and tends to be a little smaller than those at and B. P{DO (B) > mg/l DO () > mg/l} is 9.%. Fig. 3 (c) shows the correlation between DO () and DO (). The correlation coefficient takes a large value of. but the values of DO at are shifted upward. P{DO () > mg/l} is 7.% and the probability that is covered with high oxygen water is very high. On the other hand, P{DO () mg/l} is.%, and is much smaller than those at, B and B. It is well known that a relatively strong tidal current at ebb tide occurs ) along Shimabara Peninsula. The strong vertical mixing due to the tidal current may suppress the occurrence of the hypoxic water mass. From the figure, we can read that P{DO() > mg/l DO() > mg/l} is 97.%. Fig. 3 (d) shows a relation between DO (S1) and DO (). The data scatter widely in the region of DO () mg/l and DO (S1) > mg/l, and the correlation coefficient takes very small value of.. This scattering may depend on the water depth and wind stress because the mean water depth at S1 is 3.7 m. P{DO (S1) mg/l} is 1.7%. On the other hand, P{DO (S1) > mg/l} is 7.3%. Thus, the probability that S1 is covered with the hypoxic water mass is smaller than those at, B and B. This reason may be that the mean water depth at S1 is shallow and the vertical mixing is strong. From this figure, we can read that P{DO (S1) > mg/l DO () > mg/l} is 93.%. Fig. 3 (e) shows the correlation between DO (S) and DO (). The pattern of the plotted data is very similar to that of Fig. 3 (d). The data scatter widely in the region of DO () mg/l and DO (S) > mg/l, and the correlation coefficient takes very small value of.1. P{DO (S) mg/l} is 3.%. On the other hand, P{DO (S) > mg/l} is S1 S B B sites 5 7 9 1 11 1 Fig. Rates of hypoxic occurrence at sites. 5 15 1 5

1 (b) 1 B (mg/l) B (mg/l) R=. 37.% 35.%.9% 1 R=.7 5.7% 1 1 5.9% 3.1%.5% R=. 1.5% 1 1 S (mg/l) 1 1 (e) 5.% 1 5.% 1 1 R=.1 3.% 19.% 1 1 1 1 1 (c) 1 (d) 5.% 1 7.5% 1 1 1 R=. 1.1% 3.% 1 1 1 1 1 S1 (mg/l) 1 1.% 1.7% (mg/l) 9.5% 7.% 1 1 (a) (f) 1 9.% 7.% 1 15 1 9 R=.3 1.5% 3.9% 3 9 1 15 1 1 S (mg/l) 1 S1 (mg/l) Fig. 3 Correlations between DO concentrations. 7.%. Thus, the probability that S1 is covered with the hypoxic water mass is smaller than those at, B and B. This reason is the same as that in Fig. 3 (d). From this figure, we can read that P{DO (S) > mg/l DO () > mg/l} is 93.5%. Fig. 3 (f) shows a relation between DO (S1) and DO (S). The data distribute nearly symmetrically with respect to the 1:1 line. This means that the characteristics of DO at S1 and S are very similar. P{DO (S1) > mg/l, DO (S) > mg/l} is 9. % and P{DO (S1) mg/l, DO (S) mg/l} is 1.5 %. The probability that the region near the reclamation dike is covered with hypoxic water mass is about 1/5 of the case where the region is in high oxygen state. The occurrence probability of hypoxic water mass at each site is shown in Table 1. The occurrence probability takes the maximum value of 5.% at. This means that the hypoxic water mass tends to occur at the center of the bay. The occurrence probability decreases toward the bay mouth region of B, B and and the inner area of S1 and S. The hypoxic water mass at B and B occurs at the probability of.7% and.9%, respectively. At S1, S and, hypoxic water mass occurs at the probability of order of %. Table shows the probability that the concentration of DO at each site becomes larger than mg/l when DO () > mg/l. The values are 9% in the order of magnitude. In addition, the probability that the DO concentrations at the other 5 sites are larger than mg/l at the same time is 7.7% when DO () > mg/l. The high probability means the whole region of Isahaya Bay tends to be covered with high oxygen water mass if is under the high oxygen condition. Table 3 shows the probability that each site is under the hypoxic condition when DO (S1) mg/l and DO (S) mg/l. The average of the probabilities at, B and B is %. In addition, the probability that DO () mg/l, DO (B) mg/l and DO (B) mg/l is satisfied at the same time is 7.1% under the conditions of DO (S1) mg/l and DO (S) mg/l. This means that the whole region of Isahaya Bay may be covered with a hypoxic water mass at the probability of 7.1% in the case where the values of DO at S1 and S are smaller than mg/l. (3) The multiple regression analysis Assuming Eq. (1), the multiple regression analysis was conducted at sites to examine the important factors which control the DO concentration in the bottom layer. Table shows the multiple correlation coefficients R and the regression coefficients of the factors which were obtained by this analysis. We judged that the multicollinearity could be negligible since all the coefficients of correlation between the independent variables were less than.3, and concluded that the results of the multiple regression analysis were statistically significant at 95 % level because the F-values were smaller than.5 in the F-test. Table 1 Probability at which each site was covered with hypoxic water mass. S1 1.7% S 3.% 5.% B.7%.% B.9% Table Probability at which each site was in high oxygen state when DO () > mg/l. S1 S B B >.mg/l 93.% 93.5% 91.% 97.3% 9.% Table 3 Probability at which each site was in hypoxic state when DO (S1) mg/l and DO (S) mg/l. I_11 B B S1.mg/l,S.mg/l 91.% 5.9% 5.% 79.7%

Standardized DO The values of R at sites of, B, and B are larger than. and long-term variations of DO seem to be reproduced well by the multiple regression analysis. On the other hand, the values of R at S1 and S are a little small and the reproducibility may be a little low. The multiple regression coefficients of a and b are negative at all the sites. This means that the DO concentrations decrease with the increase of the degree of stratification and the water temperature. The negative correlation between DO and Δσ t is attributed to the suppression of the supply of oxygen through the water surface due to the density stratification. The negative correlation between DO and T is due to the increase of oxygen consumption by bacteria with the increase of water temperature. The coefficients of c, d and e take positive values. This means that the value of DO increases with the increase of the values of Chl.a, north-northeasterly wind speed and turbidity. The positive correlation between DO and Chl.a is due to the primary production. The positive correlation between DO and north-northeasterly wind speed is because the north-northeasterly wind transports rich oxygen water near the water surface toward the inner area and poor oxygen water in the bottom layer toward the entrance area ). We guessed that there is a negative correlation between DO and turbidity because the resuspension of sediment accelerates the oxygen consumption in the bottom layer. However, the positive correlation was obtained at all the sites in this analysis. This is a very interesting issue which we should solve in future. The regression coefficient of the water depth was nearly equal to at all sites. This suggests that the tide level variation has little influence on the long-term variation of the bottom DO concentration. The absolute values of a and b at S1 and S are smaller than those at, B, and B. The values of c at S1 and S are larger than those at, B, and B. These reasons are due to the difference of Table Multiple correlation coefficients and regression coefficients. R a b c d e f S1. -.7 -.13..1. -. S.5 -.15 -.93.33..133.. -.9 -.5.15.195.1 -.11 B.75 -.99 -.3.17.. -.3.93 -.9 -..19.15.1 -. B. -.95 -..15.13.. the mean water depth. Fig. shows the comparison between the observed results and the results obtained by the multiple regression analysis of, B and S from June to September of. The red solid lines show the observed values of DO concentration in the bottom layer. These values are standardized by using the time-averaged values and the standard deviations. The blue dashed lines show the analytical results obtained by the multiple regression analysis. We can conclude that the long-term variation of DO agrees well between the two though a good reproduction is not performed for the small scale fluctuation seen at S. It may be that the water depth at S is shallow, the DO variation is easily influenced by wind and a time-lag occurs between the variations of wind and DO. () The principal component analysis Fig. 5 shows the three main modes of the spatial variations of the DO concentration obtained by means of the principal component analysis. The first mode occurs at probability of.3% and shows the variation that the DO concentration increases or decreases uniformly in the whole region of Isahaya Bay. It means that the variation of DO along the long axis of Isahaya Bay is in-phase state and this mode corresponds to the response of the 1:1 line in Figs. 3 (a) to (e). The second mode shows the variation of DO along the long axis to be out-of-phase and occurs at probability of 1.5%. As seen in Figs. 3 (d) and (e), the DO concentration tends to be high in the inner area of Isahaya Bay. Therefore, the second mode 3-1 1-3 -1 1-3 -1 1 - -3 B S Analytical value Observation value Analytical value Observation value Analytical value Observation value Fig. Comparison between analytical results and observed results.

1st Mode nd Mode 3rd Mode.3% 1.5%.% Fig. 5 Three main modes of spatial variation of DO in Isahaya Bay. means the variation that the DO concentration is high in the inner area and is low in the entrance area. The third mode is the out-of-phase mode along the line perpendicular to the long axis. The occurrence probability is.%. This mode corresponds to the regions of {DO (S1) > mg/l, DO (S) mg/l} or {DO (S1) mg/l, DO (S) > mg/l} in Fig. 3 (f). The mode may be due to the local wind predominant in Isahaya Bay, i.e., the NNE wind and the SSW wind. 5. CONCLUSIONS In Isahaya Bay, the secular variation of hypoxic water mass and the correlation of DO concentration near the bottom were investigated using the data by the Kyushu Regional Agricultural Administration Office. In addition, the factors which control the DO concentration near the bottom and the characteristics of the spatial variation were investigated by means of multiple regression analysis and principal component analysis, respectively. The obtained results are as follows. 1. The occurrence rate of hypoxic water mass was large at, B and B between and 1. The secular variation of hypoxic occurrence at each site was nearly constant between and 1. The occurrence rate of the large-scale hypoxic water mass was largest in. In 9, the rate took the minimum value. Roughly speaking, however, the occurrence rate was nearly constant between and 1.. The hypoxic water mass tends to occur at the center of the bay. The occurrence probability decreases toward the bay mouth region and the inner area. 3. The variations of the observation value were well reproduced by the multiple regression analysis. The multiple regression analysis indicated that the density stratification decreases the bottom DO concentration, and the Chl.a concentration near the bottom increases the bottom DO concentration.. The principal component analysis revealed the three main modes of the hypoxic water mass developing in Isahaya Bay. ACKNOWLEDGMENTS: The study was funded by Grant-in-Aid for Scientific Research (A) (PI: Prof. N. Matsunaga, Kyushu Univ., No.: 17), Grant-in-Aid for Exploratory Research (PI: Prof. N. Matsunaga, Kyushu Univ., No.: 3537) and COMPAS (PI: Associate Prof. Y. Hayami, Saga Univ.). REFERENCES 1) Azuma, M.: Effects of the Isahaya Reclamation Project, In life in Ariake Sea; Biodiversity in tidal flats and estuaries (ed. by Sato, M.), Kaiyusya Co. Ltd, pp.3-337, (in Japanese). ) Yamaguchi, S. and Kyozuka, Y.: Generation mechanism of hypoxia in Isahaya Bay, Japan, Oceanography in Japan, Vol.15 (1), pp.37-51, (in Japanese). 3) Li, H., Higuchi, S. and Matsunaga, N.: The Upwelling of Water Mass with Low Dissolved Oxygen Induced at the South Coast of Isahaya Bay by the South Wind, Journal of Japan Society of Civil Engineers, Ser.B (Coastal Engineering), Vol.5, No.1, pp.-1, 9 (in Japanese). ) Matsunaga, N. and Li, H.: Wind response of hypoxic water mass in Isahaya Bay, Doboku Gakkai Ronbunshuu B, Vol., No., pp.395-, 1 (in Japanese). 5) Tada, A., Nakamura, Y., Abe, K., Tai, A., Suzuki, S. and Nakamura, T.: Influence of Fresh Water s Inflow upon Water Quality Dynamics in Isahaya Bay, Journal of Japan Society of Civil Engineers, Ser.B (Coastal Engineering), Vol., No.1, pp.3-37, 1 (in Japanese). ) Tanaka, M. and Odagiri, M.: Mechanism of the Oxygen-Depleted Water Formation near the Head of the Ariake Bay and its Numerical Modeling, Journal of Japan Society of Civil Engineers, Ser.B (Coastal Engineering), Vol., No.1, pp.111-115, 1 (in Japanese). 7) Li, H., Baba, A., Matsunaga, N. and Chiba, S.: The Wind Response Analysis of Low Salinity Water Mass in Isahaya Bay, Journal of Japan Society of Civil Engineers, Ser.B (Coastal Engineering), Vol.7, No., pp. I3-37, 11 (in Japanese). ) Komatsu, T.: Interim report of Ariake Project (1), pp.1-1, (in Japanese). (Received September 3, 13)