Temporal Variation of the Partial Pressure of CO 2 in Surface Seawater of Iyo Nada in the Seto Inland Sea, Japan

Transcription

1 Journal of Oceanography Vol. 51, pp. 681 to Temporal Variation of the Partial Pressure of CO 2 in Surface Seawater of Iyo Nada in the Seto Inland Sea, Japan YU-ICHIRO KUMAMOTO 1 *, KITAO FUJIWARA 2 and HIROYUKI TSUBOTA 3 1 School of Biosphere Sciences, Hiroshima University, Kagamiyama, Higashi Hiroshima 739, Japan 2 Faculty of Integrated Arts and Sciences, Hiroshima University, Kagamiyama, Higashi Hiroshima 739, Japan 3 Shin-Nippon Meteorological and Oceanographical Consultant Co. Ltd., Riemon, Oigawa-cho, Shita-gun, Shizuoka , Japan (Received 22 December 1994; in revised form 16 June 1995; accepted 23 June 1995) The temporal variation of the partial pressure of carbon dioxide (P CO 2 ) in surface seawater of Iyo Nada in the western Seto Inland Sea was measured by infrared absorption of carbon dioxide (CO 2 ) in air equilibrated with seawater. The variation of P CO 2 from January to late May in 1994 was relatively small (from 270 to 340 µatm). On the contrary, the P CO 2 measured in September 1993 ranged from 200 to 450 µatm. It seems that the relative high P CO 2 observed in the north and the south of Iyo Nada in September were caused by destruction of a stratification owing to tidal currents at straits between Hiroshima Bay and Iyo Nada and Hayasui Straits, respectively. The low P CO 2 observed in the central Iyo Nada in September was ascribed to a tidal front where high concentrations of chlorophylla and a relation between P CO 2 and chlorophyll-a in negative sense were found. Except the observation in the north of Iyo Nada in September, the P CO 2 in the surface seawater of Iyo Nada were lower than the atmospheric P CO 2 (about 367 µatm). The results obtained by this work and Kimoto et al. (1993) lead to the conclusion that Iyo Nada acts as a sink of the atmospheric CO 2 on the average. 1. Introduction The oceans play a crucial role in the global carbon cycle, and in particular they presently act as a net sink for atmospheric carbon dioxide (CO 2 ). A key parameter of the oceanic CO 2 system is the partial pressure of CO 2 ( P CO2 ) in the surface ocean. The difference in P CO2 between surface seawater and overlaying atmosphere defines areas of CO 2 source or sink over the ocean. Physical, chemical, and biological factors all influence the surface ocean P CO2 and the dominant ones change temporally and regionally (Codispoti et al., 1982; Weiss et al., 1982; Tans et al., 1990; Watson et al., 1991). Thus, it is necessary to clarify the temporal and regional variations of the oceanic P CO2 in order to estimate the correct CO 2 flux between the atmosphere and the surface ocean. In coastal seas, the effects of physical, chemical, and biological factors on the oceanic P CO2 are so complicated that it is difficult to estimate the CO 2 flux between the atmosphere and the surface seawater. Only a few attempts have so far been made at the CO 2 flux in coastal seas (for example, Kempe and Pegler, 1991). Recently, significant roles of the coastal sea for the global carbon cycling have been pointed out: Walsh et al. (1981) indicated that the annual loss of organic *Present address: National Institute for Environmental Studies, Division of Chemistry and Physics, 16-2 Onogawa, Tsukuba 305, Japan.

2 682 Y. Kumamoto et al. carbon from the continental shelf ecosystem to the open sea might represent the missing sink in global CO 2 reserves. Yanagi et al. (1993) found a large flux of organic carbon from Tokyo Bay to the open ocean corresponding to g C year 1 if it was applicable to the whole coastal seas. On this point of view, P CO2 measurements and estimating CO 2 flux through the air-sea interface in the coastal seas could be significant works for the global carbon budgets as well as those in the open ocean. The Seto Inland Sea in the western Japan covers an area of 21,800 km 2 and is one of the most eutrophic seas. The amount of carbon, however, derived from the atmosphere into the seawater through the air-sea interface is still uncertain. P CO2 in the surface water of the Seto Inland Sea has been measured by some researchers. Ohtaki et al. (1993) and Yamashita et al. (1993) measured diurnal and seasonal variations of P CO2 at a site in the Seto Inland Sea. Their results, however, are not a representative of the whole Seto Inland Sea because their sampling point was closed to the shore. Kimoto et al. (1993) carried out continuous measurements of P CO2, water temperature, salinity, ph, and chlorophyll-a in the Seto Inland Sea using the Japan-Korea ferry boat run. They showed large temporal variations of P CO2 in some parts of the sea. In this study, the temporal variation of P CO2 in the surface seawater of the Sea of Iyo (Iyo Nada) in the western Seto Inland Sea is presented and the factors controlling the P CO2 in the surface seawater of Iyo Nada are discussed. 2. Experiments 2.1 Samples All the data presented here were obtained during the four cruises of the R/V Toyoshio Maru, the Faculty of Applied Biological Science, Hiroshima University. Figure 1 shows the sampling locations in Iyo Nada, Hiroshima Bay, and Bungo Channel in the western Seto Inland Sea, Japan. Four track charts in Fig. 1 indicate horizontal observation lines of the four cruises, TY93-6 (September 21, 1993), TY93-11/12 (January 8 and 20, 1994), TY94-2 (May 9 and 10, 1994), and TY94-3 (May 30, 1994). During the ship was running on the tracks (the ship s speed was about 10 knots), surface seawaters were collected by a pump at a rate of 200 L min 1 from the ship bottom (about 3 m below the sea level) into a 200 L volume tank situated on the deck. At Stations A ( N, E, September 19 21, 1993) and B ( N, E, May 11 12, 1994) in Iyo Nada, surface seawaters were collected by another pump at a rate of 40 L min 1 from a depth of about 70 cm into the tank. Salinity, temperature, and concentration of chlorophyll-a of the surface seawater in the tank were monitored by a CTD system equipped with a fluorescent sensor (SBE-9, Sea-Bird Electronics Inc. and Fluorometer, Sea Tech Inc. or ACL100-D, Alec Electronics Inc.) in parallel with measurements of P CO2. The salinity measured by the SBE-9 s sensor was calibrated against that measured using an inductively coupled salinometer (YEO-KAL Electronics, Ltd.). The concentration of chlorophyll-a monitored by the Sea Tech s fluorescent sensor was calibrated by a manual analysis of chlorophyll-a collected on a glass fiber filter (GC-50, Toyo Roshi Ltd.). The salinity, temperature, and concentration of chlorophyll-a measured by the ACL100-D s sensors were calibrated against those measured by the SBE-9 s and the Sea Tech s sensors. At Stations A and B, seawater samples at several depths from 2 m depth to about 10 m above sea bottom were collected by Go-Flo samplers (General Oceanics Inc.) at 2 hours intervals. Surface seawater was collected by a polyethylene container. The samples were filtered using glass fiber filters (GC-50, Toyo Roshi Ltd.) immediately and stored in a freezer. After the

3 Temporal Variation of P CO2 in Iyo Nada 683 Fig. 1. Sampling locations in the western Seto Inland Sea, Japan. observations, concentrations of sum of nitrate and nitrite in the samples were measured in our laboratory by a flow injection method based on reduction of nitrate to nitrite and formation of an azo compound (Grasshoff et al., 1983). Just before the water sampling, vertical profiles of water temperature, salinity, and concentration of chlorophyll-a were measured by the Sea-Bird CTD system equipped with the Sea Tech s fluorescent sensor. Tidal current velocities at about 10 m depth were measured by a current profiler (ACM 2000, Alec Electronics Inc.). Other data required for computation of P CO2 including barometric pressure, wet-bulb and dry-bulb temperatures were taken at 2 hours intervals during the observations. 2.2 Measurement system of P CO2 P CO2 in surface seawater has been measured in enclosed air equilibrated with a continuous supply of seawater (for example Takahashi, 1961; Keeling, 1968; Miyake et al., 1974; Copin- Montegut, 1985; Inoue and Sugimura, 1987; Goyet and Peltzer, 1994). Our method resembles to those already used for surface seawaters. Figure 2 shows a schematic diagram of the P CO2 measuring system. The seawater in the tank was delivered by a pump at a rate of 10 L min 1 into an equilibrator which was modified of a design described by Copin-Montegut (1985). Volumes of seawater and enclosed air in the equilibrator are both 10 liters. The seawater constantly supplied from a showerhead was sprayed into the equilibrator. A constant level of seawater was maintained by a drain open to the atmosphere, which ensured that gas pressure in the equilibrator was equal to the atmospheric one. The enclosed air was recirculated through a

4 684 Y. Kumamoto et al. Fig. 2. A schematic diagram of the P CO2 measuring system. gas disperser immersed 40 cm below the surface of the seawater (the flow rate was 5 L min 1 ). An equilibrium between the enclosed gas and the seawater was attained by the exchange of CO 2 between the sprayed water droplets and the enclosed gas and between the fine dispersed bubbles of the enclosed gas and the surrounding water. Water temperature in the equilibrator was measured using a mercury thermometer. The equilibrator was made of acrylic acid resin, and was exposed to room air without special thermal insulation. However, due to the large thermal inertia of a 10-liter seawater and a short residence time of seawater (1 minute), it has been confirmed that the measured rate of temperature increase was less than 0.1 C min 1. The enclosed air was being circulated in a loop of Teflon tube by a diaphragm pump (the flow rate was 1 L min 1 ). The loop of the analytical system included the equilibrator, two 4-way valves, a glass fiber filter (GC-50, Toyo Roshi Ltd.), a 5-way ball valve, a moisture bottle, a mechanical water trap (electric cooler Model DH-205, Komatsu Electronics Inc.) at 2 C, a flow controller, a flowmeter, and a measuring cell of a non-dispersive infrared (NDIR) analyzer (Model XURA- 207, Shimadzu Corp.). A series of solenoid valves, the 4-way valves, and the 5-way ball valve allowed time sequence sampling of 5 standards of CO 2 gas (150, 250, 350, 450, and 550 ppm CO 2 in air, Nippon Sanso Ltd.), the enclosed air equilibrated with seawater, and ambient air which was collected at 10 m above the sea surface through a Tetlon hose. It should be noted that the analytical loop was opened to the atmosphere during the measurements of the standard gases and the ambient marine air. The P CO2 in the atmosphere was measured at about 1 hour s intervals during the continuous measurements of the oceanic P CO2. Reference gas (a mixture of 350 ppm CO 2 in air, Nippon Sanso Ltd.) was introduced into a reference cell of the NDIR through a moisture bottle, the water trap, a flow controller, and a flowmeter (the flow rate was 40 ml min 1 ).

5 Temporal Variation of P CO2 in Iyo Nada 685 The moisture bottles contained 200 ml of deionized water and moistened the standard gases, the reference gas, and the sample gases. The moisture in these gases was removed by the water trap up to saturated water vapor pressure at 2 C. The water vapor in the standard gases canceled errors of infrared absorption caused by the water vapor in the sample gases. The standard gases were manufactured by a gravimetric method and guaranteed 1 ppm of precision. During the P CO2 measurements the NDIR analyzer was calibrated at about 12 hours intervals using the standards gases. The calibration curves were obtained by the fourth order of polynomial regression. Repeat measurements of the standard gas (450 ppm CO 2 in air) showed a coefficient of variation of 0.1% for 1σ (n = 5). The NDIR output was from 0 to 1000 mv for 400 ppm CO 2 span ( ppm), which meant the resolution of the P CO2 measurement was 0.4 ppm. The output of the NDIR was recorded on a paper strip chart by a recorder (Model R-103, Rikadenki Kogyo Co., Ltd.). Since the mean residence time of the surface seawater in the tank was less than 5 minutes, the signal on the strip chart was read at 5 minutes intervals. 2.3 Calculation of P CO2 The signal of the NDIR output was corrected for the following remarks proposed by Copin- Montegut (1985). (1) Instrumental deviations of the output of the NDIR. (2) Difference of pressure in the NDIR cell and in the equilibrator. (3) Variations in atmospheric pressure between the calibration and the measurements. (4) Water vapor pressure above the seawater. (5) Difference of water temperature in the equilibrator and in the sea surface. (6) Non-equilibrium CO 2 distribution between the seawater and the air in the equilibrator. The instrumental deviations of the NDIR outputs were less than 1% for 12 hours, which were corrected by a simple linear interpolation. Concentrations of CO 2 (ppm) were computed by the corrected NDIR outputs using the calibration curve. The pressure in the NDIR cell might differ from the total atmospheric pressure, owing to compression from the diaphragm pump or the high pressure cylinder. The deviations between the pressure inside the NDIR and the atmospheric pressure in the opened and the closed analytical loop ( P open and P close ) were measured at an outlet of the NDIR cell using a barometer and resulted in and atm, respectively. The P CO2 in the NDIR cell is computed using P CO2 ( NDIR) = C P atm + P open ( ) 1 where C is the concentration of CO 2 and P atm is the total atmospheric pressure at the time of the calibration. The atmospheric pressure at the measurements might be different from that at the calibration. Furthermore, the air in the equilibrator was saturated with water vapor. Thus the P CO2 in the equilibrator is computed using P CO2 ( equ) = P CO2 NDIR ( ) P atm P w ( equ) P atm + P close 2 ( ) ( ) where P atm is the total atmospheric pressure at the time of the measurement and P w (equ) is the partial pressure of water vapor in the equilibrator. On the other hand, the P CO2 in the atmosphere is given by

6 686 Y. Kumamoto et al. P CO2 ( air) = P CO2 NDIR ( ) P atm P ( w air) ( ) P atm + P open 3 where P w (air) is the partial pressure of water vapor in the atmosphere. The oceanic P CO2 has to be corrected for the difference in temperature between the equilibrator and in situ. The time that the ship was running, the surface seawater was collected from about 3 m below the sea surface and the in situ temperature was continuously monitored by a thermistor sensor on the bottom of the ship. At Stations A and B, the surface seawaters were collected from about 70 cm below the sea surface into the tank on the deck. Water temperature at 70 cm below the sea surface could be represented by temperature in the tank because of a short distance from the sea surface to the tank (about 3 m). The P CO2 in situ was computed by these differences of temperature using the formula proposed by Copin-Montegut (1988). As the formula needed more than two parameters of the carbonate system in seawater, we referred to the literature data (Yamashita et al., 1993). During the continuous measurements, the air-water equilibrium in the equilibrator could not be completed because of the short residence time of seawater (1 minute). If the exchange of CO 2 between water and air in the equilibrator is a first-order reaction, the measured P CO2 of the air in the equilibrator at t time should be P CO2 = P CO2 +( P t=0 CO2 P CO2 )exp( kt) ( 4) where P t=0 CO2 is the P CO2 equilibrated with the first seawater at time t = 0 and P CO2 is the P CO2 of the second seawater newly pumped. k (s 1 ) is the rate constant of the reaction. We estimated the equilibration ratio of CO 2 between water and air in the equilibrator by a laboratory experiment as follows: A volume (10 liters) of seawater in the equilibrator was bubbled with recycled or renewed air. The recycled air was equilibrated with the seawater while the renewed one did not reach to the equilibrium because of a short residence time (2 minutes). From the difference in P CO2 between the two gases, the averaged equilibration ratio with the standard deviation was calculated as 62.6 ± 2.2% for 2 minutes. Substituting the ratio and the residence time of air in Eq. (4) yields the rate constant (k), ± s 1. Finally, the P CO2 in the surface seawater is computed using P CO2 = P CO2 ( t, p) + ( 1/k) P CO2 ( t, p) / t ( 5) where P CO2 (t, p) is the P CO2 corrected for temperature and pressure by the corrections (1) (5) and P CO2 (t, p) is the variation of P CO2 (t, p) during the interval t. The deviation between the P CO2 and the P CO2 (t, p) which arose from this correction was less than ±3 µatm for 1σ. 3. Results and Discussion 3.1 Temporal variation of P CO2 in Iyo Nada Figure 3 shows temporal variations of P CO2, water temperature, salinity, and chlorophyll-a in the surface seawater of Iyo Nada, Hiroshima Bay, and Bungo Channel on September 21, 1993 (September), January 8 and 20, 1994 (January), May 9 and 10, 1994 (early May), and May 30,

7 Temporal Variation of P CO2 in Iyo Nada 687 Fig. 3. Temporal variations of P CO2, water temperature, salinity, and chlorophyll-a in the surface seawater of Iyo Nada, Hiroshima Bay, and Bungo Channel.

8 688 Y. Kumamoto et al (late May). In Iyo Nada, the regional variation of P CO2 in September was remarkably large (200 to 450 µatm) while those in January, early May, and late May were relatively small (270 to 340 µatm). The averaged atmospheric P CO2 with the standard deviation measured in the four cruises was ± 6.6 µatm (n = 45). Temporal and regional variations of the atmospheric P CO2 were not found. The values of the oceanic P CO2 lower than the atmospheric P CO2 in January, early May, and late May suggest that the surface seawater of Iyo Nada is a sink of the atmospheric CO 2 in these seasons. The oceanic P CO2 in late May was higher than those in January and early May in Iyo Nada, which seems to correspond with the temporal variation of the surface seawater temperature. Figure 4 shows the relation between P CO2 and water temperature in the surface seawater of Iyo Nada. Diamonds, circles, triangles, and squares indicate the data obtained during the cruises in September, January, early May, and late May, respectively. The effect of temperature on P CO2 in seawater is 4%P CO2 / C, approximately (Gordon and Jones, 1973). The oceanic P CO2, however, depends not only on water temperature but also on the carbonate parameters, total alkalinity (At), total dissolved inorganic carbon (Ct), and ph. Broken and dotted lines in Fig. 4 denote P CO2 as a function of water temperature for seawaters of At 2300 µeq. kg 1 (the ratio Ct/At, R = 0.86, 0.87, 0.88, 0.89, and 0.90; salinity 33 PSU) and 2150 µeq. kg 1 (R = 0.88 and 0.89; salinity 33 PSU), respectively. These were computed using the dissociation constants obtained from Dickson and Millero (1987) for carbonic acid, from Dickson (1990) for boric acid, and from Dickson and Riley (1979) for ionization of water. The solubility of CO 2 in seawater was given by the equation of Weiss (1974). Although the concentrations of At (or Ct) are different, the lines of the ratio R (=Ct/At) of 0.89 and 0.88 are nearly parallel, respectively, which suggests the effect of temperature on P CO2 depends on the ratio R. The P CO2 measured in January and May in Iyo Nada increased with the rising of water temperature according to the theoretical slopes of the ratio R from 0.88 to 0.89, approximately, and were extrapolated to the high P CO2 in September ( µatm). Crosses in Fig. 4 show ranges of P CO2 and temperature in surface seawater of Iyo Nada in late March (A) and early July (B) 1992 (Kimoto et al., 1993). The P CO2 in July is lower than the values which are expected by the P CO2 in January and May and water temperature in July. According to the data obtained by Kimoto et al. (1993) and this work except in September 1993, it is suggested that the temporal variation of the P CO2 in the surface seawater of Iyo Nada is smaller than that expected by the variation of water temperature. This discrepancy could be caused by incorporation of Ct by phytoplanktons and dilution of seawater by land water in summer. These results suggest that the large regional variation of P CO2 in September was owing to the phenomena which did not occur in other season. The values of salinity in the surface seawater of Iyo Nada in September were more than 1 PSU smaller than those in other seasons. The large variation of P CO2 in September, however, can not be explained by the small effect of salinity (about 3 µatm P CO2 /PSU). In September 1993, the P CO2 rapidly increased in the region from N to N where water temperature fell down and salinity rose up during the cruise from Station A to the south of Hiroshima Bay. Then in the region to the north of N, water temperature increased and salinity decreased gradually. According to a tide table (Japan Maritime Safety Agency, 1993), the measurement of P CO2 on September 21, 1993, was carried out during a period of the ebb tide. This implies that a flow of seawaters of the interior of Hiroshima Bay into the north of Iyo Nada caused the increasing of water temperature and the decreasing of salinity. If so, it seems that the relative low temperature and the high salinity in the region from N to N were due to other factors. A possible explanation is a mixing between surface and deeper waters owing to

9 Temporal Variation of P CO2 in Iyo Nada 689 Fig. 4. Relation between P CO2 and water temperature in the surface seawater of Iyo Nada. Diamonds, circles, triangles, and squares indicate the data obtained during the cruises in September, January, early May, and late May, respectively. Crosses denote ranges of P CO2 and water temperature in surface seawater of Iyo Nada in late March (A) and early July (B) in 1992 (Kimoto et al., 1993). Broken and dotted lines show P CO2 as a function of water temperature for seawaters of At 2300 µeq. kg 1 (the ratio Ct/At, R = 0.86, 0.87, 0.88, 0.89, and 0.90; salinity 33 PSU) and 2150 µeq. kg 1 (R = 0.88 and 0.89; salinity 33 PSU), respectively. tidal currents at straits between Hiroshima Bay and Iyo Nada. The results obtained by the vertical observations at Station A indicate that Iyo Nada was stratified in September and the deeper water contained relatively high concentration of sum of nitrite and nitrate (see Subsection 3.2). This implies that concentration of Ct in the deeper seawater was higher than the one in the surface seawater in Iyo Nada. These discussions should amount to that the increasing of Ct led to the high P CO2 in the surface seawater of the north of Iyo Nada, which was caused by destruction of a stratification at the straits between Hiroshima Bay and Iyo Nada. In July 1992, Kimoto et al. (1993) found more than 400 µatm of P CO2 in surface seawaters of Kanmon and Bisan Straits in the Seto Inland Sea. At these straits of occurring the high P CO2, the vertical mixing of seawater is stronger than the other parts of the Seto Inland Sea (Yanagi and Okada, 1988). The relatively high concentrations of chlorophyll-a and the low P CO2 were found in the central Iyo Nada in September and in the north of Hiroshima Bay in early May. This suggests that the primary production of phytoplanktons reduced Ct and P CO2 in the surface seawater. The relation between the concentration of chlorophyll-a and P CO2 in Iyo Nada is discussed more in the next subsection by a comparison of P CO2 measured at Stations A and B in the central Iyo Nada. 3.2 Diurnal variations of P CO2 at Stations A and B P CO2 in surface seawater was also continuously measured at Stations A (September 19 21, 1993) and B (May 11 12, 1994) in the central Iyo Nada (see Fig. 1). The observations at Stations

10 690 Y. Kumamoto et al. A and B were carried out before the horizontal observations of the cruise TY93-6 (September 21, 1993) and after the cruise TY94-2 (May 9 and 10, 1994), respectively. Figure 5 shows diurnal variations of water temperature, salinity, chlorophyll-a, and P CO2 in the surface seawater at Stations A and B. Stick diagrams of tidal currents at the two stations are also shown. The P CO2 and chlorophyll-a at Station A varied temporally while those at Station B were nearly constant. Slight increases of P CO2 and water temperature were observed at Station B from 12:00 to 15:00 on May 12, which could be owing to solar radiation. On the other hand, at Station A, the variations of water temperature and salinity could not explain the large variation of P CO2. The tidal currents observed at Stations A and B were northward in the flood and southward in the ebb. More than 50 cm s 1 of the current velocity was observed at both stations, which was due to a large exchange of seawater through the Hayasui Straits (see Fig. 1). It should be noted that the surface seawater at both stations were moved by the tidal currents. In this context, Fig. 5 suggests small regional variations of P CO2 and chlorophyll-a around Station B and large ones around Station A, which remind us the small and large regional variations of P CO2 and chlorophyll-a observed in the horizontal observations of TY94-2 and TY93-6, respectively. Figure 6 shows relations between current direction and current velocity, chlorophyll-a, Fig. 5. Diurnal variations of water temperature (T), salinity (S), chlorophyll-a, and P CO2 in the surface seawater at Stations A and B in the central Iyo Nada. Stick diagrams of tidal currents at the two stations are also shown.

11 Temporal Variation of P CO2 in Iyo Nada 691 Fig. 6. Relations between current direction and current velocity, chlorophyll-a, salinity, water temperature, and P CO2 in the surface seawater of Station A. N, E, S, and W denote the northward, eastward, southward, and westward current, respectively. The tidal current at Station A was clockwise. Each line from the first N to the second N indicates a variation during one tidal cycle.

12 692 Y. Kumamoto et al. salinity, temperature, and P CO2 in the surface seawater at Station A. The salinity, temperature, and P CO2 fluctuated against the current direction regularly. According to the tide table (Japan Maritime Safety Agency, 1993), periods of the eastward and the westward currents corresponded to the high tide and the low tide, respectively. Seawaters at the high tide were represented by relatively high P CO2, high salinity, and low temperature and ones at the low tide were done by low P CO2, low salinity, and high temperature. Occurring the northward and the southward currents, the current velocity was large and salinity, temperature, and P CO2 rapidly changed. These results suggest a transition of two kinds of seawater at Station A. It also should be noted that higher concentrations of chlorophyll-a were found at the transition periods. Figure 7 shows diurnal variations of vertical profiles of water temperature, salinity, density of seawater (sigma-t), chlorophyll-a, and concentration of sum of nitrite and nitrate at Stations A (September 20, 1993) and B (May 12, 1994). The diurnal variations of P CO2 in the surface seawater at the two stations are also shown. At Station B, water temperature descended gradually along with depth. Salinity was almost constant from surface to bottom except a penetration of high saline water in the subsurface from 12:00 to 16:00. Thus, density of seawater increased gradually along with depth except from 12:00 to 16:00. Concentrations of chlorophyll-a and sum of nitrite and nitrate were low from surface to bottom. These results suggest that the seawater in the central Iyo Nada was uniform from surface to bottom in early May. On the contrary, at Station A in September, a typical stratification was found. The vertical distributions of water temperature, salinity, and density of seawater indicated presence of a thermocline, salinocline, and pycnocline, respectively. The concentration of sum of nitrite and nitrate was low above the pycnocline and high below that. The concentration of chlorophyll-a in the surface seawater changed temporally, which was able to be divided into three types: The first had the highest concentration at surface; The second had the highest concentration in sub-surface (from 10 to 20 m depth); The third s concentration was lower than those of the previous two types. It should be noted that the concentrations of chlorophyll-a in the period from 12:00 to 14:00 were low (less than 3 µg L 1 ) while the concentrations of sum of nitrite and nitrate in the surface seawater were relatively higher than those in the other time. Intensity of solar radiation might influence the concentration of chlorophyll-a in the surface seawater. The relatively low chlorophyll-a from 12:00 to 14:00, however, can not be explained by the diurnal variation of the intensity of solar radiation. If the whole water column was moved by the tidal current as the same manner as the surface seawater (see Fig. 6), it is considered that the samplings (at 2 hours intervals) in the high tide and the low tide were carried out at sites to the south and the north of Station A, respectively. In other words, the results obtained at Station A represent the regional variation of seawater in the central and the south of Iyo Nada while the horizontal observation of TY93-6 indicates that in the central and the north of Iyo Nada. These results derive schematic explanations of distributions of P CO2 in surface seawater, water temperature, salinity, density of seawater, chlorophyll-a, and concentration of sum of nitrite and nitrate in Iyo Nada in September 1993 (Fig. 8). The central Iyo Nada was a stratified region and the seawater of the north and the south of Iyo Nada could have been mixed vertically at the straits between Hiroshima Bay and Iyo Nada and the Hayasui Straits, respectively, due to the tidal currents. The concentration of chlorophyll-a was relatively high in the stratified region and low in the mixed regions. A similar distribution of chlorophylla was observed in the south of Iyo Nada in July 1991, which was owing to a tidal front formed between a region mixed vertically at Hayasui Straits and stratified one in the central Iyo Nada (Takeoka et al., 1993). These results strongly suggest that a tidal front was formed in Iyo Nada in September 1993 and that the high activity of phytoplanktons due to the tidal front reduced the

13 Temporal Variation of P CO2 in Iyo Nada 693 Fig. 7. Diurnal variations of vertical profiles of water temperature, salinity, density of seawater (sigma- T), concentration of chlorophyll-a, and concentration of sum of nitrite and nitrate at Stations A and B. The vertical observations at Stations A and B were carried out on September 20, 1993 and May 12, 1994, respectively. The diurnal variations of P CO2 in the surface seawater at the two stations are also shown.

14 694 Y. Kumamoto et al. Fig. 8. Schematic explanations of distributions of P CO2 in surface seawater, water temperature, salinity, density of seawater, chlorophyll-a, and concentration of sum of nitrite and nitrate in Iyo Nada in September P CO2 in the surface seawater of the central Iyo Nada. The relatively low concentration of chlorophyll-a in the mixed region is explained by that phytoplankton can not stay long in photic layer because of vertical mixing (Matsuda, 1990). At the tidal front, density of seawater in the sub-surface was relatively small, which implies a convergence of surface seawater. The

15 Temporal Variation of P CO2 in Iyo Nada 695 Fig. 9. Relation between concentration of chlorophyll-a and P CO2 in the surface water of Station A. Closed circles denote the data obtained from 16:00 to 20:00 in September 20. A line indicates the linear regression for the closed circles ([P CO2 ] = [Chlorophyll-a], r = ). convergence at the tidal front was confirmed by Garrett and Loder (1981) and James (1984). Figure 9 shows the relation between the concentration of chlorophyll-a and the P CO2 in the surface seawater of Station A. Although P CO2 in the seawaters contained more than 15 µg L 1 of chlorophyll-a were relatively low, the relation for all the data was obscure. This could be owing to the following two factors. First, the concentration of chlorophyll-a in the surface seawater did not represent the amount of chlorophyll-a in the photic layer. Second, the P CO2 in the surface seawater could have increased by supplying of deeper water which contained relatively higher concentration of Ct. The closed circles in Fig. 9 denote the data obtained from 16:00 to 20:00 in September 20 when the depth of the pycnocline was almost constant. A relation between P CO2 and chlorophyll-a in negative sense was found in the period ( P CO2 (µatm)/ Chl.-a (µg L 1 ) = 17.7). Watson et al. (1991) reported a relation between P CO2 and chlorophyll-a in the surface water of the North Atlantic during the spring plankton bloom and found the values of 6.6~ 16.8 for P CO2 (µatm)/ Chl.-a (µg L 1 ). Although it is difficult to compare the value of 17.7 in Iyo Nada and that in the North Atlantic directly, these relations between P CO2 and chlorophyll-a suggest the reduction of Ct and P CO2 owing to the photosynthesis of phytoplanktons in the surface seawater. 4. Conclusions The temporal variation of the P CO2 in the surface seawater of Iyo Nada is summarized as follows: (1) From January to July, the value of P CO2 increased from 270 to 340 µatm with the rising of water temperature from 12 to 21 C, approximately; (2) In September, the value of P CO2 ranged from 200 to 450 µatm, approximately. The relative high P CO2 in the north and the south of Iyo Nada could have been owing to the mixing of seawater at the straits. The low P CO2 in the central Iyo Nada was caused by the high activity of phytoplanktons due to the tidal front. The lowest (11 13 C) and highest (23 24 C) water temperature of the surface seawater in

16 696 Y. Kumamoto et al. Iyo Nada appear in March and in September, respectively (Yanagi, 1982). A seasonal stratification develops from August to October. If the large variation of P CO2 in September 1993 was owing to destruction of the developed seasonal stratification, it seems that the higher P CO2 than the atmospheric P CO2 in the north of Iyo Nada appears from August to October. The results obtained by this work and Kimoto et al. (1993) suggest that the P CO2 in the surface seawater of Iyo Nada is lower than the atmospheric P CO2 except in the limited period and region. It should be concluded that Iyo Nada acts as a sink of the atmospheric CO 2 on the average. The large variation of P CO2 in the north of Iyo Nada, however, indicates a vulnerability of P CO2 in the coastal sea to the vertical mixing of seawaters in summer. In order to estimate an exact CO 2 flux in a coastal sea, P CO2 in summer must be measured in detail. Acknowledgements We thank the captain and crew of the R/V Toyoshio Maru, Faculty of Applied Biological Science, Hiroshima University for their help during the research cruise. We also would like to express thanks to Professors S. Uye, O. Matsuda, and A. Kaneko, Hiroshima University, for providing us with opportunities of the cruise of TY93-11/12 and TY94-3. This research was mainly supported by Kansai Environmental Engineering Center Co., Inc., and partially supported by the Grant-in-Aid # from the Ministry of Education, Science and Culture, Japan, and Steel Industry Foundation for the Advancement of Environmental Protection Technology (No.29). References Codispoti, L. A., G. E. Friedrich, R. L. Iverson and D. W. Hood (1982): Temporal changes in the inorganic carbon system of the southeastern Bering Sea during spring Nature, 296, Copin-Montegut, C. (1985): A method for the continuous determination of the partial pressure of carbon dioxide in the upper ocean. Mar. Chem., 17, Copin-Montegut, C. (1988): A new formula for the effect of temperature on the partial pressure of CO 2 in seawater. Mar. Chem., 25, Dickson, A. G. (1990): Thermodynamics of the dissociation of boric acid in synthetic seawater from to K. Deep-Sea Res., 37, Dickson, A. G. and F. J. Millero (1987): A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res., 34, Dickson, A. G. and J. P. Riley (1979): The estimation of acid dissociation constants in seawater media from potentiometric titrations with strong base. I. The ionic product of water (K w ). Mar. Chem., 7, Garrett, C. J. R. and J. W. Loder (1981): Dynamical aspects of shallow sea front. Phil. Trans. R. Soc. Lond., A302, Gordon, L. I. and L. B. Jones (1973): The effect of temperature on carbon dioxide partial pressures in seawater. Mar. Chem., 1, Goyet, C. and E. T. Peltzer (1994): Comparison of the August-September 1991 and 1979 surface partial pressure of CO 2 distribution in the Equatorial Pacific Ocean near 150 W. Mar. Chem., 45, Grasshoff, K., M. Ehrhardt and K. Kremling (1983): Methods of Seawater Analysis. Verlag Chemi, Weinheim, 419 pp. Inoue, H., Y. Sugimura and K. Fushimi (1987): P CO2 and δ 13 C in the air and surface sea water in the western North Pacific. Tellus, 39B, James, I. D. (1984): A three-dimensional numerical shelf-sea front model with variable eddy viscosity and diffusivity. Cont. Shelf Res., 3, Japan Maritime Safety Agency (1993): Tide Tables Vol. 1 Japan and Its Vicinities. Japan Maritime Safety Agency, Tokyo, 448 pp. Keeling, C. D. (1968): Carbon dioxide in surface ocean waters 4. Global distribution. J. Geophys. Res., 73,

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