Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June

Journal of Oceanography, Vol. 61, pp. 435 to 445, 2005 Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June KOTO SUGURA* and SHZUO TSUNOGA Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan (Received 13 January 2004; in revised form 14 August 2004; accepted 17 August 2004) More than 14,000 measurements of surface water xco 2 were obtained during two cruises, 3 weeks apart in June 2000, along 155 E between 34 and 44 N in the western North Pacific Ocean. Based on the distributions of salinity and sea surface temperature (SST), the region has been divided into 6 subregions; Oyashio, Oyashio front, Transition, Kuroshio front, and Kuroshio extension and zones, from north to south. The surface waters were always undersaturated with respect to atmospheric CO 2. The Oyashio water was the least undersaturated: its xco 2 decreased slightly by 7 ppm, while SST increased by 2 C. The xco 2 normalized to a constant temperature decreased considerably. n the two frontal zones, a large drawdown of 30 40 ppm was observed after 18 19 days. n the Kuroshio extension zones, the xco 2 increased, but the normalized xco 2 decreased considerably. The Transition zone water may be somewhat affected by mixing with the subsurface water, as indicated by the smallest SST rise, an undecreased PO 4 concentration, and a colder and less stable surface layer than the Oyashio front water. As the uncertainty derived from the air-sea CO 2 flux was not large, the xco 2 data allowed us to calculate the net biological productivity. The productivities around 60 mmol C m 2 d 1 outside the Transition zone indicate that the northwestern North Pacific, especially the two frontal zones, can be regarded as one of the most productive oceans in the world. Keywords: xco 2, biological production rate, Oyashio front, Kuroshio front. 1. ntroduction The concentration of CO 2 (xco 2 ) in the air equilibrated with the surface water was often measured to get the air-sea CO 2 exchange flux (e.g., Goyet et al., 1998; Etcheto et al., 1999; Hood et al., 1999; shii et al., 2001; Keir et al., 2001). n the surface water, the xco 2 contents change together with total dissolved inorganic carbon (DC) contents, decreasing with biological uptake and increasing with absorption of atmospheric CO 2. The concentration of DC or xco 2 in the surface mixed layer, however, is usually affected more by the biological activity rather than the air-sea exchange (Chipman et al., 1993; Bates et al., 1995; Midorikawa et al., 2002). Furthermore, the degree of variation due to the biological activity is much larger in xco 2 than in DC and many * Corresponding author. E-mail: ksugiura@depe.titech.ac.jp Present address: Department of Environmental Science and Technology, Tokyo nstitute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. Copyright The Oceanographic Society of Japan. data on xco 2 can be obtained relatively simply, which enables us to obtain the distribution of xco 2 and its controlling factor at small spatial and temporal scales (Sabine and Key, 1998). t therefore seems promising to use xco 2, for the study of biological activity in the ocean. Of course, the direct measurements of DC in the water column together with components determining the carbonate system are useful for accurately estimating biological activity, but they are not easy and are more limited in number than the direct productivity measurements. The xco 2 (in ppm) of seawater is the concentration or mixing ratio of CO 2 by volume in the dried air equilibrated with a seawater sample in an equilibrator, which is a value measured directly against reference gases after correction for the water temperature change at the time of measurement. The xco 2 values also equal the mole fraction (in µmol mol 1 ), if it is considered as an ideal gas. As shown later, we can estimate the biological productivity from the xco 2 values. The air-sea CO 2 flux is calculated from the fugacity of CO 2, fco 2 (in µatm), or the partial pressure of CO 2, pco 2 (in µatm), corrected for atmospheric pressure, temperature and moisture con- 435

tent. The fco 2 equals the pco 2 after correction for nonideal behavior of the gas. The difference in the CO 2 concentration between air and sea, xco 2, is practically the same as fco 2 within the estimated error in this study (Weiss, 1974; Zeebe and Wolf-Gladrow, 2001). The net biological productivity across the water column (in mmol of C m 2 day 1 ) or the biological production by volume (in mmol of C m 3 day 1 ) is determined in the surface mixed layer by various methods, such as the chlorophyll-a, incubation methods using tracers such as 14 C (Fitzwater et al., 1982; Lohrenz et al., 1992), 13 C (mai et al., 2002), 18 O (Bender et al., 1987), O 2 (Nakayama et al., 2000), nutrients (Jennings et al., 1984; Minas et al., 1986; Louanchi and Najjar, 2000; Midorikawa et al., 2002), and the DC methods (shii et al., 1998, 2001, 2002; Bates et al., 1998; Lee, 2001; Murata et al., 2002). n the incubation methods, one day is required to obtain a water column productivity at an observation station. Thus, the measurements of such productivity are limited and it is almost impossible to depict their fine distribution in the ocean. Recently, maps showing the chlorophylla contents in the surface water have been successfully made by analyzing the satellite images (Antoine and Morel, 1996; Behrenfeld and Falkowski, 1997; Najjar and Keeling, 2000), but the water column chlorophyll-a contents have considerable errors and the conversion from chlorophyll-a content to net primary productivity adds further uncertainty. n this study, the fine structure of the xco 2 distribution in the surface water was observed twice in June in the western North Pacific, which has a complex hydrography and a high biological productivity (Taniguchi, 1999). We first considered the factors controlling the distribution of xco 2, such as the biological productivity, the flux of air-sea CO 2 exchange and the water temperature change, and we have next tried to estimate the biological productivities from the temporal change in the surface xco 2. The productivity (in mmol m 2 day 1 ) is a net water column biological productivity in the surface euphotic layer. f this method is successfully applied, we can easily obtain many water column biological productivy values in an observation area. 2. Methods of Observation and Chemical Analysis 2.1 Shipboard observation and sampling of seawater The observation was carried out twice in the periods 4 10 June (Cruise ) and 25 29 June (Cruise ) in 2000 on board T/S Hokusei Maru along 155 E between 34 44 N in the western North Pacific (Fig. 1). The observation line covers the mixing zone of the cold Oyashio current in the north and the warm Kuroshio extension current in the south. For the determination of the surface water temperature (SST) and the concentration of CO 2 Oyashio Oyashio front Transition Kuroshio front Kuroshio Extension Kuroshio Extension Fig. 1. Map showing the study area. Thick line denotes the observation section of 34 44 N along 155 E, where xco 2 and SST of the surface water were measured continuously. Dotted lines denote the borders of 6 subregions for both the first () and the second () cruises. From north to south: the Oyashio, the Oyashio front, the Transition, the Kuroshio front, the Kuroshio extension and the Kuroshio extension zones. (xco 2 ) in the surface water, we continuously sampled the surface seawater with a flow rate of 20 l min 1 from an inlet fitted at the bottom of the vessel, about 5 m below the sea surface. The number of data obtained exceeded 7000 for each cruise. Surface seawater samples for phosphate (PO 4 ) contents were also collected every 2 6 hours during sailing. We further occupied 11 stations to obtain discrete seawater samples every one degree of latitude from 44 N. The samples were collected from 24 depths from the surface down to 3000 m with 12 l Niskin bottles installed in a rosette-type frame together with a CTD system. 2.2 Determinations of chemical components The xco 2 in surface water was continuously measured using the method described by Nojiri et al. (1999), which was slightly modified for use on a relatively small vessel. The analytical system was also used for the determination of the atmospheric xco 2 content, which was measured twice a day. This system has a tandem-type equilibrator consisting of a bubbling water column of 73 cm high with a diameter of 7.7 cm including an air portion of 10 cm followed by a showering air column of 31 cm with a diameter of 11 cm. Continuously flowing surface water was taken and showered from the top of the equilibrator with a flow rate of 200 ml min 1. Air was passed through the equilibrator from the bottom of the bubbling water column with a flow rate of 300 ml min 1, dried, and introduced into an NDR (a non-dispersive in- 436 K. Sugiura and S. Tsunogai

Table 1. Observed water temperature (SST in C), concentration of CO 2 (xco 2 in ppm), concentration of CO 2 normalized to a temperature of 15 C (NxCO 2 in ppm), concentration of phosphate (PO 4 ), salinity and total alkalinity normalized to a salinity, 34 (NTA in µmol kg 1 ) in surface water along 155 E between 44 34 N. Entries in the column of zones, 1, 2, 3, 4, 5 and 6, refer to the Oyashio, the Oyashio front, the Transition, the Kuroshio front, the Kuroshio extension and the Kuroshio extension, respectively. The letters attached to each component, N, M, σ and CL, denote the number of data, the mean value, the standard deviation and the 95% confidence limit, respectively. Zones Latitude Observation period SST xco 2 NxCO 2 PO 4 Salinity NTA N M σ CL N M σ CL M N M σ CL N M N M Cruise (1) 44.00 43.25 8 10 June 2045 6.95 0.56 0.02 2045 365.0 11.0 0.5 513 1 1.51 1 32.92 1 2312 (2) 43.25 43.05 8 June 111 9.89 1.23 0.23 111 315.1 15.8 3.0 393 0 1 33.19 1 2304 (3) 43.05 41.45 7 8 June 1323 11.30 0.63 0.03 1323 335.5 4.3 0.2 393 7 0.56 0.09 0.07 2 34.22 1 2268 (4) 41.45 40.38 7 June 241 13.00 1.09 0.14 241 326.3 13.0 1.7 356 7 0.37 0.16 0.12 1 33.90 1 2280 (5) 40.38 37.43 5 7 June 2088 15.27 0.41 0.02 2088 311.1 5.5 0.2 307 11 0.17 0.03 0.02 4 34.50 2 2253 (6) 37.43 34.60 4 5 June 1293 17.67 0.44 0.02 1293 318.9 7.7 0.4 285 0 0 0 Whole area 7101 12.35 4.11 0.10 7101 328.6 23.4 0.6 375 26 (0.65) (0.30) (0.12) 9 33.75 6 2279 Cruise (1) 44.00 43.68 23 25 June 2719 8.96 0.21 0.01 2719 357.7 6.5 0.2 463 20 1.39 0.05 0.02 1 32.70 1 2316 (2) 43.68 43.10 25 26 June 219 12.34 0.90 0.12 219 270.1 13.5 1.8 301 0 1 33.77 0 (3) 43.10 41.48 26 June 767 12.65 0.59 0.04 767 351.4 10.0 0.7 389 12 0.52 0.16 0.09 2 33.86 1 2276 (4) 41.48 40.13 26 27 June 547 15.37 0.81 0.07 547 286.7 17.7 1.5 282 6 0.06 0.04 0.03 2 34.07 0 (5) 40.13 37.43 27 28 June 1695 17.76 1.35 0.07 1695 330.0 7.6 0.4 294 20 0.06 0.02 0.01 2 34.37 1 2246 (6) 37.43 34.80 28 29 June 1326 22.44 0.98 0.05 1326 354.1 9.6 0.5 258 17 0.04 0.01 0.01 3 34.47 2 2249 Whole area 7273 14.92 5.30 0.12 7273 325.0 24.8 0.6 331 75 (0.51) (0.57) (0.13) 11 33.86 5 2267 Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June 437

(a) SST 24 22 20 18 16 14 12 10 8 6 45 44 43 42 41 40 39 38 37 36 35 ( N) O OF TS KF KE- KE- (b) xco 2 (c) NxCO 2 500 450 400 350 300 250 200 45 44 43 42 41 40 39 38 37 36 35 ( N)...... O OF TS KF KE- KE- (d) PO 4 380 360 340 1.4 1.2 1.0 320 0.8 300 280 0.6 0.4 260 240 45 44 43 42 41 40 39 38 37 36 35 ( N) O OF TS KF KE- KE- 0.2 0.0 45 44 43 42 41 40 39 38 37 36 35 ( N)...... O OF TS KF KE- KE- Fig. 2. North-south distributions of (a) SST (in C), (b) xco 2 (in ppm), (c) xco 2 normalized to 15 C, NxCO 2 (in ppm) and (d) PO 4 (in µmol kg 1 ) observed during the two cruises, (4 10 June 2000 shown with gray color lines) and (23 29 June 2000 shown with black color lines or solid triangles) in the northwestern North Pacific along 155 E between 34 44 N. The subregions for the two cruises are masked on the bottom with following symbols; O: the Oyashio, OF: the Oyashio front, TS: the Transition, KF: the Kuroshio front, KE-: the Kuroshio extension and KE-: the Kuroshio extension. frared analyzer). The CO 2 signals were recorded every 10 seconds (Tsurushima et al., 2002). The precision was better than ±1 ppm. The volume of air inside the equilibrator was about 3 l and thus the turnover time of the air was about 10 min. Since the vessel was sailing at about 10 knots, the observed data are at most 2 sea miles astern of the vessel s recorded position. The temperature of water inside the equilibrator was measured continuously and the observed xco 2 values were corrected for the slight change in water temperature after passing the inlet. The average warming was 0.8 C. The sea surface temperature (SST) was continuously measured at ±0.01 C accuracy during sailing, in parallel to the xco 2 measurements. We determined the contents of nutrient (PO 4 ), total alkalinity (TA) and salinity (S) in the discrete water samples. The surface samples for the PO 4 determination were collected in 10 ml bottles through a bypass of the inlet line into the xco 2 system, and immediately refrigerated at 20 C. These samples were analyzed for nutrients by using the JGOFS spectrophotometric methods and an auto-analyzer (AACS, Bran + Luebber) at a laboratory on land. The data on TA and S listed in Table 1 were provided by courtesy of Dr. N. Tsurushima (personal communication). The TA values were determined by the potentiometric titration method, which had a standard deviation of 2.2 µmol kg 1 (Tsurushima et al., 2002). 2.3 Subregions of the study area divided by the basic data The study area has attracted considerable attention from many physical oceanographers (e.g., Kawai, 1972; Favorite et al., 1976; Roden et al., 1982; Yasuda et al., 1996; Kawabe and Taira, 1998), because the wide Kuroshio-Oyashio interfrontal zone or the perturbed area defined by them is not found in the North Atlantic. t includes a boundary between the cold (<10 C) and less saline (<33.0) Oyashio water in the north, and the warm (>14 C) and more saline (~34.5) Kuroshio water in the south. The surface current system in the region is illustrated in Fig. 1 of Midorikawa et al. (2002). The two water masses, however, do not come in direct contact and there is a wide frontal zone of about 3 degrees latitude (350 km) between them, as shown in Fig. 1. The concentrations of chlorophyll-a are high (0.5 0.7 µg kg 1 ) in the Oyashio water during the warm season, where nutrients 438 K. Sugiura and S. Tsunogai

are not usually exhausted and spring phytoplankton blooms are observed. Chlorophyll concentrations are low (<0.2 µg kg 1 ) in the Kuroshio water (Midorikawa et al., 2002). According to these previous studies and based on the distributions of such conservative components as salinity (S) and surface temperature (SST), the region can be divided into 6 subregions: Oyashio, Oyashio front, Transition, Kuroshio front, and two Kuroshio extension zones from the north to the south. The Kuroshio extension can be further divided into two parts, Kuroshio extension and Kuroshio extension, at around 37.4 N, where the SST changes significantly. These waters have been identified chiefly on the basis of the SST measured simultaneously with the CO 2 contents (Fig. 2(a)) and also on the basis of the salinity data obtained from the discrete surface water samples and the CTD observations. Although the SST values increased during the period between Cruises and, the increase of a few degrees Celsius occurred in a similar fashion everywhere along the north-south transect (Fig. 2(a)). The locations and water properties of the surface waters in the subregions are given in Table 1. 3. Results and Discussion 3.1 The distribution of surface xco 2 and its fine structure Figure 2 shows the north-south distributions of surface water temperature (SST), the xco 2 (xco 2 ), the xco 2 normalized to a temperature of 15 C (NxCO 2 ) and the concentration of phosphate (PO 4 ) in the surface water in the study area. According to Takahashi et al. (1993), the NxCO 2 values are calculated from the following equation (1). NxCO 2 = xco 2 exp[0.0423(t 15)] (1) where T is the observed in situ SST (in C). This approximate equation was originally written for pco 2, but the correction term is practically the same for xco 2. We can calculate the temperature dependence of xco 2 more exactly from the dissociation constants of carbonic acid, given, for example, in DOE (1994). The difference between those obtained from DOE (1994) and Takahashi et al. (1993), however, is only 3 ppm in the waters collected at 44 N where the SST is the lowest and the difference is the largest. As shown in Fig. 2(b), some surface waters in the Oyashio zone were slightly supersaturated with respect to the atmospheric CO 2 (around 370 ppm) with a maximum of 387 ppm during Cruise, but almost all the surface waters in this study area were undersaturated with a minimum of 257 ppm in the Oyashio front zone during Cruise. The xco 2 values were higher in the Oyashio Table 2. Calculated daily rate of change of total carbonate in the surface 25 m (F T in mmol m 3 day 1 ), that due to atmospheric CO 2 absorption (F A in mmol m 3 day 1 ), and that due to the biological assimilation for a unit volume (F B in mmol m 3 day 1 ) and that due to the biological assimilation for a unit area ( B in mmol m 2 day 1 ) in the surface mixed layer. The uncertainties attached are the 95% confidence limits of the mean values. The calculation is based on the differences between the two observations, SST, xco 2, NxCO 2 and DC computed from the data given in Table 1. See text for the method for estimating the dissolved inorganic carbon difference, DC, from salinity, total alkalinity and xco 2. Area t SST xco 2 NxCO 2 DC F T F A F B B (day) ( C) (ppm) (ppm) (µmol kg 1 ) (mmol m 3 day 1 ) (mmol m 3 day 1 ) (mmol m 3 day 1 ) (mmol m 2 day 1 ) (1) Oyashio 15 2.01 ± 0.02 7.3 ± 0.5 50 ± 1 18 ± 1 1.21 ± 0.03 0.06 ± 0.04 1.27 ± 0.05 40 ± 2 (2) Oyashio front 18 2.45 ± 0.26 45.0 ± 3.5 92 ± 8 49 ± 4 2.78 ± 0.24 0.50 ± 0.32 3.27 ± 0.40 102 ± 12 (3) Transition 18 1.35 ± 0.05 15.9 ± 0.7 4 ± 1 2 ± 1 0.14 ± 0.04 0.16 ± 0.10 0.30 ± 0.11 9 ± 3 (4) Kuroshio front 19 2.37 ± 0.16 39.6 ± 2.3 74 ± 4 45 ± 3 2.41 ± 0.14 0.37 ± 0.23 2.78 ± 0.27 87 ± 9 (5) Kuroshio extension 21 2.49 ± 0.07 43.0 ± 0.5 13 ± 1 10 ± 1 0.46 ± 0.04 0.26 ± 0.17 0.73 ± 0.17 23 ± 5 (6) Kuroshio extension 24 4.77 ± 0.05 35.2 ± 0.6 27 ± 1 20 ± 1 0.84 ± 0.04 0.17 ± 0.11 1.01 ± 0.11 31 ± 4 Average 19 2.57 24 1.28 0.25 1.54 48 (29) (1.51) (0.27) (1.79) (56) Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June 439

Fig. 3. Plots of xco 2 (in ppm) against SST (in C) for Cruise (a) and Cruise (b), and of NxCO 2 (in ppm) against SST (in C) for Cruise (c) and Cruise (d). Attached symbols denote subregions as follows; O: the Oyashio, OF: the Oyashio front, TS: the Transition, KF: the Kuroshio front, KE-: the Kuroshio extension and KE-: the Kuroshio extension. and the Transition zones, and lower in the two frontal zones during both cruises (Fig. 2(b) and Table 1). The mean xco 2 values in the respective zones ranged from 311 to 365 ppm for Cruise and from 270 ppm to 358 ppm for Cruise. Average xco 2 per zone was highest in the Oyashio zone and lowest in the Oyashio frontal zone during both cruises. Besides the low xco 2 of 270 ppm in the Oyashio front zone, we also should note the low xco 2 of 287 ppm in the Kuroshio front zone during Cruise should also be noted. The decrease in xco 2 between the two cruises was nearly 40 ppm in the Oyashio frontal zone and Kuroshio frontal zone, while it was only 7 ppm in the Oyashio zone (Table 2). On the other hand, it increased by 16 ppm in the Transition zone, approaching the atmospheric value. The latter increase can be almost fully explained by the increase in water temperature of 1.4 C, raising xco 2 by 20 ppm, but the surplus (4 ppm) is too small to satisfy the biological activity in the Transition zone, as discussed later (Fig. 2(c) and Table 2). n the Kuroshio extension zones, the xco 2 values increased with time, ranging from 311 to 320 ppm for Cruise and from 330 to 354 ppm for Cruise (Fig. 2(b) and Table 1). The temporal change in xco 2 increased towards the south, although the size of the xco 2 increase varied more from location to location than that of SST. The north south difference of NxCO 2 values (Fig. 2(c)) was larger than that of xco 2 values. The NxCO 2 values were higher in the Oyashio and the Transition zones containing PO 4 than in other zones during both Cruises and. t should be noted that the NxCO 2 values during Cruise were lower than during Cruise, except in the Transition zone, which means that the biological uptake of CO 2 exceeded the absorption of atmospheric CO 2 and supply of CO 2 by vertical diffusion. The decrease was 50 90 ppm in the Oyashio and the two front zones, and around 20 ppm in the two Kuroshio extension zones (Table 2). n the Transition zone, however, the NxCO 2 values (about 390 ppm) did not change with time in the undersaturated surface water. Therefore, if there were no vertical diffusion, the biological uptake of CO 2 was almost balanced by the air-sea flux in the Transition zone, but the vertical diffusion was more active there, as discussed later. This suggests that net biological CO 2 uptake was not small in the Transition zone. Except in the Transition zone, the PO 4 concentration decreased with time in all the subregions, especially in the Kuroshio front zone where it decreased by up to 0.6 µmol kg 1 (Fig. 2(d)). The PO 4 concentrations in the Transition zone were almost constant. Although the number of phosphate measurements was small and the vertical mixing was not well known, the decrease in PO 4 agreed well with that of NxCO 2. The xco 2 and NxCO 2 values are plotted against SST (Fig. 3). For Cruise, the xco 2 values decreased with increasing water temperature in the two frontal zones, from the Oyashio to the Oyashio front zones and from the Transition to the Kuroshio front zones (Fig. 3(a)). The 440 K. Sugiura and S. Tsunogai

decrease is more pronounced in the NxCO 2 vs. SST plot (Fig. 3(c)). t should be noted that all the waters were undersaturated with respect to the atmospheric CO 2 concentration. For Cruise, the xco 2 values in the Kuroshio extension zones were affected by the warming (Fig. 3(b)). The NxCO 2 mostly decreased with increasing SST, constituting two lines (Fig. 3(d)). One runs that from the Oyashio region to the south while the other runs that from the Transition zone to the south. Minima of xco 2 existed in the Oyashio and Kuroshio front zones, where PO 4 was almost exhausted. Thus, a further decrease of xco 2 may not be large in these frontal zones (Fig. 3(d)). n the Oyashio and the Oyashio front zones, the xco 2 decreased toward minima of 292 and 257 ppm with slopes of 18 and 36 ppm C-1, respectively, for Cruises and (Figs. 3(a) and 3(b)). This steepening slope and the decrease in xco 2 and NxCO 2 with increasing water temperature described above may be due to the expansion of biological production with increasing water temperature in the mixing zone. Table 1 summarizes the observed data, which shows that the xco 2 was always undersaturated with respect to the atmospheric CO 2 in June in the region. This tells us that the DC (dissolved inorganic carbon) content was increased by the air-sea CO 2 exchange. Furthermore, the NxCO 2 values decreased during the observation period, indicating the net decrease in the water column DC content. We can therefore estimate the net biological productivity in the water column as the sum of the net decrease in DC and the air-sea CO 2 flux, using an appropriate model for the surface water system. Fortunately, both terms can be added together and we can obtain the net biological productivity with a simple model, which is described in the next section. 3.2 Biological productivity estimated from the temporal variation of surface xco 2 We obtain the water column net biological productivity from the sea surface xco 2 measured twice at an interval of about three weeks, with the following assumptions. First, the surface mixed layer is vertically sufficiently well mixed. Next, the photosynthesis below the mixed layer is negligible or can be estimated. Third, the xco 2 is not significantly changed by the water exchanged with the surface mixed layer in question. Fourth, the airsea CO 2 exchange flux is negligible or can be estimated. Fifth, the production of CaCO 3 does not affect the observed xco 2 or can be estimated. Last, the concentrations of chemical components change in proportion to salinity, but the xco 2 values are not changed by the salinity change principally caused by evaporation and precipitation. The mixed layer depth is defined as an upper boundary of the layer where the σ θ gradient is greater than 0.01 m 1 (Tsurushima et al., 2002). Based on the estimated (a) 23.5 24.0 24.5 25.0 25.5 26.0 26.5 (σ θ) 0 10 20 30 40 50 60 70 80 (m) (b) 0 10 20 30 40 50 60 70 80 (m) 44 43.25 42.5 41.75 41.04 40.25 39.53 38.75 38 44 43.25 42.5 41.74 41 40.25 39.5 38 37.25 36.5 35 Fig. 4. Vertical profiles of density (σ θ ) at stations observed during Cruise () (a) and Cruise () (b). The open, solid and gray symbols refer to the Oyashio and the two front zones, the Transition zone, and the Kuroshio extension zone, respectively, and the other simple symbols such as crosses refer to the Kuroshio extension zone. mixed layer depths of 20 49 m for Cruise and 16 24 m for Cruise, we have assumed that the mixed layer depth is 25 m for all the water columns in this season in this study area. n the Transition zone, the σ θ gradient was around 0.02 m 1 at most, which was about one third of that in the Oyashio zone (Fig. 4). The vertical eddy diffusivity is estimated as 0.02 0.03 cm 2 s 1 by inserting 8.0 ± 4.5 m s 1 for the observed wind velocity and a σ θ gradient of 0.01 m 1 into the equation of Denman and Gargett (1983). The DC gradient is 3 µmol kg 1 m 1 at most at the pycnocline. These values give a vertical diffusive flux into the mixed layer of less than 1 mmol m 2 d 1. This diffusive flux is less than the uncertainty in the primary productivity, which is calculated below. The Ekman upwelling seems to be negligible, because the study area is neither a coastal region nor a center of the subarctic gyre. When the vertical diffusion and advection could not be neglected, the net biological productivities obtained later from the DC change will be the lower limit, be- Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June 441

cause the DC concentration increases with depth in the pycnocline. During the period from spring to autumn from 1998 1999 at a western subarctic time-series station KNOT (44 N, 155 E), the biological productivity in the surface 25 m layer accounted for 80% of the total productivity in the euphotic layer on an average (mai et al., 2002). Sweeney et al. (2000) also calculated the biological productivity including the surface layer below the pycnocline. Considering the results, we have assumed that the productivity in the surface mixed layer is 80% of the total productivity. For the production and dissolution of CaCO 3, the particulate flux of CaCO 3 from the surface layer was observed to be 0.3 mmol m 2 d 1 in the western North Pacific (Tsunogai et al., 1990; Tsunogai and Noriki, 1991). This flux is one order of magnitude smaller than the uncertainties in the biological productivity shown below. Based on these assumptions, we have estimated the biological productivity of the surface water column in the western North Pacific, 34 44 N along 155 E, in June 2000 using the observed S, TA, SST, and xco 2 data. The difference in dissolved inorganic carbon (DC), DC, is expressed by the following equation (2): DC = DC() DC() (2) where DC() and DC() are the DC normalized to a salinity (S av, an average of two salinities observed at Cruises and ) for Cruises and, respectively, according to the above last assumption for the salinity change given above. The normalization is essential to eliminate the effect of evaporation or precipitation of water and to include only the effect of biological uptake. We have calculated the DC contents in the surface water from the observed water temperature (SST) and carbon dioxide content (xco 2 ). We have also used a mean value of total alkalinity (TA av ) as well as salinity (S av ) for the calculation, according to the above fifth assumption for the production of CaCO 3. The error resulting from the salinity normalization is 3% of DC per unit of salinity (say, 35 for salinity of 34). DC() = f(s av, TA av, SST(), xco 2 ()) (3) DC() = f(s av, TA av, SST(), xco 2 ()). (4) Table 2 shows the S av, TA av values used and the calculated results. n these calculations we have used the figures given by Dickson and Millero (1987) for the equilibrium constants of CO 2 species in seawater. The daily change in the DC for a unit volume, F T (mmol m 3 d 1 ), is obtained as follows: F T = DC/ t (5) where t is the period between the two observations in days. The mean air-sea CO 2 exchange flux during the observation period for a unit volume of the surface mixed layer of 25 m thick, F A (mmol m 3 d 1 ), is given by the following equation: F A = kα( fco 2 )/25 kα( xco 2 )/25 (6) where the constant, k, is the CO 2 transfer velocity (m d 1 ), α is the CO 2 solubility in seawater (mmol m 3 ppm 1 ) (Weiss et al., 1974), and all these quantities including fco 2 and xco 2 are those averaged for the observation period and contain considerable errors. The air-sea difference, xco 2 (ppm), is obtained by subtracting the atmospheric xco 2 from surface water xco 2. Here we define the efflux to the atmosphere as positive and the influx to the sea as negative. The mean wind velocity and its 95% confidence limit during two cruises, 8.0 ± 4.5 m s 1, are calculated from the 13 wind velocities observed on 8 different days during the two cruises at 10 m above the sea level ranging from 3.0 14.5 m s 1 (Hokusei Maru s logbook). The wind velocities give the air-sea CO 2 transfer velocity of 3.6 ± 2.3 m d 1 by introducing them into both the two equations of Liss and Merlivat (1986) and Wanninkhof (1992). At present, we have no evidence for the more appropriate equation giving gas transfer velocity and the difference between the two equations is included in the uncertainty in the average velocity. From the gas transfer velocities and the observed xco 2 values, we obtain the air-sea exchange fluxes, F A, of 0.06 0.50 mmol m 3 d 1 with uncertainties of 0.04 0.32 mmol m 3 d 1 (Table 2), which are less than 36% of the biological production rates, except for the Transition zone as described below. The biological production rate, F B, is obtained from Eq. (7), F B = F T F A. (7) The inventory change in the DC due to the biological activity in the whole water column, the net biological productivity B, is calculated from the following equation (8): B = F B 25 (1/0.8). (8) The results obtained are also listed in Table 2, including the estimation errors. The error depends on the assumptions given above. f the assumptions hold true and the region is productive, like this study area, the error is less than 15% or 5 10 mmol m 2 d 1 (Table 2). The small error is due to the air-sea CO 2 flux being additive to the estimated DC decrease. The assumption of vertical mixing across the pycnocline is serious, as discussed below. 442 K. Sugiura and S. Tsunogai

The largest productivities are obtained in the two frontal zones. The estimated productivities of 102 and 87 mmol m 2 d 1 agree well with those observed during the phytoplankton bloom in the world oceans, e.g., 90 107 mmol m 2 d 1 in the North Atlantic (Chipman et al., 1993; Martin et al., 1993), 69 78 mmol m 2 d 1 in the Ross Sea, Antarctic (Smith et al., 2000), 67 mmol m 2 d 1 in the Antarctic Polar Frontal Zone (Hiscock et al., 2003), and 83 133 mmol m 2 d 1 observed at 44.7 N, 156 E during the spring bloom of phytoplankton in May 1999 (Murata et al., 2002). There are some papers that report that the spring bloom starts in May at the northern end of this study area (mai et al., 2002; Murata et al., 2002), and our observation was carried out in early to late June. Midorikawa et al. (2002) have found that the xco 2 in the frontal zone was low until September in the region 48 38 N, 165 E. According to the pco 2 data compiled by Kamiya (2001), the surface water pco 2 values do not show a definite seasonal variation and are always undersaturated by (300 ± 50) µatm in the Oyashio Kuroshio mixing zone, although the region is closer to the Japanese coast. Moreover, the particulate fluxes observed with sediment traps do not show a steep peak in spring to summer, although they are larger in summer than in winter (Noriki et al., 1999; Honda et al., 2002). All these findings suggest that the large biological production, matching that of the spring bloom, continues from May to September in this study zone. f the biological production in the surface 25 m continues at a rate of 70 80 mmol m 2 d 1, as described above, the monthly consumption of PO 4 amounts to 0.8 0.9 µmol l 1 month 1. This means that all PO 4 in the surface water is exhausted and the blooming ends within about 1 month, unless PO 4 is supplied from the outside. Therefore, the biological production may be maintained by renewing the water containing nutrients. As described in the previous section, there are two lines in the diagrams showing the SST dependence of xco 2 or NxCO 2, one of which starts and descends from the Oyashio to the Oyashio front, the other running from the Transition zone to the Kuroshio front (Figs. 3(a), (b), (c) and (d)). The waters at the starting points of the lines contain more nutrients and are nearer the original water, bringing about the blooming of phytoplankton in this study area. The original water is probably the Oyashio water coming from the northern North Pacific flowing southwestward along the Krill slands, including the subsurface water mixed by vertical diffusion and upwelling. Besides the front zones, the Oyashio zone gives a moderately large biological productivity (40 mmol m 2 d 1 ) which is nearly the same as the maximum rate observed at the station KNOT (44 mmol m 2 d 1 ) in spring (mai et al., 2002). This may indicate that the moderate biological production will continue for a long period from spring to summer or early autumn, because the water contains sufficiently high nutrients. The moderate production may be due to vertically less stable water column, which is inconvenient for phytoplankton (Fig. 4), a lower water temperature, lacking in some minor nutrients such as Fe (Harrison et al., 1999), or some other factors, but the remaining nutrients in the Oyashio water will be exhausted sooner or later, before arriving at the subtropical gyre. The Kuroshio extension zones show almost uniform productivities (23 and 31 mmol m 2 d 1 ) which are 60 80% of that in the Oyashio region, despite the gradual SST increase towards the south. The moderate productivity was likely to have decreased soon, as the nutrients were almost exhausted and the water column was more strongly stratified during Cruise (Fig. 4). We obtained the lowest productivity of 9 mmol m 2 d 1 in the Transition zone (Table 2). However, the low productivity may be only apparent due to mixing of the subsurface water with high xco 2. The suggestion of mixing in subsurface water in the Transition zone is supported by the following four observations. First, Fig. 4 shows that the pycnocline of the Transition zone was the least stable of all zones. n addition, the PO 4 concentration (0.5 µmol kg 1 ) did not decrease during the observation period in June, although it had a wide range of values (Fig. 2(d)). Thirdly, the daily increase in SST was only 0.08 C day 1 in the Transition zone, while the overall mean for all zones was 0.15 C day 1. Lastly, the surface water in the Transition zone was colder than water in the Oyashio frontal zone further north. The mixing would have added water rich in CO 2 to the mixed layer. As a result, our calculation has underestimated biological productivity. The remaining and supplied nutrients will be later used for the biological assimilation, especially in the frontal zones (Midorikawa et al., 2002). The mean biological productivity in June was 56 mmol m 2 d 1, for all regions with the exception of the Transition zone (Table 2). The productivities in the western North Pacific region are large, but vary widely from region to region. 4. Summary and Conclusions 1) The observed xco 2 varied widely in the perturbed area in the western North Pacific in June and it was extremely low in the two frontal zones, where it decreased with water temperature. 2) Almost all the surface waters were undersaturated with respect to the atmospheric CO 2, and were absorbing the atmospheric CO 2 even as they were warming. 3) The normalized xco 2 decreased between Cruises and, indicating that the biological productivity was larger than the air-sea CO 2 flux, except in the Spatial and Temporal Variation of Surface xco 2 Providing Net Biological Productivities in the Western North Pacific in June 443

Transition zone. 4) n the Transition zone, the surface water was probably mixed with the cold subsurface water containing much nutrients as well as DC. 5) A method for estimating the net biological productivity has been successfully used, which utilizes the temporal change in the surface xco 2. f the assumptions hold true and the study area is productive, the error is less than 15%. 6) Applying the method to this study area in June, except in the Transition zone, we have found extremely large productivities of 90 mmol m 2 d 1 in the Oyashio front and the Kuroshio front zones. 7) The subsurface water supplied to the Transition zone as well as the Oyashio water may maintain the high biological productivity from May to September. Acknowledgements This work was conducted with the help of many people. n particular, we are greatly indebted Dr. S. Watanabe for his encouragement through the work and Dr. Y. Nojiri for obtaining the xco 2 data as well as Dr. N. 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