Spatial Variability in the Primary Productivity in the East ... - CiteSeerX

Journal of Oceanography, Vol. 53, pp. 41 to 51. 1997

Spatial Variability in the Primary Productivity in the East China Sea and Its Adjacent Waters T. HAMA, K. H. SHIN* and N. HANDA** Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan (Received 20 September 1995; in revised form 15 May 1996; accepted 29 May 1996)

Primary productivity in the East China Sea and its adjacent area was measured by the 13C tracer method during winter, summer and fall in 1993 and 1994. The depth-integrated primary productivity in the Kuroshio Current ranged from 220 to 350 mgC m–2d–1, and showed little seasonal variability. High primary productivity (above 570 mgC m–2d–1) was measured at the center of the continental shelf throughout the observation period. The productivity at the station nearest to the Changjiang estuary exhibited a distinctive seasonal change from 68 to 1,500 mgC m–2d–1. Depth-integrated primary productivity was 2.7 times higher in the shelf area than the rates at the Kuroshio Current. High chlorophyll-a specific productivity (mgC mgChl.-a–1d–1) throughout the euphotic zone was mainly found in the shelf area rather than off-shelf area, probably due to higher nutrient availability and higher activity of phytoplankton at the subsurface layer in the shelf area.

The East China Sea receives large amounts of dissolved inorganic nutrients from the Changjiang, which can result in high primary productivity compared with areas outside of the shelf. However, only limited knowledge was available hitherto on the dynamics of primary productivity in the East China Sea and its adjacent area (Fei et al., 1987; Xiuren et al., 1988). Fei et al. (1987) obtained 200–1,500 mgC m–2d–1 from shelf edge to Changjiang estuary in summer, the productivity simply increasing toward the inner part of the East China Sea. Concerning the Changjiang estuary, Xiuren et al. (1988) observed the detailed distribution of primary productivity and reported that the maximum of primary productivity located about 100 km offshore from the Changjiang river mouth. These studies indicate a higher primary productivity in the East China Sea. However, the relationships between the primary productivity and the environmental factors are not yet fully understood. The Marginal Sea Flux Experiment (MASFLEX) was initiated in the Western Pacific to assess both the dynamics of material cycling in the East China Sea and the effect of the East China Sea as a source of organic material to the surrounding open ocean. As the primary production can be the main source of organic compounds, spatial and seasonal changes in the primary productivity provide the basic information by which we can understand the biogeochemical cycle in the East China Sea and its adjacent area. In the present study, we carried out 13C uptake experiments in the East China Sea and the adjacent area to assess the spatial and

1. Introduction Photosynthetic production by phytoplankton in the euphotic layer supplies organic material and energy to the aquatic food chain, and its spatial and seasonal variations affect the dynamics of material cycling in aquatic environments. Further, a possible decrease in pCO2 in the surface sea water by phytoplankton photosynthesis (Watson et al., 1991; Robertson et al., 1993) can accelerate the transfer of CO2 from the atmosphere to sea water. Thus, primary production in the euphotic zone is one of the most important processes, not only in material cycling in aquatic environments but also in carbon cycling in relation to global change. Accumulated knowledge of primary productivity in the oceanic environment has revealed that productivity varies spatially, and high primary productivity has been reported in continental shelf and upwelling areas where the supply of nutrients to the euphotic zone can be abundant (Berger, 1989; Longhurst et al., 1995). Although shelf regions account for only 8% of the total area of the world’s oceans, their contribution to global primary productivity has been estimated at from 18 to 33% (Wollast, 1991).

*Present address: Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan. **Present address: Aichi Prefectural University, Mizuhoku, Nagoya 467, Japan.

41 Copyright  Oceanographic Society of Japan.

Keywords: ⋅ Primary productivity, ⋅ East China Sea, ⋅ Kuroshio Current, ⋅ phytoplankton, ⋅ 13C tracer method.

seasonal variability of primary productivity and their relation to the physical and chemical factors. 2. Materials and Methods The primary productivity was determined by the 13C tracer method during three cruises of the Kaiyo, a research vessel of Japan Marine Science and Technology Center (Table 1). Most experiments were carried out along the PN line in the three cruises, except at SST-2 in the winter cruise, which was located just northwest (0°02′ N and 0°08′ W) of PN-3, instead of PN-3 (Fig. 1). PN-3 and SST-2 are located

about the center of the Kuroshio Current, and PN-5 was a shelf edge station. As the shelf area, the experiments were done at PN-8, 10 and 12. PN-12 is located about 100 km east from the Changjiang river mouth. Water temperature and salinity were measured by Sea Bird 911 Plus. Inorganic nutrients were determined by the Bran + Leubbe TRAACS800 SYSTEM. These measurements were carried out throughout the PN line (12 stations) as well as at the stations where the productivity experiments were conducted. Chlorophyll-a (Chl.-a) concentration was determined by fluorometry (Turner Designs, 10R) after

Table 1. Incubation experiments in winter (K92-09), summer (K94-04) and fall (K93-05) cruises. Shelf

Self edge

Off-shelf Kuroshio Current

Station Depth (m)

PN-12 45

PN-10 48

PN-8 84

PN-5 125

SST-2 1,070

PN-3 1,031

PN-1 1,004

Winter cruise (K92-09)

Mar. 2 1993

—

Mar. 4 1993

Feb. 26 1993

Feb. 21 1993

—

—

Summer cruise (K94-04)

Aug. 26 1994

Aug. 18 1994

Aug. 24 1994

Aug. 16 1994

—

Aug. 6 1994

—

Fall cruise (K93-05)

Oct. 23 1993

Oct. 19 1993

Oct. 21 1993

Oct. 17 1993

—

Oct. 11 1993

Oct. 10 1993

Fig. 1. Station locations along PN line in the cruises of K92-09, K93-05 and K94-04. The broken and dotted lines show the Kuroshio Current during the cruises in winter (– – –), summer (- - -) and fall (· · ·) (Department of Hydrography, 1993a, b, 1994). 42

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extraction with N,N-dimethylformamide (Suzuki and Ishimaru, 1990). Underwater photon flux was measured by a 4π scalar sensor (Li-cor, LI-193SB), and the daily surface irradiance was monitored by a 2π scalar sensor (Li-cor, LI190S). Water samples for the incubation experiments were collected with a Go-Flo water sampler attached to a nyloncoated Kevlar rope from 6 layers (100, 45, 20, 9, 4 and 0.8% of surface irradiance (I0)) in the euphotic zone before dawn. Water samples from the 100% of I0 layer were usually collected at the time when the water sampler was just submerged under the water surface, and its depth is cited as 0 m in the present paper. However, the water sampler was submerged to about 5 m depth due to sea surface rough conditions at SST-2 and PN-5 during the winter cruise and these waters were used as the samples for 100% of I0 . Samples were drained from the water sampler into 600 ml polycarbonate bottles (three bottles for light incubation and one for dark incubation). After addition of 13C-NaHCO3 (13C concentration in the dissolved inorganic carbon was about 10 at.-%), the bottles were incubated in an on-deck incubator filled with circulating sea water under the corresponding light intensity, which was regulated by neutral density filters. Incubation experiments were started just before sunrise and continued for 24 h. Bottles were placed in the dark at the end of the incubation period and particulate matter was filtered onto precombusted (450°C, 4 h) glass fiber filters (Whatman GF/F) under dim conditions. It took less than 1 h to filter all samples. The concentration and 13C at.-% of particulate organic carbon were determined by a mass spectrometer (ANCAMS, Europe Scientific), and the productivity was calculated after the method of Hama et al. (1983); isotope discrimination was not considered in this study. The primary productivity was corrected for the rate obtained from the dark bottle. The mean value of three light incubation bottles is given in this study. The percentage coefficient of variation of the three incubation bottles ranged from 1.8 to 22% in this study and was less than 10% in the majority (78%) of the samples. This variability among incubation bottles was almost comparable with the results of the 13C tracer method applied previously (e.g., Hama et al., 1991). A trapezoidal integration was applied to calculate primary productivity throughout the euphotic zone. 3. Results 3.1 Physicochemical conditions along PN line The detailed physicochemical conditions during the three cruises were shown elsewhere (Watanabe et al., 1995). We thus confine ourselves here to a brief description, including water temperature, nitrate concentration and light conditions.

Fig. 2. Distribution of water temperature (a) and concentration of nitrate (b) along PN line during the winter cruise (K92-09).

3.1.1 Winter cruise Surface water temperature in the Kuroshio Current (SST-2) was higher than 22°C even in winter, and the thermocline is found below 150 m depth (Fig. 2a). Water temperature decreased rapidly from the shelf edge (PN-5) to the shelf area. No thermocline was found in the shelf area, attesting to the vigorous vertical mixing of water masses on the continental shelf. Nitrate was almost undetectable in the surface layer of the Kuroshio Current (Fig. 2b), but nitrate concentrations of greater than 1 µM were measured even at the surface at the shelf edge station. Increased concentrations were found throughout the shelf area: up to 5.8 µM at PN-12. The minimum of the nitrate concentration (1.3 µM) was found at PN-8. The euphotic depth (1% of I0), which was 80 m in the Kuroshio Current (SST-2) and at the shelf edge (PN-5) stations, tended to be shallower toward the shelf area. High turbidity was observed at PN-12, resulting in a rapid decrease in light intensity. Thus, the euphotic depth at this station was quite shallow (7 m). 3.1.2 Summer cruise Surface water temperature increased to higher than 28°C from PN-1 to PN-10 during the summer cruise (Fig. 3a). Relatively lower water temperatures of less than 27°C

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43

Fig. 3. As Fig. 2 but during summer cruise (K94-04).

Fig. 4. As Fig. 2 but during fall cruise (K93-05).

were observed at the surface of PN-12, indicating the effect of river water. This is supported by the lower salinity in the surface layer at PN-12 (31.6 PSU) compared with values at the surface at PN-1 through PN-10 (33.9–34.4 PSU); the detailed distribution of salinity is not shown in this study. A seasonal thermocline developed throughout the PN line, and the depth of the thermocline became shallower toward the Changjiang estuary. The concentration of nitrate was not measurable in the surface layer throughout the PN line, and the observed nitracline coincides with the thermocline (Fig. 3b). The euphotic depths were 80–100 m at PN-3 and 5. At shelf stations, on the other hand, shallower euphotic depths (18–62 m) were observed. Except at PN-12, the euphotic depths were deeper than the nitracline, which probably resulted in the development of a subsurface chlorophyll maximum (SCM), as will be shown below. 3.1.3 Fall cruise The surface water temperature at the Kuroshio Current was still high (more than 26°C) but it decreased to 25°C at PN-5 (Fig. 4a). At the shelf area, water temperature gradually decreased from the shelf edge to PN-12. A distinct thermocline was observed from PN-1 to PN-5, whereas the thermocline became indistinct at the shelf station. Only a small difference of temperature (2°C) was observed between the surface and bottom water at PN-12. Nitrate concentration in the surface water was nearly undetectable

through the PN line except at the station between PN-8 and 10 (Fig. 4b). A small amount of nitrate (0.25 µM) was measured even at the surface at PN-12. The euphotic depth in the off-shelf area and at shelf edge stations was about 70 m, whereas it ranged from 23 m (PN-12) to 32 m (PN-8) in the on-shelf area.

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3.2 Chl.-a and primary productivity Chl.-a concentration and the primary productivity in winter are summarized in Table 2. Chl.-a concentration did not show any distinctive spatial variability throughout the PN line, from 0.44 to 0.70 mg m–3 at the surface water. Chl.a was evenly distributed throughout the euphotic zone, except at PN-5. Primary productivity showed maximum values at the surface layer except at PN-5, where the highest productivity was measured in a 9% I0 layer. The highest primary productivity at the surface was found at the center of the East China Sea (PN-8) where it was 2.8 times higher than that at the Kuroshio Current (SST-2). At PN-12, light intensity decreased rapidly with water depth due to high turbidity. Thus, the primary productivity at this station showed a rapid decrease with water depth. In summer, SCM was generally observed throughout the stations, except at PN-12 (Table 3). This SCM generally developed at depths of a few percent of I0 , and its depth

Table 2. Concentration of Chlorophyll a, productivity and Chl. a specific productivity in the winter cruise (K92-09). Station

Depth (m)

Chl. a (mg m –3 )

Productivity (mgC m –3 d–1 )

Chl. a specific productivity (mgC mgChl. a–1 d–1 )

SST-2

5 10 25 35 50 80 0–80*

0.44 0.43 0.44 0.44 0.47 0.43 36

7.9 6.9 4.0 3.0 1.9 0.52 270

18 16 9.1 6.8 4.0 1.2 7.5

PN-5

5 10 25 35 50 80 0–80*

0.44 0.43 0.40 0.40 0.11 0.10 22

2.4 2.7 4.2 5.8 2.1 0.38 220

5.5 6.3 11 15 19 3.8 10

PN-8

0 8 15 20 30 45 0–45*

0.70 0.70 0.75 0.78 1.0 0.90 38

22 22 21 16 4.6 1.6 570

31 31 28 21 4.6 1.7 15

PN-12

0 1 2 3 4 7 0–7*

0.54 0.50 0.51 0.50 0.51 0.50 3.6

14 15 12 11 9.3 2.8 68

26 30 24 22 18 5.6 19

*Depth-integrated values for Chl. a (mg m–2), productivity (mgC m–2d–1) and Chl. a specific productivity (mgC mgChl. a–1d–1).

corresponded to the nitracline where the concentration of nitrate increased. Subsurface maxima in productivity were also observed at these four stations. At the Kuroshio Current and shelf edge stations (PN-3 and 5), the productivities at the SCM layer were almost comparable with those observed in the surface layer. On the other hand, the productivity in the SCM layer were highest throughout the water column at PN8 and 10, accounting for values more than twice as high as the rates at the surface layer. The concentration of Chl.-a at PN-12 in summer showed the highest value throughout three cruises, the concentration at the surface being 47 times higher than at the adjacent station (PN-10). The profile of Chl.-a concentration at PN12 was different from those observed in other stations in this season; the highest concentration was noticed in the surface layer. The euphotic depth at PN-12 was shallower than the nitracline and this probably related to little development of

SCM at PN-12. The primary productivity per water volume also showed the highest rate at PN-12 throughout the observation period. The Chl.-a specific productivity, however, was generally lower than values obtained at other stations in this season. Chl.-a concentration in fall showed spatial variability (Table 4). Concentrations in the off-shelf area (PN-1 and 3) and at the shelf edge (PN-5) were low, ranging from 0.11 to 0.39 mg m–3 and 0.30 to 0.57 mg m–3, respectively. At the shelf area, on the other hand, Chl.-a concentrations higher than 1 mg m–3 were found at PN-8 and 12. Relatively low concentrations were measured at PN-10 compared with PN8 and 12. Primary productivity in the off shelf area in fall were almost comparable with those in winter, with rates ranging from 4.0 to 5.6 mgC m–3d–1 in the surface water. High primary productivity was found in the shelf area. At PN-8 and

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45

Table 3. As Table 2 but in summer cruise (K94-04). Station

Depth (m)

Chl. a (mg m –3 )

Productivity (mgC m –3 d–1 )

Chl. a specific productivity (mgC mgChl. a–1 d–1 )

PN-3

0 17 35 50 65 97 0–97*

0.12 0.066 0.092 0.13 0.58 0.13 21

6.3 4.3 2.9 2.5 5.6 0.13 350

53 65 32 19 9.7 1.0 17

PN-5

0 15 27 40 53 80 0–80*

0.12 0.12 0.15 0.30 0.57 0.25 23

4.3 6.1 3.1 3.1 4.7 0.68 300

36 51 21 10 8.2 2.7 13

PN-8

0 10 23 32 45 62 0–62*

0.28 0.23 0.35 0.42 0.70 0.25 25

8.2 9.5 11 11 20 2.5 710

29 41 31 26 29 10 28

PN-10

0 10 21 31 40 44 0–44*

0.30 0.26 0.30 0.89 0.70 0.67 22

9.8 11 9.8 28 26 4.4 710

33 42 33 31 37 6.6 32

PN-12

0 3 5 8 10 18 0–18*

220 150 160 91 28 3.6 1500

16 21 23 7.0 4.7 3.0 13

14 7.0 7.1 13 6.0 1.2 120

*Depth-integrated values for Chl. a (mg m–2), productivity (mgC m–2d–1) and Chl. a specific productivity (mgC mgChl. a–1d–1).

12, especially, the rate at the surface increased to more than 80 mgC m–3d–1. As was observed with the Chl.-a concentration, the primary productivity at PN-10 was lower than those at other shelf stations, and the rate at the surface accounted for only 12 to 19% of the values at PN-8 and 12. 3.3 Depth-integrated productivity Depth-integrated primary productivity along the PN line during three cruises clearly demonstrated the distinctive spatial and seasonal variability, and ranging from 68–570, 46

T. Hama et al.

300–1,500 and 220–1,100 mgC m–2d–1 in winter, summer and fall, respectively (Tables 2–4). The highest primary productivity in winter was found in the center of the shelf (PN-8), being about 2.5 times higher than those in the Kuroshio Current (SST-2) and the shelf edge station (PN-5). The primary productivity at PN12, which was the nearest station to the Changjiang river mouth, was quite low (68 mgC m–2d–1). This was primarily due to the fact that the radiation decreased rapidly with water depth.

Table 4. As Table 2 but in fall cruise (K93-05). Station

Depth (m)

Chl. a (mg m –3 )

Productivity (mgC m –3 d–1 )

Chl. a specific productivity (mgC mgChl. a–1 d–1 )

PN-1

0 8 20 40 50 70 0–70*

0.23 0.23 0.23 0.25 0.25 0.37 18

5.6 5.6 5.6 4.2 3.6 0.77 290

25 24 25 17 15 2.1 16

PN-3

0 7 20 40 50 70 0–70*

0.21 0.11 0.19 0.19 0.18 0.39 14

4.0 6.2 3.8 2.7 2.2 0.48 220

19 55 20 14 12 1.3 16

PN-5

0 5 15 35 45 65 0–65*

0.53 0.55 0.57 0.27 0.57 0.30 30

15 14 13 6.1 4.5 1.2 510

28 25 23 23 7.9 4.0 17

PN-8

0 4 7 14 20 32 0–32*

1.2 1.4 1.2 1.1 1.1 0.53 34

81 95 56 36 5.6 1.7 1100

68 68 47 33 5.1 3.1 32

PN-10

0 4 7 15 20 30 0–30*

0.61 0.67 0.65 0.70 0.87 0.58 21

15 17 13 13 7.8 2.4 320

25 25 20 19 9.0 4.1 15

PN-12

0 4 7 10 14 23 0–23*

2.1 2.1 1.8 1.1 1.3 0.57 32

120 110 56 27 19 1.2 1000

57 52 31 25 15 2.1 31

*Depth-integrated values for Chl. a (mg m–2), productivity (mgC m–2d–1) and Chl. a specific productivity (mgC mgChl. a–1d–1).

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In contrast to the extremely low primary productivity at PN-12 in winter, the highest primary productivity (1,500 mgC m–2d–1) was found at PN-12 in summer; this rate was 22 times higher than that in winter. The rates at other shelf stations (PN-8 and 10) were comparable, and the contributions of phytoplankton of SCM were high at those two stations. In fall, primary productivity higher than 1,000 mgC m–2d–1 was found at PN-8 and 12. Whereas the rate at PN-10 was low compared with other shelf stations, being almost comparable with the rates at off-shelf stations. In the Kuroshio Current (SST-2 and PN-3) and its adjacent area (PN-1), low primary productivity ranging from 220 to 350 mgC m–2d–1, was noticed in all seasons. Although seasonal variability was less extensive than in the shelf area, the rate obtained in summer was about 1.5 times higher than the rates in winter and fall. 4. Discussion 4.1 Difference in primary productivity between shelf and off-shelf waters Fifteen 13C uptake experiments were carried out along the PN line during three cruises for the MASFLEX study. Eight of these experiments were done inside the continental shelf (PN-8, 10 and 12), three at the shelf edge (PN-5), and the remaining four in the off-shelf area (mainly at the Kuroshio Current). Depth-integrated productivities in the shelf area are considerably higher than in other areas (Table 5). The mean primary productivity obtained from the shelf area throughout the observation was 750 (±450 as 1σ) mgC m–2d–1, whereas the rates obtained from the off-shelf and from the shelf edge station were 280 (±54) and 340 (±150) mgC m–2d–1, respectively. The average primary productivity within the shelf area was 2.2 and 2.7 times higher than those in shelf-edge and off-shelf areas, respectively. The differences in the coefficient of variation for all three areas (Table 5) reflect the extent of the seasonal variability of primary productivity at each area. The low ratio in the off-shelf area shows the lower variability of primary productivity in the Kuroshio Current. This was primarily due to the small seasonal change in the environmental factors affecting the primary productivity such as

nutrient concentration and light conditions in the Kuroshio Current. Fei et al. (1987) also reported a low seasonal variability in the primary productivity in the Kuroshio Current within the range form 100 to 200 mgC m–2d–1. However, the rates obtained in this study in the Kuroshio Current were considerably higher than the rates found by Fei et al. (1987). This may be partly due to the “clean” technique used in this study. Such higher measured productivity due to improved protocols has been suggested in oligotrophic waters (Laws et al., 1987; Marra and Heinemann, 1987). In the shelf area, on the other hand, the high seasonal variability of primary productivity is confirmed by the high SD of productivity, suggesting that the variability of the environmental factors to highly extent. An extremely high temporal change in productivity was found at PN-12, the nearest station to the Changjiang river mouth. The high primary productivity found at the surface layer at PN-12 in summer coincided with the low salinity water mass. The observed low salinity was evidence of a flow of fresh water from the Changjiang, which reportedly contains high concentration of inorganic nutrients (Edmond et al., 1985; Xiuren et al., 1988). Thus, the high productivity at PN-12 in summer resulted from the nutrient supply from the Changjiang. However, a low concentration of nitrate was measured in the surface layer at PN-12 in contrast to the high phytoplankton biomass, indicating that the supplied nutrient had almost been consumed by phytoplankton at the time of our observation. This indicates that the direction of the plume and/or the amount of the flow from the Changjiang changes with time in the estuary area, and this results in the high temporal and spatial changes in the primary productivity in the estuary area through the variability of nutrients supply. In winter, on the other hand, high turbidity due to suspended particles originating from the Changjiang and the vertical mixing throughout the water column (Xiuren et al., 1988) reduced the availability of light at PN-12, and this resulted in the low primary productivity, as reported for the estuaries of major world rivers, such as Amazon (DeMaster and Pope, 1996) and Zaire (Cadee, 1978). Xiuren et al. (1988) obtained a low productivity values (5–40 mgC m–2d–1) within a 100 km area from the river mouth of the Changjiang. Although the productivity in winter at PN-12, obtained in the present study, was much lower than the values at other

Table 5. Mean values of depth-integrated Chl. a, productivity and Chl. a specific productivity in off-shelf (PN-1, PN-3 and SST-2), shelf edge (PN-5), and shelf (PN-8, PN-10 and PN-12) stations in three cruises. Standard deviations are shown in the parenthesis.

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Area

Chl. a (mg m –2 )

Productivity (mgC m –2 d–1 )

off-shelf ( n = 4) shelf edge ( n = 3) shelf ( n = 8)

22 (±9.6) 25 (±4.4) 37 (±35)

280 (±54) 340 (±150) 750 (±450)

Chl. a specific productivity (mgC mgChl. a–1 d–1 ) 14 (±4.4) 13 (±3.5) 23 (±8.4)

stations, the result of Xiuren et al. (1988) indicated that productivity in the area nearer to the river mouth than PN12 could be lower than that at PN-12. The results obtained from PN-12 in our study clearly showed that primary productivity in the Changjiang estuary is greatly affected by the discharge from the Changjiang due to the availability of nutrients and light. In order to evaluate primary productivity in the East China Sea through four seasons, the estimation of the rate in spring will be important. In our summer cruise we found that nutrients (especially nitrate in the euphotic zone), which were abundant in winter, were already exhausted. This indicates that inorganic nitrogen was consumed during the spring. Although a brief study has reported the spatial distribution of Chl.-a in spring (Zunle, 1991), spring primary productivity has not been elucidated. An accurate estimation of primary productivity is therefore needed in spring to evaluate annual productivity in the East China Sea. 4.2 Relation between Chl.-a and productivity in the euphotic zone The depth-integrated productivity obtained in this study showed a positive relationship with depth-integrated Chl.-a concentration through the euphotic zone (Fig. 5) and this indicates that the water column productivity depended primarily on phytoplankton biomass. However, the Chl.-a specific productivity in the euphotic zone was not constant among three areas and the Chl.-a specific productivity obtained from the shelf area was considerably higher than

Fig. 5. Relationship between Chl.-a standing stock and depthintegrated productivity of the euphotic zone in off-shelf, shelf edge and shelf waters.

those from off-shelf and shelf edge areas (Table 5). The regional differences in the Chl.-a specific productivity from shore to off-shore region were also reported by Eppley et al. (1985) who used the Chl. concentration at top attenuation depth (1% depth of the I0 being 4.6 optical depth) of the euphotic zone instead of using the depth-integrated value throughout the euphotic zone used in this study. Incubation experiments in this study were carried out under natural light conditions and it is possible that the rates obtained were affected by the prevailing weather conditions. Chl.-a specific productivities through the euphotic zone are plotted against the surface photon flux during each incubation experiment (Fig. 6). The Chl.-a specific productivity values lower than 19 mgC mgChl.-a–1d–1 obtained from 11 stations tended to increase with surface photon flux, indicating that the Chl.-a specific productivity values at these 11 stations were likely affected by the daily irradiance. In the Southern California Bight, Eppley et al. (1985) observed that the ratio of water column productivity/near surface Chl.-a correlated well with the daily irradiance at irradiance levels less than 50 E m–2d–1 and they concluded that the ratio was primarily limited by the surface irradiance. The relation between Chl.-a specific productivity lower than 19 mgC

Fig. 6. Relationship between surface irradiance during the incubation experiments and the Chl.-a specific productivity of the euphotic zone in off-shelf, shelf edge and shelf waters. The regression line shows the relationship between the surface irradiance and Chl.-a specific productivity lower than 19 mgC mgChl.-a–1d –1. The Chl.-a specific productivities higher than 28 mgC mgChl.-a–1 d–1 obtained from four stations (PN-8 and 10 in summer and PN-8 and 12 in fall) are not included in the regression line.

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mgChl.-a–1d–1 and daily irradiance obtained in this study, was, thus, comparable with the result observed in the southern California Bight (Eppley et al., 1985). As is clearly shown in Fig. 6, four plots of Chl.-a specific productivity higher than 28 mgC mgChl.-a–1d–1 deviated greatly from the relation obtained from the stations with the Chl.-a specific productivity lower than 19 mgC mgChl.-a–1d–1. These high Chl.-a specific productivity values were measured at shelf stations in summer and fall (PN-8 and 10 in summer and PN-8 and 12 in fall). The surface irradiance during the incubation experiments at these four stations was within the range recorded at the other 11 stations where the relation between the Chl.-a specific productivity and the surface irradiance was noticed. Thus, other factors than irradiance probably affected the Chl.-a specific productivity in the euphotic zone. High rates at PN-8 and 10 in summer were mainly due to the high Chl.-a specific productivity at SCM layer (Table 3). Although SCM were observed at off-shelf stations (PN3 and 5) as well as at PN-8 and 10, Chl.-a specific productivity values in the SCM layer of PN-3 and 5 were much lower than the rate in the surface layer of each station. By contrast, the Chl.-a specific productivities in the SCM layer at PN-8 and 10 were almost comparable with those in the surface layer, showing the high photosynthetic activity of the SCM layer. Thus, the high Chl.-a specific productivity through the euphotic zone at PN-8 and 10 can be attributed to high ratios in the SCM layer, though the ratios in the surface layer at these two stations were comparable to or lower than those at off-shelf (PN-3) and shelf-edge (PN-5) stations. It is possible that the difference of the activity in the SCM layer between off-shelf and shelf stations was partly related to the nutrient availability in the SCM layer. SCMs were generally found at 4% depth of I0 from PN-3 to PN-10 in summer (at PN-10, a high Chl.-a concentration was also found at 9% depth of I0) as shown in Table 3. The concentrations of nitrate at 4% depth of I0 at off-shelf and shelf edge stations were quite low (Fig. 3b). At PN-8 and 10, on the other hand, the 4% depth of I0 coincided with the nitracline, indicating higher nutrient availability compared with the SCM layer at PN-3 and 5. Considering that high nutrient availability caused the higher Chl.-a specific productivity (Takahashi et al., 1973; Malone, 1977), the high Chl.-a specific productivity in the SCM layer at PN-8 and 10 in summer likely reflected the high nutrient availability. The high Chl.-a specific productivity more than 31 mgC mgChl.-a–1d–1 through the euphotic zone at PN-8 and 12 in fall attributed to the high Chl.-a specific productivity in the surface layer and not to the subsurface layer as observed in summer. The Chl.-a specific productivity observed in the 100 and 45% layer of I0 at PN-8 and 12 ranged from 52 to 68 mgC mgChl.-a–1d–1, which is considerably higher than the values in the surface layer at off-shelf and

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shelf edge stations (Table 4). During the fall cruise, a small amount of inorganic nitrogen was found at the surface of offshelf stations, in contrast to the nitrate and ammonium measured at the surface of PN-8 and 12, which ranged form 0.10 to 0.37 µM (nitrate plus ammonium). This indicates that the higher Chl.-a specific productivity at PN-8 and 12 reflects the higher nutrient availability at the surface layer of both stations, as is also suggested for SCM layer of PN-8 and 10 in summer. It is notable that Chl.-a specific productivities obtained from 100 and 45% layer of I0 at PN-10, which is located between PN-8 and 12, were low and almost comparable with those at off-shelf stations. The relation between salinity and water temperature at PN-10 indicates that the water mass originated form the south (Kondo, 1985; Watanabe et al., 1995), probably representing a branch of the Taiwan Warm Current (Chen et al., 1994). The water mass from the south at PN-10 was characterized by lower nutrient concentrations than the surrounding waters. The concentration of silicate was much lower at PN-10 (0.85 µM at the surface) than the representative high concentration of the coastal current at PN-8 and 12 (3.6 and 8.3 µM, respectively). Although the concentrations of inorganic nitrogen were measurable at PN-8 and 12, as mentioned above, no inorganic nitrogen was detected at the surface of PN-10. Thus, the low Chl.-a specific productivity at PN-10, as well as low concentrations of Chl.-a and primary productivity compared with surrounding stations in fall (Table 4), was probably due to the low nutrient availability at this station. 5. Conclusion 13 C tracer experiments in the present study revealed that the carbon uptake rate in the East China Sea was an average of 2.7 times higher than in the Kuroshio Current, indicating the importance of the East China Sea as the source of organic matter to the surrounding open ocean. However, Furuya et al. (personal communication), in experiments using the dissolved oxygen method, suggested that the community production in the East China Sea is not very high; the organic matter produced is almost completely decomposed within the euphotic zone. Studies including estimations of the decomposition rate of organic carbon in the water column by heterotrophic organisms, and of the carbon flux to sea floor in the East China Sea and the Okinawa Trough, etc., in the MASFLEX project, can afford significant information on the importance of the East China Sea to the carbon cycle in the Western Pacific. Acknowledgements This work was supported by Special Coordination Funds for Promoting Science and Technology of the Science and Technology Agency of Japan. The authors wish to thank the scientists on board the K92-09, K93-05 and K94-04 for their cooperation and valuable discussions, as well as the

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