Seasonal variations in nutrients and chlorophyll-a ... - Springer Link

Ocean Science Journal, Vol. 41 No. 3, 125-137(2006)

Article

http://osj.kordi.re.kr

Seasonal Variations in Nutrients and Chlorophyll-

a Concentrations in the

Northern East China Sea

Dongseon Kim*, JeongHee Shim, and Sinjae Yoo

Marine Environment Research Department, KORDI, Ansan P.O. Box 29, Seoul 425-600, Korea

Received 24 July 2006; Revised 4 September 2006; Accepted 26 September 2006

Abstract − Nutrients, chlorophyll-a, particulate organic carbon

(POC), and environmental conditions were extensively investigated in the northern East China Sea (ECS) near Cheju Island during three seasonal cruises from 2003 to 2005. In spring and autumn, relatively high concentrations of nitrate (2.6~12.4 µmol kg− ) and phosphate (0.17~0.61 µmol kg− ) were observed in the surface waters in the western part of the study area because of the large supply of nutrients from deep waters by vertical mixing. The surface concentrations of nitrate and phosphate in summer were much lower than those in spring and autumn, which is ascribed to a reduced nutrient supply from the deep waters in summer because of surface layer stratification. While previous studies indicate that upwellings of the Kuroshio Current and the Changjiang (Yangtze River) are main sources of nutrients in the ECS, these two inputs seem not to have contributed significantly to the build-up of nutrients in the northern ECS during the time of this study. The lower nitrate:phosphate (N:P) ratio in the surface waters and the positive correlation between the surface N:P ratio and nitrate concentration indicate that nitrate acts as a main nutrient limiting phytoplankton growth in the northern ECS, contrary to previous reports of phosphate-limited phytoplankton growth in the ECS. This difference arises because most surface water nutrients are supplied by vertical mixing from deep waters with low N:P ratios and are not directly influenced by the Changjiang, which has a high N:P ratio. Surface chlorophyll-a levels showed large seasonal variation, with high concentrations (0.38~4.14 mg m− ) in spring and autumn and low concentrations (0.22~1.05 mg m− ) in summer. The surface distribution of chlorophyll-a coincided fairly well with that of nitrate in the northern ECS, implying that nitrate is an important nutrient controlling phytoplankton biomass. The POC:chlorophyll-a ratio was 4~6 times higher in summer than in spring and autumn, presumably because of the high summer phytoplankton death rate caused by nutrient depletion in the surface waters. 1

1

3

3

*Corresponding author. E-mail: [email protected]

Key words − nutrients, chlorophyll-a, POC, seasonal variations,

East China Sea

1. Introduction The East China Sea (ECS), the largest marginal sea in the western North Pacific, includes a large area of shallow continental shelf with enormous inputs of terrestrial nutrients. The ECS is bounded to the east by the Kuroshio Current, and there is extensive exchange between the East China Sea and the Kuroshio through frontal processes (Chern . 1990; Chen 1995). The sea is bounded to the west by continental China and receives tremendous river runoff from the Changjiang, which has an annual mean discharge of 924 km yr− (Tian . 1993). The runoff from the Changjiang shows large seasonal variation, with a maximum in summer and a minimum in winter (Beardsley 1985). The river discharge forms a water type of the Changjiang diluted water (CDW) by mixing with saline ambient waters. In winter, the CDW flows southward along the Chinese coast in a narrow band because of the low river discharge and the prevailing northeasterly wind. In summer, however, the CDW propagates across the shelf because of the combined effect of high discharge and the prevailing southerly wind. During this time, the CDW covers most of the northern part of the ECS (Le 1988; Hu 1994; Su and Weng 1994). Annual fluxes of nitrate, phosphate, and silicate from the Changjiang to the ECS are approximately 6×10 , 1.3×10 , and 12×10 mol, respectively (Edmond . 1985). The et al

et al.

3

1

et al

et al.

10

10

et al

9

126

Kim, D. et al.

river water has very high concentrations of nitrate and silicate but a low concentration of phosphate. Thus, the nitrogen: phosphorus (N:P) ratio of the river water is much higher than the Redfield ratio of 16 (Tang . 1990). In general, phytoplankton growth is limited by phosphate in freshwater environments and by nitrate in marine and estuarine environments (Raymont 1980). Since the Changjiang, with its extremely high N:P ratio, is a main source for nutrients in the ECS, however, previous studies have suggested that phosphate is a limiting nutrient to phytoplankton growth in the ECS (Harrison . 1990; Wong . 1998; Chen . 1999; Gong . 2003). The ECS is a highly valuable fishing ground owing to its extensive continental shelf area, and many studies have examined the distributions of water types, nutrients, chlorophyll- , and primary production in this area (Gong . 1996; Hama . 1997; Wong . 1998; Gong . 2003; Wang . 2003; Chen . 2004). In the Chinese coastal areas near the Changjiang, surface waters are nutrient rich, and therefore have high concentrations of chlorophyll- and high primary production rates. Farther from the coastal areas, chlorophyll- and primary production levels gradually decrease because of limited nutrient supplies. Primary production in the ECS shows pronounced spatial and seasonal variations (Chen . 2001; Chen and Chen 2003; Gong . 2003). In the shelf area, primary production is low in both summer and winter. Strong stratification in summer prevents deep waters from supplying nutrients to the surface waters, and et al

et al

et al

et al

et al

a

et al

et al

al

et al

et al

et

et al

a

a

et al

et al

Fig. 1.

low irradiance in winter constrains primary production. However, few studies have examined the seasonal variation in nutrients and chlorophyllin the northern ECS near Cheju Island. Most published data have been limited to the central and southern ECS (Chen . 2001; Chen and Chen 2003; Gong . 2003). The Three Gorges Dam is now under construction along the middle portion of the Changjiang and is expected to be operational by 2009. The completion of this dam will have a large impact on the coastal ecosystem, including decreases in nutrient and suspended solids discharges (Humborg . 1997; Milliman 1997). Because of its extensive shelf area, the ECS is also highly susceptible to environmental changes associated with global warming, such as an increase in surface seawater temperature (SST) and sea level rise. In light of these factors, it is essential to conduct long-term monitoring of environmental changes in the ECS. This study aimed to monitor and identify the main factors controlling the spatial distribution and seasonal variations in nutrients and chlorophyllin the northern ECS. a

et al

et al

et al

a

2. Materials and Methods Measurements for this study were performed during three cruises of the R/V in summer (26 August to 3 September 2003), spring (29 April to 8 May 2004), and autumn (31 October to 8 November 2005). The study area was in the northern ECS which covers 31°30~34°0N

Study area and sampling stations (filled circle) in the northern East China Sea.

Eardo

127

Seasonal variations in nutrients and chlorophyll-a

and 124°0~127°30E (Fig. 1). Vertical profiles of temperature and salinity were measured with a SeaBird conductivitytemperature-depth sensor (CTD; SBE 9/11 plus, SeaBird Inc., Bellevue, WA, USA). Seawater samples were collected for the analyses of nutrient, chlorophyll- , and particulate organic carbon (POC) analyses using a Rosette sampler with 10-L Niskin bottles mounted on the CTD assembly at seven water depths (0, 10, 20, 30, 50, 75, and 100 m). Water samples for nutrient analysis were filtered through GF/F filter paper (25 mm, Whatman, Middlesex, U.K.), placed in acid-cleaned polyethylene bottles, and frozen at −20°C. Nitrate+nitrite (hereafter referred to as nitrate), phosphate, and silicate concentrations were measured using a flow injection autoanalyzer (model QuikChem AE, Lachat, Loveland, CO, USA) and standard colorimetric procedures (Strickland and Parsons 1972) and calibrated using brine standard solutions (CSK Standard Solutions, Wako Pure Chemical Industries, Osaka, Japan). Based on duplicate analyses, the precision of the nitrate, phosphate, and silicate measurements was 3, 3, and 5%, respectively. Water a

Fig. 2.

samples for chlorophyll- analysis were filtered through GF/F filter paper (47 mm, Whatman); the filters were then immediately frozen with liquid nitrogen. The chlorophyllconcentration in the extracted filtrate mixed with 90% acetone for 24 h was determined using a Turner-designed fluorometer (10-006R, Turner BioSystems, Sunnyvale, CA, USA). For POC measurements, 1.0-L water samples were filtered onto GF/F filters (25 mm, precombusted at 550°C, Whatman), and the filters were then stored at −20°C. After elimination of remaining inorganic carbon from the filters by fuming with concentrated HCl (Strickland and Parsons 1972), POC concentrations were measured using a Carlo Erba CNS analyzer (precision of < 10%; Milan, Italy). a

a

3. Results and Discussion Seasonal Variations in Nutrients

The surface distributions of salinity, nitrate, phosphate, and silicate in the northern ECS in spring (May) are shown in Fig. 2. The surface salinity increased eastward

Surface distribution of salinity (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) in the northern East China Sea in spring. 1

1

1

128

Kim, D. et al.

from 32.1 to 34.5 psu (Fig. 2a). Wang . (2003) identified ECS water with a salinity of less than 33.0 psu as modified water created by mixing with freshwater from the Changjiang. However, Gong . (1996) defined the CDW as a water mass with a salinity of less than 31 psu. Because the salinities of the Kuroshio and Taiwan warm currents, which are the principal water masses in the ECS, do not drop below 33.7 psu (Su and Weng 1994; Chen . 1995), it is reasonable to define the CDW as water with a salinity of less than 33.0 psu. Water with a salinity of less than 33.0 psu covered the western part of the study area, occupying about one third of the entire study area (Fig. 2a). The surface concentrations of nitrate were very low, less than 1.0 µmol kg− in most of the study area, except in the western part where the nitrate concentration increased gradually westward up to 9.5 µmol kg− (Fig. 2b). The surface distribution of phosphate was rather different from that of nitrate (Fig. 2c). Phosphate concentrations et al

et al

et al

1

1

Fig. 3.

below 0.1 µmol kg− were observed only in the southern part of the study area, and high concentrations above 0.4 µmol kg− were found in the western part. The surface concentrations of silicate were relatively high in the western and northern part and low in the eastern part, with a range of 4.5 to 22.5 µmol kg− (Fig. 2d). In spring, nitrate, phosphate, and silicate concentrations were relatively high in the western part of the study area. Figure 3 illustrates the vertical distributions of water density, nitrate, phosphate, and silicate along transect C in spring. The upper 40 m of the surface layer were well mixed in the study area, except around 125°E where the water density varied from 24.5 at the surface to 25.3 at a depth of 40 m (Fig. 3a). Nutrient concentrations were relatively high and vertically constant west of 125°E (Fig. 3). In contrast, to the east, nutrient concentrations were very low in the upper 30 m and increased gradually below that depth. Waters west of 125°E were fully mixed from the surface 1

1

1

Vertical distribution of density (σ ) (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) along transect C in the northern East China Sea in spring. t

1

1

1

129


Fig. 4.

Surface distribution of salinity (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) in the northern East China Sea in summer. 1

to the bottom because of the shallow water depth (< 50 m). In the bottom waters, nutrient concentrations were generally enriched because of the benthic remineralization of organic matter. Vertical mixing from the surface to the bottom, therefore, resulted in high nutrient concentrations in surface waters in spring. In summer (August), surface salinities were significantly reduced compared with those in spring, ranging from 27.7 to 32.8 psu (Fig. 4a). Less saline surface waters (< 29.0 psu) occurred in a band in the western part of the study area. The surface waters in the eastern part (the Kuroshio water area) were also influenced by the CDW, with salinities reduced to less than 33.0 psu. Although all the surface water was influenced by CDW, nitrate concentrations were not enriched in the surface waters and were relatively low, i.e., 0.13~3.2 µmol kg− (Fig. 4b). The surface distribution of nitrate did not mirror that of salinity. Surface phosphate concentrations were also relatively low, ranging from 0.08 to 0.25 µmol kg− (Fig. 4c). The surface concentrations of 1

1

1

1

silicate were also relatively low compared with those in spring, ranging from 4.0 to 12.6 µmol kg− (Fig. 4d). Wang . (2003) found a linear relationship between dissolved inorganic nitrogen concentrations and salinity in surface waters with a salinity of less than 31.0 psu in summer, indicating that dissolved inorganic nitrogen was conserved during the mixing of Changjiang water with seawater. In our study area, however, salinity did not have a distinct relationship with any nutrients in surface waters with a salinity of less than 33.0 psu in summer (Fig. 5); this result indicates that nutrients were not conservative during the mixing of the Changjiang with the seawater. Because our study area was at least 260 km from the mouth of the Changjiang, it would take more than 10 days for the river plume to reach the study area (Lie . 2003). High integrated productivity values were observed in summer in the coastal area near the Changjiang with less saline and nitrate-enriched surface waters (Gong . 2003). Apparently, the enhanced growth of phytoplankton is 1

et al

et al

et al

130

Fig. 5.

Fig. 6.

Kim, D. et al.

Salinity vs. nitrate (a), phosphate (b), and silicate (c) in the surface waters in summer.

Vertical distribution of density (σ ) (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) along transect C in the northern East China Sea in summer. t

expected to occur in the river plume in the coastal area near the Changjiang, resulting in a large uptake of nutrients. Thus, even though the river plume was transported from the river mouth to our study area, the plume nutrients may have already been exhausted by biological uptake. This was a main reason why nutrients were not conserved during the mixing of Changjiang water with seawater in

1

1

1

our study area. In summer, the surface water was well stratified, with a large density gradient in the upper 20 m because of the influence of the Changjiang (Fig. 6a). Vertical mixing between surface and deep waters was thus completely restrained, suppressing the transfer of nutrients from deep to surface waters. Nitrate, phosphate, and silicate concentrations had steep gradients in the upper 20 m

131


Fig. 7.

Surface distribution of salinity (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) in the northern East China Sea in autumn. 1

1

(Fig. 6), resulting from the completely reduced vertical mixing in the upper layer. In our study area, therefore, the very low concentrations of nutrients in the surface waters in summer are attributed to the reduced supply of nutrients from the Changjiang and deep waters. In autumn (November), the surface salinity increased eastward, ranging from 30.3 to 34.3 psu (Fig. 7a). Surface water with a salinity of less than 33.0 psu covered the northwestern part or about half of the study area. Surface nitrate concentrations were greatest, i.e., 12.4 µmol kg− , at the westernmost extent of the study area and decreased gradually eastward to below 1.0 µmol kg− (Fig. 7b). The nitrate 1.0 µmol kg− isopleth spanned the study area in a north to south direction. The surface distribution of nitrate was rather different from that of salinity. Low phosphate concentrations below 0.1 µmol kg− were only observed in the eastern part of the study area, the region influenced by the Kuroshio; high concentrations above 0.3 µmol kg− 1

1

1

1

1

1

were found in the western and southern parts of the study area (Fig. 7c). Surface silicate concentrations were distributed similarly to surface nitrate, with high concentrations in the westernmost part of the study area and decreasing gradually eastward (Fig. 7d). Nitrate, phosphate, and silicate concentrations in autumn were relatively high in the western part of the study area, as in spring. Water density and nitrate, phosphate, and silicate concentrations in autumn also exhibited similar vertical distributions to those observed in spring (Fig. 8). The upper 50 m of the surface layer were well mixed across the entire study area, showing a vertically uniform distribution of water density (Fig. 8a). The maximum concentrations of nutrients were found in the westernmost part of the study area and decreased gradually eastward, with vertically uniform distributions in the upper 50 m (Fig. 8). Nutrient concentrations in the upper 50 m were very low east of 126°E, where waters were influenced by the Kuroshio (Fig. 8). As in

132

Fig. 8.

Kim, D. et al.

Vertical distribution of density (σ ) (a), nitrate (µmol kg− ) (b), phosphate (µmol kg− ) (c), and silicate (µmol kg− ) (d) along transect C in the northern East China Sea in autumn. 1

t

spring, waters at all depths were fully mixed in the west because of the shallow water depth (< 50 m). Therefore, vertical mixing in the west also resulted in high surface water nutrient concentrations in autumn. In the three seasons of observations, surface water nutrient concentrations were not significantly influenced by the Changjiang, which has been considered a main source of nutrients for the ECS (Gong . 1996; Wong . 1998; Gong . 2003; Wang . 2003). et al

et al

et al

et al

Nutrients Limiting Phytoplankton Growth

Many researchers have suggested that phosphorus is a limiting nutrient in the ECS. Harrison . (1990) postulated that phosphorus was a limiting element for primary production in the ECS where salinity was less than 32 psu. Wong . (1998) reported excess nitrate up to 6 µM and a phosphate concentration below 0.07 µM in an area covering a third to a half of the ECS. Chen . et

al

et al

et al

1

1

(1999) proposed that nitrate was not taken up by phytoplankton despite a profound nitrate abundance, and phosphorus insufficiency was considered to limit the nitrate uptake because of a high N:P ratio (213) and a low phosphate concentration (0.03 µM). Gong . (2003) suggested that the rate of primary production in the ECS was regulated by the availability of nutrients, especially phosphate, from summer to autumn. To determine nutrient limitations on phytoplankton growth, surface N:P ratios were calculated (Fig. 9). The N:P ratios in the study area in all three seasons were much lower than the Redfield ratio (16), except in the western part of the study area in autumn. In summer, especially, the surface N:P ratios were less than 10 for the entire study area (Fig. 9b), with an average of 5.7, only a third of the Redfield ratio. In the marine environment, an N:P ratio substantially lower than 16 is considered to indicate nitrogen limitation of phytoplankton growth (Raymont 1980; Hecky and Kilham 1988; Kilham et al

133


Fig. 10.

Fig. 9.

Surface distribution of N/P ratio in spring (a), summer (b), and autumn (c).

and Heckey 1988). Thus, in the present study area, phytoplankton growth appears to be limited by nitrogen deficiency in the three seasons examined, except for the western part of the study area in autumn. The surface N:P ratios were plotted against the surface nitrate and phosphate concentrations (Fig. 10). The surface N:P ratios were positively correlated with surface nitrate concentrations in all three seasons, and were more closely correlated with nitrate concentrations below 6.0 µmol kg− (Fig. 10a). In particular, for surface nitrate concentrations below 2.0 µmol kg− , the surface N:P ratios were always

1

1

N/P ratio vs. nitrate (a) and phosphate (b) in the three seasons.

much lower than the Redfield ratio, implying that phytoplankton growth was constrained as nitrate concentrations dropped below 2.0 µmol kg− . The surface N:P ratios were not correlated with surface phosphate concentrations (Fig. 10b). Phytoplankton growth in the study area was therefore likely constrained by nitrogen deficiency rather than phosphorus deficiency. The lower N:P ratio in the surface waters and the positive correlation between the surface N:P ratio and nitrate concentration suggest that nitrate acts as a main limiting nutrient to phytoplankton growth in the study area. This result contrasts with previous findings in which phosphate limited the growth of phytoplankton in the ECS. The difference is that most surface water nutrients are supplied by vertical mixing with deeper waters having low N:P ratios, not directly by the Changjiang with its high N:P ratio. 1

134

Fig. 11.

Kim, D. et al.

Surface distribution of chlorophyll-a (mg m− ) in spring (a), summer (b), and autumn (c). 3

Seasonal Variations in Chlorophyll-a

Surface chlorophyllconcentrations showed large seasonal variations, with high levels in spring and autumn and low values in summer (Fig. 11). In spring, surface chlorophyllconcentrations exceeded 2.0 mg m in the western and southern parts of the study area (Fig. 11a). In the southern part, where the surface chlorophyllconcentrations were relatively high, concentrations of surface nitrate and phosphate were very low. In the western part, however, chlorophyll- , nitrate, and phosphate all had high concentrations. In a

a

-3

a

a

Fig. 12.

Vertical distribution of chlorophyll-a (mg m− ) along transect C in spring (a), summer (b), and autumn (c). 3

summer, the surface chlorophyllconcentrations were generally low, less than 1.0 mg m− in the entire study area (Fig. 11b). The surface distribution of chlorophyllwas fairly well correlated with that of nitrate (Fig. 4b). Both surface chlorophyll- and nitrate distributions displayed relatively high concentrations in the western part of the study area and decreased gradually eastward. However, the surface distribution of chlorophyllwas not well correlated with that of phosphate (Fig. 4c). In autumn, surface chlorophyllconcentrations were high, i.e., more a

3

a

a

a

a

135


than 3.0 mg m− , in the western part and decreased gradually eastward (Fig. 11c). The surface distribution of chlorophyllwas fairly well matched with those of nitrate and phosphate (Fig. 7). With the exception of the southern part of the study area in spring, the surface distribution of chlorophyll- generally coincided with that of nitrate, implying that nitrate is an important nutrient regulating phytoplankton biomass. Figure 12 shows the vertical distributions of chlorophyllin the three studied seasons. In spring, high concentrations of chlorophyllexceeding 2.0 mg m− were observed at water depths of 10~20 m in the area of 125~125.5°E (Fig. 12a); this area had moderate nitrate concentrations ranging from 1.0 to 3.0 µmol kg− (Fig. 3b). Although high nitrate concentrations were observed from 124 to 124.5°E, chlorophyllconcentrations were moderate, ranging from 1.0 to 2.0 mg m− (Fig. 12a). In the area of 126~127°E, both chlorophyll- and nitrate concentrations were low in the upper 30 m. At 125~125.5°E with high chlorophyllconcentrations, seawater density showed a relatively large vertical variation compared with that in other areas, implying reduced vertical mixing (Fig. 3a). Thus, the high chlorophyllconcentrations at 126~127°E in spring likely resulted from a relatively high stability in the surface layer that prevented phytoplankton from dropping rapidly below the euphotic layer. In summer, high concentrations of chlorophyllexceeding 2.0 mg m− were observed at a depth of 10 m in the western part of the study area (Fig. 12b). The maximum concentration of chlorophyllwas found in increasingly deeper waters with distance eastward, and reached a depth of 30 m in the easternmost part of the study area. The depth at which chlorophyllconcentrations were maximized coincided fairly well with the nitrate 3.0-µmol kg− isopleth (Fig. 6b), implying that the vertical distribution of chlorophyllin summer is mostly determined by the available nitrate. In autumn, high concentrations of chlorophyllwere confined to the upper 10 m in the area of 124.5~125.5°E; at depths greater than 10 m, chlorophyllconcentrations were less than 1.0 mg m− over the entire study area (Fig. 12c). Although nitrate concentrations were high in the western part of the study area, chlorophyllconcentrations were very low, i.e., < 0.5 mg m− , for the entire water column (Fig. 12c). Seawater density showed a relatively large vertical variation in the area of 124.5~125.5°E compared with that in other areas (Fig. 8a). As in spring, therefore, the high chlorophyllconcentrations at 3

a

a

a

3

a

1

a

3

a

a

a

3

a

a

a

1

a

a

a

3

a

3

a

Fig. 13.

POC vs. chlorophyll-a in the surface water in spring (a), summer (b), and autumn (c). POC/Chl ratio is calculated from the slope of the best regression line (R and p value =0.45 and < 0.05 in spring, 0.73 and < 0.01 in summer, 0.68 and < 0.05 in autumn). 2

124.5~125.5°E in autumn were a result of the relative stability of the surface layer. The main factors controlling

136

Kim, D. et al.

the vertical distribution of chlorophyllin the study area varied seasonally; in spring and autumn, the water column stability was the primary main factor, while in summer, the availability of nitrate was the main factor. Chlorophyll- concentrations were well correlated with POC concentrations in surface water (Fig. 13). A regression analysis of POC and chlorophyll- (chl- ) concentrations in surface waters indicated that the POC:chlratio was 49 in spring, 278 in summer, and 69 in autumn. Chang . (2003) estimated that POC:chl- ratios in the ECS vary from 13.0 in coastal waters to 92.8 in offshore waters. In coastal waters of the northeastern ECS, POC:chl- ratios were estimated to be 19~41 (Yamamoto 1995). The POC: chl- ratios estimated in our study area for spring and autumn were similar to previous estimates for the ECS. However, the POC:chlratio estimated for summer was much higher than previous estimates. POC concentrations did not differ considerably in the three seasons and were within the range of 100 to 400 mg m− . However, chlconcentrations were much lower in summer than in spring and autumn (Fig. 13), which explains why the POC:chlratio estimated for summer was much higher than those for spring and autumn. POC includes living and non-living carbon reservoirs, but chl- represents living phytoplankton biomass. The high POC:chlratio in summer, therefore, appears to reflect a high summer phytoplankton death rate caused by nutrient depletion in the surface waters. a

a

a

a

a

et

al

a

a

a

a

3

a

a

a

a

Chen, C.T.A., R. Ruo, S.C. Pai, C.T. Liu, and G.T.F. Wong. 1995. Exchange of water masses between the East China Sea and the Kuroshio off northeastern Taiwan. Cont. Shelf Res., , 19-39. Chen, Y.L., H. Lu, F. Shiah, G. Gong, K. Liu, and J. Kanda. 1999. New production and f-ratio on the continental shelf of the East China Sea: Comparison between nitrate inputs from the subsurface Kuroshio Current and the Changjiang River. Estuarine Coast. Shelf Sci., , 59-75. Chen, Y.L., H.-Y. Chen, W.-H. Lee, C.-C. Hung, G.T.F. Wong, and J. Kanda. 2001. New production in the East China Sea, comparison between well-mixed winter and stratified summer conditions. Cont. Shelf Res., , 751-764. Chen, Y.L. and H.-Y. Chen. 2003. Nitrate-based new production and its relationship to primary production and chemical hydrography in spring and fall in the East China Sea. DeepSea Res. II, , 1249-1264. Chen, Y.L., H.-Y. Chen, G.-C. Gong, Y.-H. Lin, S. Jan, and M. Takahashi. 2004. Phytoplankton production during a summer coastal upwelling un the East China Sea. Cont. Shelf Res., , 1321-1338. Chern, C.-S., J. Wang, and D.-P. Wang. 1990. The exchange of Kuroshio and East China Sea shelf water. J. Geophy. Res., , 16017-16023. Edmond, J.M., A. Spivact, B.C. Grant, M.H. Hu, Z.X. Chen, S. Chen, and X.S. Zeng. 1985. Chemical dynamics of the Changjiang estuary. Cont. Shelf Res., , 17-36. Gong, G.-C., Y.-L. Chen, and K.-K. Liu. 1996. Chemical hydrography and chlorophy a distribution in the East China Sea in summer: implications in nutrient dynamics. Cont. Shelf Res., , 1561-1590. Gong, G.-G., Y.-H. Wen, B.-W. Wang, and G.-J. Liu. 2003. Seasonal variation of chlorophyll a concentration, primary production and environmental conditions in the subtropical East China Sea. Deep-Sea Res. I, , 1219-1236. Hama, T., K.H. Shin, and N. Handa. 1997. Spatial variability in the primary productivity in the East China Sea and its adjacent waters. J. Oceanogr., , 41-51. Harrison, P.J., M.H. Hu, Y.P. Yang, and X. Lu. 1990. Phosphate limitation in estuarine and coastal waters of China. J. Exp. Mar. Biol. Ecol., , 79-87. Humborg, C.V., A.C. Ittekkot, and B.V. Bodungen. 1997. Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure. Nature, , 385-388. Hecky, R.E. and P. Kilham. 1988. Nutrient limitation of phytoplankton in freshwater and marine environment: A review of recent evidence on the effects of enrichment. Limnol. Oceanogr., , 796-822. Hu, D. 1994. Some striking features of circulation in Huanghai Sea and East China Sea. p. 27-38. In: Oceanology of China Seas, vol. 1., ed. by D. Zhou, Y.-B. Liang, and C.-K. Zeng. 15

48

21

50

24

93

4

16

Acknowledgments We are indebted to captain and crews of the R/V who were most helpful in all our shipboard operations. The authors also would like to thank Drs. Sung-Tae Jang, Jae-Hoon Noh, Hyung-Gu Kang for assistance in the field work and Ms. Jeong-Ah Lee for chlorophyll- measurement. This work was supported by the KORDI projects PM33600 and PE92400. Eardo

a

References

50

53

140

386

Beardsley, R.C., R. Limeburner, H. Yu, and G.A. Cannon. 1985. Discharge of the Changjiang (Yangtze River) into the East China Sea. Cont. Shelf Res., , 57-76. Chang, J., F.-K. Shiah, G.-C. Gong, and K.P. Chiang. 2003. Cross-shelf variation in carbon-to-chlorophyll a ratios in the East China Sea, summer 1998. Deep-Sea Res. II, , 1237-1247. 4

50

33

137


Kluwar Academic, Norwell. Kilham, P. and R.E. Hecky. 1988. Comparative ecology of marine and freshwater phytoplankton. Limnol. Oceanogr., , 776-795. Le, K. 1988. A preliminary study of the basic hydrographic features and the current structures off the Changjiang River mouth in the dry season. Prog. Oceanogr., , 387-400. Lie, H.-J., C.-H. Cho, J.-H. Lee, and S. Lee. 2003. Structure and estward extension of the Changjiang River plume in the East China Sea. J. Geophy. Res., , 22. Milliman, J.D. 1997. Blessws dam or dammed dams? Nature, , 325-327. Raymont, J.E.G. 1980. Plankton and productivity in the oceans, Phytoplankton. Vol. 1. Pergamon Press, Oxford. 489 p. Strickland, J.D.H. and T.R. Parsons. 1972. A practical handbook of seawater analysis. Fisheries Research Board of Canada, Bulletin No. 167, 2nd edition. 310 p. Su, Y. and X. Weng. 1994. Water masses in China Sea. p. 3-16. In: Oceanology of China Sea, ed. by D. Zhou, Y.-B. Liang, and C.K. Zeng. Kluwer Academic Publishers, Netherlands. 33

21

108

386

Tang, R., H. Dong, and F. Wang. 1990. Biogeochemical behavior of nitrogen and phosphate in the Changjiang estuary and its adjacent waters. p. 322-334. In: Biogeochemical study of the Changjiang estuary, ed. by G. Yu, J.M. Martin, J. Zhou, H. Windom, and R. Dawson. China Ocean Press, Beijing. Tian, R.C., F.X. Hu, and A. Saliot. 1993. Biogeochemical processes controlling nutrients at the turbidity maximum and the plume water fronts in the Changjiang Estuary. Biogeochem., , 83-102. Wang, B.-D., X.-L. Wang, and R. Zhan. 2003. Nutrient conditions in the Yellow Sea and the East China Sea. Estuarine Coast. Shelf Sci., , 127-136. Wong. G.T.F., G.-G. Gong, K.-K. Liu, and S.-C. Pai. 1998. ‘Excess Nitrate’ in the East China Sea. Estuarine Coast. Shelf Sci., , 411-418. Yamamoto, T. 1995. Contribution of micro- and nanophytoplankton cell carbon to particulate organic carbon in the East China Sea during May 1980. J. Faculty Appl. Biol. Sci., , 147-160. 19

58

46

34