Distributions of Nutrients in the East China Sea and the South China Sea Connection

Transcription

1 Journal of Oceanography, Vol. 64, pp. 737 to 751, 2008 Review Distributions of Nutrients in the East China Sea and the South China Sea Connection CHEN-TUNG ARTHUR CHEN* Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, Republic of China (Received 10 December 2007; in revised form 20 May 2008; accepted 2 June 2008) Surface maps of nitrate, phosphate and silicate of the East China Sea (ECS) have been constructed and are described. Reports on exchanges of material between the ECS and the South China Sea (SCS) through the Taiwan Strait are reviewed. Recent advances seem to have reversed the earlier view that the SCS exports nutrients to the ECS through the Taiwan Strait. This is because the northward flow of seawater in the summer carries little nutrient. On the other hand, the waters flowing southward along the coast of China in winter carry orders of magnitude higher nutrient concentrations. The outflow of subsurface waters from the SCS, however, is the major source of new nutrients to the ECS continental shelves because these subsurface waters flow out of the Luzon Strait, join the northwardly flowing Kuroshio and enter the Okinawa trough. Around 10% of the nutrients exported from the SCS through the Luzon Strait upwell onto the ECS shelf. These inputs are larger than the aggregate of all the rivers that empty into the ECS, contributing 49% of the externally sourced nitrogen, 71% of the phosphorous, and 54% of the silica for the ECS. Keywords: Nutrients, Kuroshio, East China Sea, South China Sea, teleconnection, fluxes. 1. Introduction Marginal seas are generally important fishing grounds because they offer high primary and secondary productivity, which is supported by high nutrient concentrations (Guo, 1994; Iseki, 1995; Tang and Su, 2000; Wei et al., 2003). The fisheries, however, cannot be sustained without new production, which relies upon externally sourced nutrients. New production also helps to drive down the CO 2 concentration in the marginal seas. Accordingly, the marginal seas may be an important sink of atmospheric CO 2 (Gong et al., 1996; Tsunogai et al., 1997; Zhang et al., 1999; Ma et al., 1999; Chen, C. T. A. et al., 2001, 2003; Chen, 2003, 2004). Recognizing the importance of the continental margins in terms of the cycling of carbon and associated elements, the Joint Global Ocean Flux Study (JGOFS, established in 1988 as a core project under the International Geosphere Biosphere Program, IGBP) adopted continental margin studies as an operational element (JGOFS, 1992). Five years later, another IGBP core project, the * address: ctchen@mail.nsysu.edu.tw Copyright The Oceanographic Society of Japan/TERRAPUB/Springer Land-Ocean Interaction in the Coastal Zone (LOICZ), was established to elucidate the coastal ecosystems in detail. The first of four over-arching objectives of LOICZ was to determine the fluxes of materials among land, sea and atmosphere (IGBP, 1994). Indeed, the very first LOICZ report was entitled, Coastal seas: a net source or sink of atmospheric carbon dioxide? (Kempe, 1995). Operationally, the first two of the six LOICZ foci were as follows. Is the coastal zone a sink or source of CO 2? and What are the mass balances of carbon, nitrogen and phosphorus in the coastal zone? (Crossland et al., 2005). The JGOFS/LOICZ Continental Margins Task Team (CMTT) was established to study, among other things, the carbon and nutrient flux in the marginal seas (Chen, C. T. A. et al., 1994) to determine the contribution of continental margins and seas to CO 2 sequestration and the horizontal fluxes of carbon, nitrogen and phosphorus across the ocean-continental margin boundary. The CMTT draws upon experience of the East China Sea (ECS), which is one of the most extensively studied seas in the world. Scientists from Japan have been studying the ECS for over a century (Iseki, 1995). More recently, China, Taiwan and South Korea have undertaken 737

2 Fig. 1. Study area with schematic major currents in (a) summer and (b) winter that link the East and South China Seas identified. extensive projects in the ECS, which happens to be the tenth largest marginal sea in the world. The Bohai, the Yellow and the East China Seas have a total area of km 2, of which around km 2 is continental shelf, and is one of the most productive parts of the world s oceans. Two of the largest rivers in the world, the Changjiang (Yangtze River) and the Huanghe (Yellow River), empty into the shelf with large, ever-increasing nutrient inputs. For instance, the total nitrogen input from the Changjiang is approximately kg in 1997, which is a threefold increase over 1968 levels (Chen et al., 1993; Zhang, 1996a, b; Yan and Zhang, 2003; Zhang and Su, 2006). East of the ECS continental shelf lies the Okinawa Trough, which is along the flow path of the Kuroshio. Numerous studies have focused on exchanges between the Kuroshio and the ECS shelves (e.g. Zheng et al., 1991; Yanagi and Takahashi, 1993; Yanagi, 1994; Hsueh et al., 1996; Lie et al., 1998; Yanagi et al., 1998; Seung, 1999; Lie and Cho, 2002; Isobe et al., 2004; Isobe and Beardsley, 2006; Guo et al., 2006; Matsuno et al., 2006; Lee and Matsuno, 2007). These are not discussed unless the results relate to nutrients. The southern ECS connects with the South China Sea (SCS) through the Taiwan Strait (Fig. 1). Essentially all previous relevant publications have established year-round northward currents in the Taiwan Strait (Niino and Emery, 1961; Mao and Guan, 1982; Weng and Wang, 1984; Beardsley et al., 1985; Chen, C. S. et al., 1994). Nutrients have also been found to be transported from the SCS to the ECS (Liu et al., 2000; Chung et al., 2001). A few studies have indicated that Chinese coastal currents flow southward in winter (Hu et al., 1999c; Chang and Isobe, 2003). However, only very recently, a handful of investigations began to demonstrate that the net northward flux is close to zero (Wang and Chern, 1988), or that even a net southward flux may exist in winter (Chen, 2003). These findings imply that nutrients may actually be transported from the ECS to the SCS through the Taiwan Strait, at least in winter. However, the SCS transports nutrients to the ECS year-round by way of the Kuroshio (Chern et al., 1990; Wong et al., 1991; Liu et al., 1992; Gong et al., 1995; Chen et al., 1995; Chen, 1996, 2005; Chen and Wang, 1998). Unlike the ECS, the SCS, the largest marginal sea in the world at km 2, has attracted much less attention than the ECS. Results concerning the distribution of nutrients in the SCS have only started to appear in the literature over the past decade or so (Chen et al., 2001, 2003, 2006). Data are still limited and maps of nutrient distributions in the SCS cannot yet be produced. The following is a review of what is known about the distribution of nutrients in the ECS. Inventories of N and P are calculated for the first time. Finally, we describe the interconnection between the ECS and SCS, and fluxes of nutrients. 2. Distribution of Nutrients in the East China Sea Numerous investigations of nutrients in the ECS have been conducted. Chinese scientists started this research, mainly in the estuaries, in the 1930s (e.g. Wu and Tang, 1937; Chu and Young, 1950; Xin, 1953; Li et al., 1964; Wei, 1966), but most of these studies were not published in the open literature until the late 1970s (Wang, 1974; Gu and Lin, 1976; Ji, 1979; Li, 1979a, b; Chen et al., 1979; Li, 1980), and essentially all of them were pub- 738 C. T. A. Chen

3 lished in Chinese. More publications, some by non-chinese scientists written in English, started to appear in the 1980s, but these were still mostly confined to reiverine exports or to the estuaries and coastal areas (Wang and Yao, 1981; Gu et al., 1981, 1982a, 1982b; Gu, 1982; Wang et al., 1983; Gan et al., 1983; Tian, 1983; Aller et al., 1983; DeMaster et al., 1983; Edmond, 1983; Huang et al., 1983a, 1983b, 1986; Lu et al., 1985, 1993; Sun et al., 1986; Fujian Oceanological Institute, 1988; Hu et al., 1989; Shen et al., 1989; Tang et al., 1990). Although most work was still published in Chinese, some papers started to appear in the western literature (Carbon Cycle Research Unit, 1982; Grant et al., 1983; Milliman et al., 1984; Edmond et al., 1985; Cauwet and Mackenzie, 1993; Zhang and Liu, 1994; Zhang and Gu, 1994; Chen et al., 2002). Eventually the Chinese scientists ventured into the more open waters and the Japanese, Korean as well as Taiwanese scientists all became increasingly active in studying the ECS (Shen et al., 1984; Li, 1990; Zhang, 1991; Lu et al., 1991; Tsunogai, 1991; Chung et al., 1991, 1999; Ito et al., 1994; Uzuka et al., 1994; Yanagi, 1994; Yang et al., 1994, 1999; Hong et al., 1995a, b; Tsurushima et al., 1996, 1999; Wang et al., 1998; Ma et al., 1999; Wang and Wang, 1999; Wang, 1999, 2000; Liu, 2001; Hu and Yang, 2001; Wang et al., 2001, 2003; Ichikawa et al., 2003; Hung et al., 2003, 2007; Gao et al., 2004). To the best of the author s knowledge the most comprehensive information on nutrients on the ECS continental shelf, at least in terms of temporal variability, was published in the Marine Atlas of the Bohai Sea, the Yellow Sea and the East China Sea: Chemistry (Wang, 1991). Largely based on this atlas, but with information from off Korea, with the addition of Taiwan and the Okinawa Trough, Chen (2008) produced several maps of nutrients. These cover N, P and Si in the surface and bottom layers for summer and winter, but only modified surface maps are discussed here. Because NO 2 is frequently measured along with NO 3, which is much higher in concentration, the NO 3 + NO 2 values reported below are shown as NO 3. Further, because the NH 4 concentration is orders of magnitude lower than NO 3 + NO 2, it is neglected (Shin et al., 2003). Data from the Bohai Sea are essentially the same as those given in the most comprehensive atlas of Wang (1991), as are most data near the Chinese coast and in most of the Yellow and East China Seas. Distributions of phosphate and silicate in the eastern part of the Yellow Sea and south of Korea are mainly based on Wang (1991), modified based on KORDI (1987; based on data from 27 stations in August, 1983 and 35 stations in February, 1986), KORDI (1993; based on data from 39 stations in 1991), Lee et al. (1998 based on data from 149 stations between ) and Wang et al. (2000; based on data from 26 stations in 1995). The distribution of nitrate in these regions is based mainly on KORDI (1987, 1993), Lee et al. (1998) and Wang et al. (2000). Distributions of nutrients in the East China Sea proper are based mainly on Wang (1991), modified based on the data of Chen, M. P. et al. (1992), Chen et al. (1995, 1996), Chen (1996), Wang and Chen (1998), and Guo et al. (2003). Distributions of nutrients in the Taiwan Strait are based on Wang (1991), Fujian Oceanological Institute (1988), Hu et al. (1999a, c), Chen (2003) and Guo et al. (2003). Distributions of nutrients around Taiwan are based on Wang (1991), Chen et al. (1995), Chen and Hsing (2005) and several unpublished reports of Chen, C. T. A. In the summer, mild southerly monsoon winds bring in moist air, leading to high river discharge that empties into the generally warm seas (Fig. 2(a)). Since the Changjiang is a major source the highest NO 3 concentrations in the ECS can be found near the Changjiang estuary and in the river plume which extends towards the east and propagates across the shelf towards Jeju Is. (Cheju Is.) and beyond (Fig. 2(c), Tian et al., 1993; Su and Weng, 1994; Zhang et al., 1997; Duan et al., 2000; Ma, 2001; Shen et al., 2001; Liu et al., 2002; Wang et al., 2002; Liu, S. M. et al., 2003a; Pang et al., 2003; Wang and Wang, 2006). The plume, which has a NO 3 concentration higher than 1 µmol/dm 3, is generally confined to the region with salinity below about 31 (Figs. 2(b) and (c)). Otherwise, an NO 3 concentration greater than 1 µmol/dm 3 can only be found in the circulation cell off SW Korea (>2 µmol/dm 3 ), in the westernmost part of the Bohai Sea (>1 µmol/dm 3 ; Zhang, 1993; Cui et al., 1994; Cui and Song, 1996; Hong et al., 1999; Xing and Liu, 1999; Yu et al., 1999, 2000; Shan et al., 2000; Zou and Zhang, 2001; Zhao et al., 2002; Liu, C. et al., 2003; Zhang and Su, 2006), in various pockets off southeastern China (>10 µmol/dm 3 ) and in an upwelling area off NE Taiwan (>1 µmol/dm 3 ; Gong et al., 2003). Upwelling, riverine and aeolian input are all operative (Gu and Lin, 1976; Gu, 1991; Chung et al., 1998; Lin et al., 1999; Liu, S. M. et al., 1999, 2003b; Fang and Mu, 2001; Fu and Shen, 2002; Gao et al., 2003; Guo et al., 2004; Xia et al., 2006). River influenced areas are easy to identify because of high nitrate but low salinity. The upwelling areas with high nitrate concentrations, however, may display high salinities, e.g. in the area NE of Taiwan (S > 33.5; NO 3 > 1 µmol/dm 3 ; Figs. 2(b) and (c)). The Kuroshio-influenced areas, especially in the Okinawa Trough where the salinity is near 34 or above (Fig. 2(b)), display very low NO 3 concentrations (Fig. 2(c); <0.2 µmol/dm 3 ) where nitrogen fixation may be an important source of nitrogen (Saino and Hattori, 1980). The Kuroshio Branch west of Kyushu has relatively higher salinity (>32) than waters further to the west (Fig. 2(b)), and displays nitrate concentrations below 1 µmol/dm 3 (Fig. 2(c)). Northward intrusion of the Kuroshio in the ECS proper, notably at Nutrients in the East China Sea 739

4 Fig. 2. Distribution of surface water (a) temperature, (b) salinity, (c) nitrate concentration, (d) phosphate concentration and (e) silicate concentration in the Bohai, East China and Yellow Seas in August (data taken from KORDI, 1987, 1993; Fujian Oceanlogical Institute, 1988; Wang, 1991; Wong et al., 1991; Chen, D. X., 1992; Chen, M. P. et al., 1992; Chen et al., 1995, 1996; Chen, 1996, 2003; Wang and Chen, 1998; Lee et al., 1998; Hu et al., 1999a, b; Wang et al., 2000). 30 N, 124 E, also results in an upward bulge in the salinity contours (Fig. 2(b)). A bulge in nitrate contours (Fig. 2(c)) with concentrations below 1 µmol/dm 3 in the south also exists in roughly the same area. Nitrate concentrations are generally low (<1 µmol/dm 3 ) in the Taiwan Strait because in summer the northward-flowing SCS and Kuroshio Branch waters occupy the Strait. These waters are low in nutrients in the euphotic zone. Noteworthy is that the pockets of high nutrients off the SE coast of China are not necessarily due to upwelling, although coastal upwelling certainly occurs in summer with favorable SW winds (Hong and Dai, 1994; Lü et al., 2006; Wang and Wang, 2007). This conclusion is based on the fact that bottom waters outside these pockets contain lower concentrations of nutrients. It is tempting to conclude that these pockets are probably remnant winter water with local rivers supplementing the already high nutrient concentrations. The pattern of PO 4 distribution in summer (Fig. 2(d)) is similar to that of NO 3, in that high values are found in 740 C. T. A. Chen

5 Fig. 3. Distribution of surface water (a) temperature, (b) salinity, and (c) nitrate concentration, (d) phosphate concentration, and (e) silicate concentration in the Bohai, East China and Yellow Seas in February (T, S, and nutrient data taken from KORDI, 1987, 1993; Fujian Oceanological Institute, 1988; Wang, 1991; Wong et al., 1991; Chen, D. X., 1992; Chen et al., 1995, 1996; Chen, 1996, 2003; Wang and Chen, 1998; Lee et al., 1998; Hu et al., 1999c; Guo et al., 2003). the Changjiang estuary (>1 µmol/dm 3 ; Tian et al., 1993; Zhang, 1996a; Duan et al., 2000; Liu et al., 2002), off SW Korea (>0.4 µmol/dm 3 ), in western Bohai Sea (>0.2 µmol/dm 3 ; Liu and Zhang, 2000), in isolated areas off the coasts of SE China (>0.2 µmol/dm 3 ) as well as in the upwelling area off northeastern Taiwan (>0.1 µmol/dm 3 ; Chen, 1994; Chen et al., 1999; Gong et al., 2003). A notable difference is that PO 4 is rapidly consumed in the Changjiang River Plume, and the concentration is reduced to below 0.2 µmol/dm 3 (Fig. 2(d)) when the salinity becomes higher than 29.5 (Fig. 2(b)). Note that NO 3 is still higher than 1 µmol/dm 3 (Fig. 2(c)) in the salinity range between 29.5 and 31 (Fig. 2(b)) in the Changjiang River Plume. This is because the N/P ratio of the Changjiang water exceeds 100 (Chen and Wang, 1999). Since phytoplankton in the ECS, in common with other areas in the world oceans, consume N and P with a Redfield ratio of 16 (Chen et al., 1996), P is exhausted while N remains in the plume. As a result, P seems to be the limiting nutrient in the ECS (Wong et al., Nutrients in the East China Sea 741

6 Table 1. Inventory of nitrogen and phosphorous in the East China Sea. N (10 9 moles) P (10 9 moles) Si (10 9 moles) Summer inventory* Winter inventory* Summer-to-winter accumulation* in the ECS Total annual river outflow to the ECS** Annual flux from SCS to ECS shelves** *Unpublished data, C. T. A. Chen. **Taken from Chen and Wang (1999). 1998; Wang et al., 2003; Wang and Wang, 2006). In the Taiwan Strait, however, there seems to be some P left in coastal waters, whereas P is low (<0.1 µmol/dm 3 ) in the middle. A tongue of low-p water extends northward (Fig. 2(d)). The PO 4 concentrations are very low (<0.05 µmol/dm 3 ; Fig. 2(d)) in the Kuroshio waters with a salinity of higher than about 34 (Fig. 2(b)). The main features of the SiO 2 distribution (Fig. 2(e)) are more similar to those of NO 3 than those of PO 4. The SiO 2 values are high in both the Changjiang estuary (>50 µmol/dm 3 ) and plume (>5 µmol/dm 3 ; Zhang, 1996a) and the pockets off SE China (>25 µmol/dm 3 ). The region with SiO 2 higher than 5 µmol/dm 3 occupies roughly the same area as the region with NO 3 higher than 1 µmol/dm 3. The high SiO 2 area off SW Korea, however, seems to extend further westward than the case for N and P (Fig. 2(e)). Additionally, in the Bohai Sea, the area near the Huanghe estuary, and not the westernmost part of the Bohai Sea, has the highest SiO 2 concentration (Xin and Liu, 1999; Liu and Zhang, 2000). The Kuroshio waters with salinity higher than 34 (Fig. 2(b)) have a SiO 2 concentration below 3 µmol/dm 3 (Fig. 2(e)). The Kuroshio Branch, which occupies the eastern Taiwan Strait, also displays SiO 2 concentrations below 5 µmol/dm 3. The distribution of nutrients in the surface layer in winter differs distinctly from that in summer. Water temperatures are now much lower (Fig. 3(a)) whereas salinity values are generally higher (Fig. 3(b)). Cooling and stronger wind-induced mixing, supplemented by coastal upwelling (Qiao et al., 2006), cause the concentrations generally markedly to exceed those in summer (Figs. 3(c) (e)). In general, higher temperature (Fig. 3(a)), or higher salinity (Fig. 3(b)) corresponds to lower nutrient concentrations (Figs. 3(c) (e)). On the other hand, lower temperature or salinity corresponds to higher nutrient concentrations. Since the by now smaller Changjiang River Plume flows southwest under the influence of the strong NE monsoon, a jet with high nutrient concentrations (NO 3 up to 25 µmol/dm 3, PO 4 up to 1.6 µmol/dm 3 and SiO 2 up to 30 µmol/dm 3 ) flows southwestward along the coast of SE China. This is in direct contrast to the still northward flowing, nutrient-poor Kuroshio Branch waters with NO 3 below 2 µmol/dm 3, PO 4 below 0.2 µmol/dm 3 and SiO 2 below 5 µmol/dm 3 (Figs. 3(c) (e)) found in the eastern Taiwan Strait. Waters in the main path of the Kuroshio (S > 34.5; Fig. 3(b)) display even lower nutrient concentrations (NO 3 < 0.2 µmol/dm 3 ; PO 4 < 0.05 µmol/dm 3 ; SiO 2 < 3 µmol/dm 3 ; Figs. 3(c) (e)). The area previously occupied by the Changjiang River Plume in summer still displays high nutrient concentrations, probably due to the vertical mixing which brings up nutrient-laden deep and bottom waters. It is noteworthy that the Central Yellow Sea, under the influence of the warm (Fig. 3(a)), salty (Fig. 3(b)) but nutrient-poor (Figs. 3(c) (e)) Yellow Sea Warm Current, itself the shoreward extension of the Kuroshio, displays low nutrient concentrations, as is the area influenced by the Taiwan Warm Current, indicated by the bulge of high salinity contours near 30 N, E (Fig. 3(b)). The eastern Taiwan Strait displays even lower nutrient concentrations (Figs. 3(c) (e)). Nutrient concentrations in the Taiwan Strait may increase in the future, because the operation of the Three Gorges Dam will result in a reduction of river flow in summer but an increase between January and April (Chen, C. S. et al., 2003). As a result, more nutrient-rich Changjiang Diluted Water may reach the Taiwan Strait. 3. Inventories of N and P in the East China Sea Importantly, vertical mixing is not the only reason why the surface nutrient concentrations in the winter exceed those in the summer. This is because the inventories of nutrients such as N and P are larger in winter ( moles N and moles P) than in summer ( moles N and moles P). Briefly, inventories were obtained by first calculating average concentrations of N and P in the following layers 0 5 m, 5 15 m, m, m and m. These values are then multiplied by the area and summed to obtain the inventories (Chen, in preparation). The net accumulation of N and P between summer and winter on the continental shelves from the coast to a depth of 200 m amounts to 742 C. T. A. Chen

7 moles N and moles P (Table 1). In the case of P the net accumulation is several times larger than the quantities supplied by the rivers in a year ( moles; Table 1). As discussed later, a large external source must exist on the ocean side (Chen, 1996, 2000; Chen and Tsunogai, 1998; Lee and Matsuno, 2007). It is interesting to note that the construction of the Three Gorges Dam has been projected to affect the distribution of nutrients in the Changjiang River estuary (Shen et al., 1992) and beyond (Chen, 2000). It has been reported that the ecosystem has changed somewhat since the Dam started its first phase operation in 2003 (e.g. Jiao et al., 2007). Since more water will be released in winter after the third and final phase operation in 2009, southward flow of the Changjiang Diluted Water is expected to increase. More nutrients may therefore be transported from the ECS to the SCS, through the Taiwan Strait, in the future. 4. Interconnection between the East and South China Seas The SCS has received much less attention than the ECS. Accordingly, less is known about the distribution of nutrients. No basin-scale atlas is known to this author. The obvious link between the East and South China Seas is the Taiwan Strait, where a branch of Kuroshio tends to flow northward throughout the year. In summer, monsoon winds blow in the same direction, causing coastal and SCS waters to flow northward together with the Kuroshio Branch. Consequently, waters across the Taiwan Strait all flow northward (Chuang, 1986; Chen and Zhang, 1992; Song, 2006; Liao et al., 2007), thus transporting nutrients from the SCS to the ECS. Part of the Kuroshio Branch, however, may rejoin the Kuroshio north of Taiwan in summer (Chen et al., 1995; Hsin et al., 2008). In winter, however, strong NE monsoon winds push the nutrient-rich waters southward along the Chinese coast, while some nutrient-poor Kuroshio Branch waters may manage to flow northward on the eastern part of the strait (Chen and Hsing, 2005; Chen and Wang, 2006). In the last two decades or so, the trend seems to be that investigators are reporting continual reduction in net northward fluxes. Wang and Chern (1988), Hu et al. (1999c) and Chen and Wang (1999) all reported small northward flows, with the last reporting a net northward flow of only 0.2 Sv in winter. Lie and Cho (1994) concluded that the Kuroshio Branch is stagnant in the Taiwan Strait in winter and flows out only intermittently from winter to early spring. Jan and Chao (2003), Ko et al. (2003), Teague et al. (2003), Lin et al. (2005) and Chen and Sheu (2006) all concluded that no persistent northward flow occurs in the Taiwan Strait in winter, while Song (2006) reported a net southward flow. Figure 4 displays the surface seawater temperature Fig. 4. Temperature of surface water in February, 2001, based on NOAA satellite data. in February, 2001 as an example. Clearly, waters that are warmer than 22 C (in yellow and purple) in the Kuroshio region flow past the eastern coast of Taiwan and move onto the ECS shelf northeast of Taiwan. However, on the other side of Taiwan this warm water can be found only in the SE Taiwan Strait. The entire western Taiwan Strait and most of the northern Strait are occupied by waters that are cooler than 18 C (shown in light blue and dark blue), revealing the aforementioned fact that the nutrient-rich, cold Chinese coastal waters flow from the ECS to the SCS. However, the nutrient-poor, warm Kuroshio Branch does not flow freely from the SCS towards the ECS. As a result of these new findings, the report that asserted that nutrient fluxes from the Taiwan Strait..., ranging from less than half to more than double the Kuroshio inputs (Liu et al., 2000) is almost certainly erroneous, as has already been elucidated by Chen (2003). The flux estimates by Chen and Wang (1999) of mol N, mol P and mol Si from the SCS to the ECS through the Taiwan Strait also seems to be in error. In fact, the ECS may actually transport nutrients to the SCS through the Strait because the southward-flowing coastal waters in winter have much higher nutrient concentrations than the northward-flowing Kuroshio Branch Nutrients in the East China Sea 743

8 Fig. 5. Plots (a) θ/s, (b) θ/aou, (c) θ/cfc 11 and (d) θ/cfc 12 based on the data of Swift et al. (1990) in and near the central Okinawa Trough. The θ/s plot of ORI-462 Station CM6 near the ridge northeast of Taiwan is also given in (a). The θ/cfc 11 and θ/cfc 12 data for the intermediate waters are enlarged in (e) and (f), while the θ/s, θ/aou, θ/cfc 11 and θ/cfc 12 data for the tropical waters are enlarged in (g) (j). Station locations mentioned are given in Fig. 2(a) (taken from Chen, 2005). and SCS water. Unfortunately, no updated estimates of nutrient fluxes through the Taiwan Strait have been made. A word of caution should be sounded, in that the above fluxes are based on Chen and Wang (1999). As mentioned above, fluxes through the Taiwan Strait need revision, which may alter the ECS nutrient budgets to a certain extent. New bottom-mounted ADCP data in the Taiwan Strait are being produced. Hopefully, new nutrient budgets will be forthcoming before too long. Exchanges of nutrients through the Taiwan Strait, however, are much smaller than the flux via the Kuroshio east of Taiwan because the Taiwan Strait is rather shallow, with a sill depth of no more than 60 m. Consequently, only surface waters, which are typically nutrient-poor, are exchanged here between the ECS and the SCS. However, subsurface, nutrient-rich tropical and intermediate waters of the SCS flow out of the SCS through the Luzon Strait, to form the western part of the Kuroshio (Chen and Huang, 1996; Chen and Wang, 1998). The main body of the Kuroshio originates from the North Equatorial Current. A clear subsurface front separates waters originating in the SCS from the main body of the Kuroshio, which has higher salinity at the Smax (the tropical water) but lower salinity at the Smin (the intermediate water; Nitani, 1972; Chen and Huang, 1996). In addition, the SCS tropical and intermediate waters (SCSTW and SCSIW) originate from the Kuroshio tropical and intermediate waters (KTW and KIW). As a result, the SCSTW and SCSIW have higher apparent oxygen utilization (AOU), but lower Chlorofluorocarbons 11 and 12 (CFC 11 and CFC 12) concentrations (Fig. 5). These salinity and CFC signatures demonstrate that stations on the western part of the Kuroshio are observing water from the SCS (Chen, 2005). As the subsurface SCS/Kuroshio waters upwell onto the ECS continental shelf, nutrients are transported from the SCS to the ECS. Chen and Wang (1999) estimated such annual fluxes as mol N, mol P and mol Si (Table 1) and these contribute 49% of the externally sourced nitrogen, 71% of the phosphorous, and 54% of the silica for the ECS. Note 744 C. T. A. Chen

9 that although a large portion of the upwelled waters originate from the SCS, there is no doubt that some Kuroshio waters are mixed in. As a result, not all upwelled nutrients originate from the SCS. Importantly, these fluxes exceed the total annual river inputs of mol N, mol P and mol Si to the ECS (Table 1; Zhang et al., 1997; Chen and Wang, 1999). The subsurface SCS waters are therefore the major sources of nutrients, particularly P, to the ECS shelf. By way of comparison, Chen et al. (2001) estimated that the SCS Intermediate Water (350 1,350 m) exports 1, mol N, mol P and 3, mol Si out of the SCS a year. Up to 10% of these exports end up on the ECS continental shelf. 5. Conclusions The summer and winter distributions of N, P and Si in the surface waters of the East China Sea have been discussed. In general, coastal waters, especially near the Changjiang estuary, are higher in nutrient concentrations whereas waters of, and influenced by, the Kuroshio are very low in nutrients. Nutrient concentrations are higher in winter. In fact, there is a net accumulation of moles N and moles P on the shelves from summer to winter. These nutrients come mainly from upwelling of subsurface Kuroshio waters which in turn come from the outflowing Tropical and Intermediate Waters of the South China Sea. Part of the SCS export to the ECS returns to the SCS through the Taiwan Strait in winter by way of the China coastal currents. Acknowledgements The author would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC Z , M MY3. Aim for the Top University Plan (95C 0312) also supported this research. Two anonymous reviewers provided valuable comments which strengthened the manuscript. References Aller, R. C., C. H. Wang, J. C. Jin, F. Tsai and Y. N. 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