The South China Sea (SCS) is a large semi-enclosed marginal sea to the west of the tropical Pacific, and connected to the western Pacific through the Luzon Strait. The surface water circulation and hydrography in the SCS are strongly influenced by the East Asian monsoon system. The seasonal changes of the monsoon system induce changes in the mixed layer thickness, upwelling, primary production, and associated biogeochemistry. In order to understand the characteristics of the Kuroshio intrusion and the source strength of radium isotopes from the coastal zone, we carried out surface water and water column samplings for Ra-228 and Ra-226 measurements in the northern SCS and the Luzon Strait areas. The Ra isotopes were much higher in the SCS surface waters than in the open ocean surface waters because the SCS was enclosed mostly by landmasses which are known as sources of these nuclides. Higher surface water activities are seen in the northern (shelf and slope) area; lower values are observed in the southern (deep basin) area; the lowest values appear in the eastern (Luzon Strait) area. Large temporal and spatial variations were also observed probably due to the source strength of radium isotopes from the coastal zone and intrusion of the Kuroshio Current. The vertical 228Ra profiles are remarkably similar, showing high values in the surface layer and fairly uniform below about 500m depth but with an increase toward the bottom due to input from the underlying sediments. The shallow water profile on the shelf shows higher 228Ra values due to both vertical and horizontal mixing of the shelf water and additional source from the shore zone. The 226Ra profiles in the northern SCS are quite similar to those in the northwest Pacific both in pattern and magnitude, showing lowest values at the surface and an increase with depth although more scattered. 226Ra activities in the shallow water (less than 1000m depth) are higher in the northern SCS than in the northernwest Pacific Ocean, but they are quite comparable below this depth. The Ra-228/Ra-226 activity ratios of the surface water decrease from the coastal zone and estuaries toward the basin and the Luzon Strait (from 4.11 to 1.03), indicating the former as a main source area for Ra-228. The vertical 228Ra/226Ra activity ratios decrease rapidly from the surface at 2.3 to a depth of 1500m at 0.5 below which the values are less than 0.5. This pattern is similar to that of the open ocean but these values are much higher than those in the open oceans where the surface water values are 0.5 or less and the deep water values are less than 0.1. This suggests a strong input of Ra-228 relative to Ra-226 from the bottom sediments into the deep water of the semi-enclosed SCS.

Luzon Strait is the deepest channel for water exchange between the Northern South China Sea (NSCS) and the West Philippine Sea (WPS). It is important to investigate the seasonal and spatial distributions and the biogeochemical processes of organic carbon, nitrogen and phosphorus in Luzon Strait. During the summer season, the flow of Kuroshio water into the SCS through the Bashi Channel was restricted due to the prevailing southwest monsoon. However, during the winter season, the flow of Kuroshio water into the SCS through the Bashi Channel was enhanced due to the prevailing northeast monsoon. The characteristics of water types across the Bashi Channel depend highly upon the water exchange between the WPS and the NSCS. Distributions of dissolved organic carbon (DON), nitrogen (DON) and phosphorus (DOP) in the euphotic zone generally show an increasing trend from the WPS to the NSCS. The same distribution was pattern found for particulate organic carbon (POC) and nitrogen (PN). The stichomythic ratio (C/N/P) of dissolved organic matter in the euphotic layer was lower in spring than in autumn. The reason may be that the Kuroshio water flowing through the Luzon Strait is much less in spring than in autumn. The ratios of DIN/DIP were much lower than the Redfield ratio (16) suggesting a status of N-limitation in the euphotic zone. The DOC/DON ratios, however, were much higher than the Redfield ratio (6.6). These results implied that DOM might have played an important role in modulating nutrient cycling and food web dynamics in the euphotic zone of study area. The subsurface water (100-600m) of the NSCS, west of Luzon, was more enriched with POM than that of the WPS. However, the situation was reversed in the intermediate water (600-1500m). Correlations are significant between POC and Chl-a in spring and summer, suggesting that phytoplankton abundance may primarily control the distribution of POC in the euphotic zone. The DOC/POC ratio was inversely correlated with Chl-a in both spring and summer. The ratios generally decreased to a constant value as the Chl-a concentration increased to a higher level, implying a higher biological contribution for POC than for DOC. Correlations were also significant between TEP/POC and Chl-a. The ratio of TEP-C/POC in the euphotic layer showed a decreasing trend from the NSCS to the WPS, implying a significant influence of phytoplankton productivity on TEP distribution. During the study period, the integrated gross production (IGP) and integrated dark community respiration (IDCR) in the study area were in the range of 3056~7094 mg C m-2 d-1 and 3372~8901 mg C m-2 d-1 in autumn, respectively; 1740~5338 mg C m-2 d-1 and 2628~7685 mg C m-2 d-1 in spring, respectively; 2149~6110 mg C m-2 d-1 and 4391~8896 mg C m-2 d-1 in summer, respectively. During the autumn and summer season, there were no significant correlations between GP (DCR) and temperature, PAR, salinity, Chl-a, DOC and POC, possibly resented from the effect of typhoon. During the spring season without the typhoon effect, there were significant correlations between GP (DCR) and salinity, Chl-a, DOC and POC. The ratio of IGP/IDCR is an indicator of net ecosystem production, with>1 for the autotrophic system and <1 for the heterotrophic system. The ratio was <1 for all stations indicating a heterotrophic system. However, the ratio was slightly higher in autumn than in spring and summer.

The purposes of this study are to understand and compare: the distributions of the particulate and dissolved 210Po and 210Pb in the northern South China Sea and in the east and west sides of the Luzon Strait; the associated biogeochemical processes, including their mechanisms and rates involved in the area; and the disequilibrium between 226Ra and the 210Pb as observed in profiles. Seawater samples were collected from three stations in the northern South China Sea (station I, J, and F) and one station to the east of the Luzon Strait (station S5) using a Rosette sampling system during three Ocean Researcher I cruises (ORI-688 in July, 2003, ORI-707 in February, 2004, and ORI-734 in October, 2004). The dissolved 210Pb in surface water generally displays higher activities (around 14-28 dpm/100kg) relative to deep water, reflecting atmospheric input. The dissolved 210Pb profile as seen at station F displays a maximum of 26 dpm/100kg at 200 m and lower values at about 10 dpm/100kg below 1500 m. Highest value at 28 dpm/100kg was observed at S5 surface water. It is probably due to being on the path of the Kuroshio which has accumulated atmospheric 210Pb continually from the northern Equatorial Current on its way. The mean residence time with respect to particle scavenging is about 0.62 years for total 210Po within a 50 m mixed layer, and ranges between 0.34 to 1.13 years for total 210Pb in the mixed layer. 210Po is deficient relative to 210Pb in most profiles, suggesting that 210Po is more readily scavenged. The mean scavenging residence time is about 87 years for total 210Pb in the deep water at station F, and 60 years at station J. This residence time is longer than that in both the Pacific and East China Sea deep waters, i.e. a lower scavenging rate in the South China Sea. As the upwelling rate is much higher in the South China Sea than in the Pacific, it may effectively reduce the sinking rate of the sinking particulates, resulting in higher 210Pb residence time in the water columns of the South China Sea.

Pb-210 and Po-210, a parent-daughter pair, are particle-reactive radionuclides. Pb-210 tends to be associated with inorganic particles but Po-210 prefers organic particles. In the context of these characteristics the purpose of this study is to determine Po-210 and Pb-210 in the surface water plankton of the northern South China Sea (SCS) and the Luzon Strait (LS) areas in order to understand their temporal and spatial distributions and the extent of their radioactive disequilibrium. As the LS has provided a pathway for the exchange between the Kuroshio and the SCS waters, the study area has the characteristics of an open ocean and a marginal sea. The plankton Po-210 activities in the study area are about 10~400 dpm/g, but may reach 1200dpm/g in an El Nino year, the effect of which on Po-210 is not clear at present. The Po-210 in the plankton is mainly affected by the surface water Po-210 , biomass concentration, and the Po-210 in the surface water as well as plankton transported from other area(s). If the biomass concentration (as indicated by Chlorophyll-a) increases, the Po-210 in the plankton decreases, i.e. they are inversely correlated. Based on the planktonic Po-210 distribution, the Kuroshio water which has both high Po-210 and high Po-210-bearing plankton has evidently intruded into the northern SCS. The Pb-210 activities of in the plankton vary from 5 to 25dpm/g; the variation trend is similar to that of Po-210 but with much lower activity, resulting in a Po-210/Pb-210 ratio much greater than unity. Compared to the suspended particles, the plankton is highly enriched in Po-210 but it strongly repels Pb-210. Based on earlier studies and this one, the extent of Po-210 enrichment (as indicated by the Po-210/Pb-210 activity ratio) in various organisms increases sequentially from plankton to mussel, large swimmer and then to marine fish, i.e. the higher the level of organism in the food chain, the higher the ratio becomes. However, the Po-210 and Pb-210 activities per unit mass of these organisms are generally lower than those of plankton. Organisms of higher food-chain level may accumulate Po-210 in the digestive system, but part of it may be excreted. Po-210 accumulation does not occur in other parts of these organisms. In terms of adsorption, the plankton with greater surface area to volume ratio, can adsorb more Po-210 resulting in a higher specific activity, whereas large organisms with smaller surface area to volume ratio can adsorb less Po-210 yielding a lower specific activity. The enrichment of Po-210 in various organisms must have been achieved by absorption and adsorption with unknown proportion. Based on a simple box model calculation for the northern SCS water within the upper 100m layer, the excess Po-210 in the planktonic biomass can account for about 70% of the total deficit in this layer. But in the LS area, the excess and the deficit are balanced. This suggests that the Po-210 deficit in this surface water is due to absorption and adsorption by organisms as evidenced by their large Po-210 enrichment.

Abstract In this study, we have analyzed systematically the oxygen isotopic compositions of South China Sea (SCS) and Luzon Strait (LS) seawater so that a comprehensive interpretation of their temporary and spatial variability can be delineated. The oxygen isotopic compositions of the samples collected in the two areas were determined using the Epstein-Mayeda technique, and the overall precision of the d18OSMOW measurements is +0.1 ?. Our data suggest that LS seawater is a mixture of SCS and Kuroshio waters. The precipitation (1.5 mm) on SCS in April 1998 (during ORI517 cruise) is much less than that (169.5 mm) in April 1999 (during ORI546 cruise). As LS and SCS waters are concerned, the profiles of d18OSMOW generally share the same trend as those of salinity. The d18OSMOW values of the SCS surface water decrease toward the south, while those of the LS surface water decrease toward the west. These suggest that the effect of the Kuroshio water decreases are it moves from northeast toward southwest after its intrusion through LS. The salinity of Kuroshio water reaches the maximum at 34.92 with the corresponding d18OSMOW value of 0.42 ?, whereas the salinity of the representative SCS surface water is 33.34 and the corresponding d18OSMOW is -0.25 ?. As these two data were selected as end members, the estimated proportion of the intruding Kuroshio water in the composition of LS water could be high up to 80 % at 121.5o E. The plot of d18OSMOW versus salinity for cruises OR517 and 546 shows a difference between the slopes of the two regression lines, indicating the effect of variability in precipitation and the depth of the mixed layer. The average d18OSMOW value for surface water decreases toward the west as the water moves from Western Philippine Sea (WPS) to LS and SCS. The average d18OSMOW values for the surface, subsurface, intermediate, and deep waters are listed in the corresponding order in parentheses preceded by the studied area as follows: WPS (0.29 ?, 0.25 ?, -0.06 ?, -0.10 ?); LS (0.15 ?, 0.17 ?, -0.02 ?, -0.08 ?); SCS (-0.03 ?, 0.18 ?, -0.05 ?, -0.08 ?).

This study analyzed historical hydrographic data consist of 95 years of NODC data and 18 years of NCOR data. Variations of upper-layer current in Luzon Strait and its neighboring Northern South China Sea are investigated by the method of dynamical topography. On the other hand, higher salinity characteristic of the North Pacific Tropical Water (NPTW) is used to trace the water mass distribution and its seasonal variations in the studied area. The result shows that the maximum of x-component velocity (along 120.25˚E, relative to 400 m ) in the Luzon Strait occurs in the middle of the strait, the flow direction is westward and the speed increases toward the surface. At the north and south ends of the strait flows are eastward and the maximum speed core is at the depth of 0~50 m. Horizontal distribution of flow fields indicates that intrusion of the north Pacific waters into the South China Sea through the Luzon Strait in the whole year. Westward bifurcation of the intrusion flows occurs at 20~21˚N. A cyclonic eddy exists in the South China Sea all year-round, and its core is located at about 18˚N, 118˚E. This eddy strengthens as the monsoon prevails with its speed reached in January and February. Annual mean upper-layer transport (0~ 400 m) of the Luzon Strait is estimated to be about 3.5 Sv (positive value means westward) with a maximum value of about 6.5 Sv in December and a minimum value of 1.1 Sv in June. The Upper-layer current of the SCS is dominated by the monsoon. When the northeast monsoon prevails, the currents are affected by the Ekman effect to form high sea surface height in the north and low sea surface height in the south to produce a westward current. On the other hand, strong two coexistent wind stress curls with reversing signs during the northeast monsoon produce a westward current along the line of zero curl in the middle of the strait. The distribution of the NPTW(£m£c=23.5¡ã25.5 kg/m3,S >34.5 psu) is mostly at a depth of 120¡ã130 m in the South China Sea. NPTW were traced at 125 m depth, and the result indicates that this water mass enters the South China Sea through the Luzon strait all year-round. The intrusion path is along the continental slope of south China. The extent of intrusion reaches the maximum between December and February, and the water mass can spread into the South China Sea basin.

This study utilized, for the first time, the d18Osw as a tracer to investigate the seasonal variations of circulation in the Taiwan Strait (TS), which is the predominant sea passage with an average depth of 60 m connecting the East China Sea (ECS) and the South China Sea (SCS). The result shows that the circulation system in TS is mainly influenced by the inter-mixing among the China Coastal Water (CCW), the SCS water (SCSW), and the Kuroshio Water (KW). In spring, the KW dominates in TS, whereas the CCW is still observed in northwest TS. During the summer, SCSW replaces the KW and becomes the major water type in the TS, yet the KW is found to be restricted in the southwest part and the bottom of the TS. Due to the larger discharge from rivers (mainly the Yangtz River), the CCW has a more extensive distribution in the TS in summer than other seasons. In fall and winter, the CCW occupies the northern part of TS due to the stronger northeastern monsoon which limits the intrusion of the KW through the Luzon Strait to the northern TS. The two distinct water types inevitably form a front in the central TS. The hydrographic variations at Penghu Channel (PHC) were also explored in this study. The d18Osw indicates that the perennial intrusion of the KW into the PHC is varying throughout different seasons. This intrusion is found strongest in fall and winter. In summer, the upper layer of PHC is occupied chiefly by SCSW, while the KW remains at the bottom layer in PHC. By including an additional inflow of 0.5Sv from TS to ECS, this study further reconstructed a box model of the ECS, which was previously furnished by Lin(1999). The new estimates suggest that ~0.38*104 km3/year of the Kuroshio surface water (0-50m) and ~1.54*104 km3/year of the upwelled Kuroshio subsurface water (50-150m) are transported to the ECS, while ~3.83*104 km3/year of the ECS water are exported to the western Pacific Ocean.