Distribution of the degree of nutrient consumption in the northern Taiwan Strait Li-Yu Hsing and Chen-Tung Arthur Chen Institute of Marine Geology and Chemistry, National Sun Yat-sen University Fig.2 Potential temperature (  ) vs salinity for ORII-806. Solid lines are the South China Sea (SCS) and Western Philippine Sea (WPS) water. Fig.7 Potential temperature (  ) vs salinity for ORI-631. Solid lines are the South China Sea (SCS) and Western Philippine Sea (WPS) water. ORII-806 ORI-631 The general features of upwelled water are cold, salty and nutrient-rich. However, factors such as the air-sea exchanges of heat affect temperature, and freshwater input from rivers and groundwater, precipitation and evaporation affect salinity. Further, nutrients may be consumed by biological productivity. Takahahsi et al. (1986) first developed the concept of Aging Index of Upwelling (AIU) to identify upwelling more clearly. As biologically-important elements exist mostly in the dissolved inorganic forms in young upwelled waters, and in old upwelled waters they exist in the particulate organic forms, the aging status of upwelled waters can be expressed as the relative percentages of biologically-important elements in the organic and inorganic forms (Chen, 2000). The value of AIU is nearly zero in the young upwelled waters and is almost one in the old upwelled waters. Since AIU can not directly identify the age of upwelled water, we choose to use the degree of nutrient consumption (DNC) based on the same concept. In this case, we choose DNC N which is the relative percentage of dissolved inorganic nitrogen (DIN) and organic nitrogen (particulate and dissolved organic nitrogen, PON and DON) : DNC N = ( PON+DON ) / ( PON+DON+DIN ). The hydrology in summer (the R/V Ocean Researcher II 806 cruise, ORII-806) in the northern Taiwan Strait (Fig.1) was comprised mainly of two different water masses (Fig. 2). The T/S diagrams for Sts. D-I were consistent with those found in the southern Taiwan Strait with a clear South China Sea (SCS) origin, but those at Sts. A-C could not be found in the southern Taiwan Strait, although they were also of the SCS origin. The logical explanation is that waters at Sts. A-C were SCS waters carried by the Kuroshio off eastern Taiwan (Chen and Wang, 1998). A front divided the two water masses. According to the cross-sections of temperature, salinity and SiO 2 (Figs. 3, 4, 5), there was a front at St. C. It seems that subsurface waters at either St. B or D could have upwelled at St. C. However, based on the T-S diagram, it is clear that the hydrology at St. C was completely different from St. D, so the water at St. C could not have come from St. D. From the DNC N distribution (Fig6), it seems that subsurface waters at Sts. A and B first downwelled then upwelled at St. C. As shown in Fig.7 (ORI-631), in winter the water in the northeastern Taiwan Strait was comprised of both the China Coastal Current (CCC) which was cold and fresh, and the water from the Kuroshio which flowed off eastern Taiwan. Again, waters at Sts. B1 and C1 could not have come from the Taiwan Strait as such a T/S correlation did not exist in the southern Taiwan Strait. Therefore, they were altered Kuroshio water after mixing with the CCC. There was also a front at St. C1. According to the cross- sections of temperature, salinity and DNC N (Figs. 8, 9, 10), the water at St. C1 could have upwelled from either St. B1 or D1. Suppose subsurface waters at St. D1 upwelled at St. C1, they should have provided low nutrient- water to St. C1, but instead, higher nutrients were found at St. C1 (Fig. 11). For this reason, subsurface waters at St.B1 probably upwelled at C1. Both in summer and winter, waters at and east of St. C (C1) in the northeastern Taiwan Strait were affected by the Kuroshio off eastern Taiwan. There was a front that divided the Kuroshio water, the SCS water that flowed through the Taiwan Strait and the CCC water (in winter). This implies that not all currents in the Taiwan Strait flow northward, although most of them do in summer. There is a current in northeastern Taiwan Strait which flows southward during both summer and winter. Fig.1 ORII-806 (sampled in August) and ORI-631(sampled in December) station locations. Introduction Fig.3 Cross-section of temperature for ORII-806. Fig.5 Cross-section of SiO 2 for ORII-806. Fig.6 Cross-section of DNC N for ORII-806. Fig.4 Cross-section of salinity for ORII-806. Fig.8 Cross-section of temperature for ORI-631. Fig.10 Cross-section of DNC N for ORI-631. Fig.11 Cross-section of SiO 2 for ORI-631. Fig.9 Cross-section of salinity for ORI-631. In summer In winter Conclusion