Phosphorus speciation and budget of the East China Sea
Introduction
The East China Sea (ECS), located at 26–31°N and 121–126°E, is surrounded by China to the west, the Kuroshio Current to the east, Taiwan and the Taiwan Strait to the south, and the Yellow Sea to the north. The ECS continental shelf is one of the largest shelves in the world. The combined continental shelf area of the ECS and the Yellow Sea is 0.9×1012 m2 in area, or 3% of the world’s shelf. The suspended load and water discharge to the region are 1.5×1015 g yr−1 and 1.0×1012 m3 yr−1, representing 26% and 10% of the total world discharge, respectively (Milliman and Meade, 1983; Milliman et al., 1987). Due to the high discharge nutrients from the Changjiang and Yellow Rivers, the inner and middle shelves of the ECS form one of the highest primary production areas in the world (Hama et al., 1997; Gong et al., 2003). Although shelf regions account for only 8% of the total area of the world's oceans, this area contributes from 18 to 33% of global primary production (Wollast, 1991).
The ECS receives a rich supply of phosphorus from the discharge of the Changjiang river, 11.74×109 mol P yr−1 (Qu et al., 1993) and from the upwelling of the Kuroshio Intermediate Water (KIW), 14.3×109 mol P yr−1 (Chen, 1996). However, Wong et al. (1998) investigated the nutrient distribution in the ECS in July of 1992 and found an ‘excess nitrate’ concentration of up to 6 μM and a dissolved inorganic phosphate (DIP) concentration of <0.07 μM in an area covering one-third to one-half of the ECS, where salinities were found to be <30.5 psu in the top 10–15 m layer. These authors suggested that the ECS is an oligotrophic environment, since low-P availability measured as DIP, that are considered to be limiting factors for phytoplankton production. In contrast, a more recent study indicated that primary production in the ECS is limited by N deficiency in the summer and by sun light in the winter (Chen et al., 2001).
The Kuroshio Edge Exchange Processes (KEEP) program focuses on the interactions between the ECS and the Kuroshio and has been carried out for more than a decade (Wong et al., 2000). With the exception of DIP, the other phosphorus forms, such as dissolved organic P (DOP) and particulate P, in the ECS have been rarely investigated in the KEEP study. However, recent studies have shown that the concentration of DOP can exceed that of DIP in the surface waters of many coastal and oceanic environments (Orrett and Karl, 1987; Bentzen et al., 1992; Bjorkman and Karl, 1994; Conley et al., 1995; Karl and Yanagi, 1997; Monaghan and Rutternberg, 1999; Wu et al., 2000). For example, it was found that the surface concentrations of DOP around the Sargasso Sea near Bermuda accounted for 94–99% of the total dissolved phosphorus (TDP) (Wu et al., 2000). These studies indicate that algae and bacteria are capable of utilizing DOP compounds as a source of phosphorus nutrition in marine environments.
Phosphorus is an element with high particle affinity. The removal of DIP from solution by adsorption onto particles has long been suggested (Froelich, 1988) and observed in field studies (Fox, 1989; Balls, 1994; Zwolsman, 1994). However, particulate inorganic P (PIP) may be resupplied in the water column during resuspension (Edmond et al., 1985; Berner and Rao, 1994; Balls, 1994). Based on these facts, studies on marine ecosystems are incomplete if they do not take other phosphorus species present in the water column into account. In order to accurately quantify the phosphorus geochemistry in the ECS, DIP, DOP and PIP in the water column were measured. In addition, the phosphorus budget of the ECS was calculated by employing a box model based on water and phosphorus mass balance. Because this study focused on the measurements of phosphorus, iron and suspended particulate matter (SPM), and other parameters, such as nitrite, nitrate, organic nitrogen, silicate and primary production, were not determined.
Section snippets
Sampling
Sampling stations (Fig. 1) were placed in a rectangular grid so as to encompass the ECS. Stations O1–O5 were located along the Okinawa Trough, through which the Kuroshio Water flows. Stations M1–M3 and M4–M8 were situated across the northern ECS and the northern Taiwan Strait, respectively. Stations I1–I6 were located along the coast of China. To facilitate interpretation of the results, the sampling stations were divided into three groups based on their locations and bathymetry of the sampling
DIP and DOP
The profiles of DIP and DOP for each station are shown in Fig. 2. The concentrations of DIP and DOP in the ECS ranged from 0.05–3.01 to 0.01–0.54 μM, respectively. The concentrations of DIP and DOP at stations in the inner shelf were significantly higher than those in the middle and the outer shelves. These results are consistent with those of a previous study conducted by Chen et al. (2001), which showed that the concentrations of DIP in surface waters along the coast of China and in the
Discussion
Our results show that DIP was the dominant form of TDP in the inner and middle shelf stations in the ESC. In contrast, the DOP concentration generally surpassed the DIP concentration in the surface layer (<100 m) of the outer shelf stations. Overall, the profile of DOP in the outer shelf stations showed enrichment in the surface layer and depletion at deeper depths, the inverse of the DIP profiles. Compared to DIP, knowledge of DOP distribution in coastal and oceanic waters is limited. Recent
Conclusion
Concentrations of DIP, DOP and PIP (w/w) in the ECS ranged from 0.05 to 3.01 μM, 0.01–0.54 μM and 0.18–13.94 μmol g−1, respectively. The concentrations of these parameters, especially PIP, decreased seaward. With the exception of surface water in the outer shelf (<100 m), DIP was the major form of phosphorus and generally accounted for more than 60% of the total P pool (TDP+PIP). Because DIP concentrations in the surface layer (<100 m) of the outer shelf were relatively low (<0.1 μM), DOP
Acknowledgements
The author is grateful to Dr. Jahnke, R.A. and to an anonymous referee for their constructive comments and suggestions which led to significant improvements to the manuscript. Thanks are also given to Prof. G.C. Gong who kindly invited me to join the cruise and to the assistance of the captain and crew of the R/V Ocean Researcher I during sampling. This research was financially supported by the National Science Council of the Republic of China under grants NSC 90-2611-M-019-007 and
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