Lead in the southern East China Sea

Lead in the southern East China Sea

Fei-Jan Lin, Shih-Chieh Hsu, Woei-Lih Jeng*

Abstract

At present, in most oceans the lead (Pb) biogeochemical cycling has been disturbed by anthropogenic Pb through atmospheric input. The Pb concentrations in the upper water posi-tively correlate with atmospheric input ¯uxes of Pb. The North Paci®c is a€ected greatly by atmospheric substances via long-range transport from eastern Asia, especially from Mainland China. Mainland China may export considerable amounts of pollutants into the seas via rivers and the atmosphere owing to its recent fast growth in industry and economy. The East China Sea lies in an important geographical positionÐa transit between Mainland China and the western North Paci®c. However, no data are available for seawater concentrations of Pb, a representative element with anthropogenic origin. In this work seawater samples from both 5 and 30±50 m water layers of 15 stations occupied over a cyclonic eddy in the southern East China Sea were analyzed for particulate Pb (PPb) and dissolved Pb (DPb). The Mean con-centration of DPb (128 ng/l) in the southern East China Sea upper waters (450 m) is approximately several times higher than those in the Paci®c; the high DPb concentrations in the southern East China Sea waters correspond to much higher atmospheric supplies of Pb to the East China Sea. Thus, this study partly ®lls the `data gap' of the marginal seas. Also, it indicates that the East China Sea may be considerably contaminated by deposited polluted aerosols. Spatial distributions of DPb in the surface water show a tendency of increasing con-centrations with distance o€shore, that depends on the magnitudes of atmospheric Pb inputs and on particle scavenging processes. In contrast to DPb, spatial distributions of PPb basically display an `'-like picture and a tendency of decreasing concentrations with distance o€shore. These are related to riverine and scavenging sources and to the drive by the eddy. Additionally, the residence times of DPb in the surface water were estimated to be about 2 years, agreeing well with the reported data. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: East China Sea; Dissolved lead; Particulate lead; Biogeochemical cycle; Residence time; Scavenging

0141-1136/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-1136(99)00076-8

* Corresponding author. Tel.: +886-2-23636040, ext. 301; fax: +886-2-23626092. E-mail address: wljeng@iodec1.oc.ntu.edu.tw (Woei-Lih Jeng).

1. Introduction

In most oceans the biogeochemical cycling of lead (Pb) has been disturbed by anthropogenic Pb since the beginning of the Industrial Revolution, and the dis-turbance has become more serious since the inception of consuming alkyl-leaded gasoline. The fact was recorded in sediments and corals (Patterson, 1987; Shen & Boyle, 1989; Veron, Lambert, Isley, Linet & Grousset, 1987). Pb is a particle-reactive element with a strong anity for certain particles such as clay minerals, Mn oxide, Fe oxyhydroxide and carbonate (Li, 1981; Sturchio et al., 1997; Turekian, 1977; Whit®eld & Turner, 1987). Most of the Pb discharged from rivers is removed from overlying water in estuaries or coastal seas (Elbaz-Poulichet, Holliger, Huang & Martin, 1984; Schaule & Patterson, 1981). Atmospheric transport thus becomes the major passage for many anthropogenic constituents and Pb from land to o€shore seas. Certainly, the Pb ¯uxes into the open ocean are dominated by the atmospheric supplies (Nriagu & Pacyna, 1988; Patterson & Settle, 1987).

The western North Paci®c receives a large in¯ux of mineral particles and pollutants from eastern Asia, especially from Mainland China through long-range atmospheric transport (Duce, Arimoto, Ray, Unni & Harder, 1983; Gao, Arimoto, Zhou, Merrill & Duce, 1992). The East China Sea (ECS), one of the largest marginal seas in the world, is situated in a transit between Mainland China and the western North Paci®c. Massive quantities of terrestrial materials are emitted to the atmosphere in Mainland China (which has a rapidly developing economy and industrial base) and are depos-ited on the ECS shelf (Gao et al., 1996, 1997). Mainland China has been su€ering from serious air pollution and acid rain owing predominantly to coal combustion by nearly two thousand power plants especially in eastern China, and by most house-holds especially in northern China (Qian & Zhang, 1998; Zhao & Sun, 1986). The regional (even global) atmospheric environment could be in¯uenced by these large discharges of SO2(around 20 Mt/year at present), greenhouse gases (ranking as third

largest generator of the world), and trace constituents (including Pb) (Dod et al., 1986). These pollutants could a€ect Chinese marginal seas (particularly the ECS) and the Paci®c Ocean through the westerlies transport. It can therefore be expected that the ECS Pb system has gradually been a€ected by eolian inputs of Pb.

Pb concentrations in sediments have been increasing in coastal regions o€ Main-land China since about 1980, based on the measurements of Pb pro®les and the dating of sediment cores (Huh & Chen, 1999). However, the impact on seawater composition corresponding to these deposited eolian constituents is not known. This work attempted to determine the extent of the contamination by eolian inputs from the analysis of seawater Pb. At present, no data are available for dissolved Pb (DPb) concentrations in seawater of the ECS. Flegal and Patterson (1983) have suggested that the DPb concentrations in the surface oceans correspond directly to and cor-relate positively with magnitudes of atmospheric ¯uxes of Pb into the oceans. This work could examine that suggestion by taking advantage of the ECS with its unique position, neighboring the western North Paci®c.

A total of 30 seawater samples were collected from both 5 and 30±50 m water layers of 15 stations occupied over a cyclonic eddy in the southern ECS o€ northeastern

Taiwan, and were analyzed for both particulate Pb (PPb) and DPb. The southern ECS is a dynamically energetic marginal sea based upon its geological, physical, chemical and biological features (Chen Lee, 1995; Hsueh, Wang & Chern, 1992; Liu et al., 1992; Wong, Pai, Liu, Liu & Chen, 1991). The Kuroshio Current ¯ows along the eastern coast of Taiwan and collides with the shoaling ECS shelf when it approaches the northeastern tip of Taiwan. As a result, it causes a variety of phenomena, one of which is the development of a cyclonic eddy. Eddies can act as an important vehicle, exchanging seawater constituents between distinct water masses, particularly coastal waters and o€shore waters (Hayward & Mantyla, 1990; The Ring Group, 1981). However, the spatial distributions of metals over eddies are poorly understood (Bishop & Fleisher, 1987; Sakamoto-Arnold, Hanson, Huizenga & Kester, 1987). Determining the in¯uences of the cyclonic eddy on the spatial dis-tributions of both PPb and DPb in the upper water is another goal of this work. 2. Materials and analysis

Seawater samples were taken from both 5 and 30±50 m water layers of the southern ECS o€ northeastern Taiwan during August 1994 on board National Tai-wan University's R/V Ocean Researcher I. They were taken along three NE±SW parallel transects containing 15 hydrographic stations (Fig. 1) and collected using 20-l acid-pre-cleaned GO-FLO bottles on a CTD rosette. Details of stations and hydrographic parameters are given in Table 1. When the rosette was recovered, seawater was transferred into 20-l PE bottles (which were rinsed twice with the sea-water sample before ®lling) with a ®lling and venting closure that could prevent contamination from deck air. About 20±40 l of seawater were pressure-®ltered through a pre-cleaned and pre-weighed 142-mm 0.4-mm Nuclepore polycarbonate ®lter using a Sartorius PTFE pressure ®lter holder and a Master¯ex1_{wriggle pump.}

One liter ®ltrate was stored in acid-pre-cleaned 1-l PP bottles and acidi®ed to pH<2 with Suprapur HNO3(Merck, Germany) for subsequent analysis of dissolved trace

metals in land-based laboratory. Particulate-laden ®lters were stored in plastic petri dishes. Seawater and particulate samples were kept in a refrigerator and a desicca-tor, respectively, until analysis.

After drying and weighing, particulate-laden ®lters were processed by the total digestion method using mixed acids of Suprapur HF, HNO3and HClO4(all from

Merck, Germany). The seawater ®ltrates were concentrated by using Chelex-100 resin (100±200 mesh and ammonium form) columns after adjusting pH to 6.5 in our land-based laboratory. Resin columns were pre-cleaned with 2 N Suprapur HNO3

and DDW twice before use. The elution also used 2 N Suprapur HNO3. All

treat-ments for the particulate and seawater samples were carefully processed on class 100 laminar ¯ow bench.

Metals were analyzed using a Hitachi Zeeman graphite furnace atomic absorption spectrophotometer (model Z-8100) equipped with an autosampler SSC-200. Each sample solution (digestion and elution solutions) was analyzed in triplicate at least. The quality of determination was controlled by analysis of BCSS-1 standard

reference samples (National Research Council, Canada). Our Pb data (19.2‹0.3 mg/ g, n=5) fell at the lower end of certi®ed value (22.7‹3.4 mg/g). By spiking a small volume of known amounts of Pb (Pb standard-Merck) into four separated batches of 1 l duplicatedly puri®ed seawater and by subsequent pre-concentration pro-cedures, the recovery and precision of Pb for pre-concentration were obtained to be 77‹3 and 4% (n=5), respectively. No corrections were made on the DPb data because the recovery of 77% was good enough for our purposes and the reproduci-bility was excellent.

To estimate the procedure blank, 20-l seawater samples, which have been puri®ed by passing through a Chelex 100 column twice to eliminate trace metals, were taken on board as `blank samples'. The blank samples were processed by the same pro-cedures, including shipboard ®ltration and subsequent pre-concentration for ®ltrate in land-based laboratory, as `real samples'. The Pb levels in blank samples detected in this study were much lower than those in real samples by at least one order of magnitude.

Fig. 1. Sampling locations. Inset is the regional map. The seawater collection is made along three trans-ects: the southern East China Sea (ECS) shelf and slope and the Southern Okinawa Trough. Each transect contains ®ve stations. An o€shore island, Pengchiayu Island (shown as a star), is the collection station of marine aerosols, providing the eolian Pb ¯uxes used in the discussion of the study. Bathmetric map is also shown. TS, Taiwan Strait.

3. Results and discussion

The circulation patterns, particularly the summertime pattern, of the study region are shown in Fig. 2. The generating mechanism of the eddy has been discussed elsewhere (Tang, Hsueh, Yang & Ma, 1999), as have the circulation patterns of the study region (Hsu, Lin, Jeng & Tang, 1998).

Table 1

Station locations, temperature, salinity and both dissolved and particulate Pb (DPb and PPb) as well as particulate Al (PAl) and particulate Mn (PMn) concentrations in the southern East China Sea surface seawater

Station

No. Latitude(_N) Longitude₍_{E) (}Temperature_C) Salinity_(psu) DPb_(ng/1) PPb_(ng/1) PAI a (mg/l) PMn b (ng/l) 5 m 1 25_17.880 ₁₂₁_48.670 _{25.2 34.04 99 15.8 35.1 456} 3 24_54.010 ₁₂₂_12.040 _{24.1 34.14 83 12.1 28.2 315} 4 24_42.310 ₁₂₂_24.210 _{26.4 33.78 46 10.5 9.5 446} 5 24_54.290 ₁₂₂_36.250 _{27.4 34.01 246 3.9 4.3 38} 6 25_06.280 ₁₂₂_24.300 _{25.1 34.13 224 12.8 26.9 385} 8 25_29.900 ₁₂₂_00.050 _{24.6 34.33 41 4.2 5.1 43} 9 25_41.880 ₁₂₂_12.040 _{24.8 34.28 242 5.3 9.1 91} 11 25_18.350 ₁₂₂_35.930 _{25.9 34.08 220 23.2 31.5 428} 12 25_06.460 ₁₂₂_48.450 _{27.6 34.15 177 9.2 1.0 10} 13 25_17.920 ₁₂₂_59.870 _{28.0 34.22 90 1.6 0.2 9} 14 25_29.990 ₁₂₂_48.090 _{27.2 34.14 184 9.8 1.1 10} 16 25_54.180 ₁₂₂_23.920 _{26.6 34.16 96 5.5 3.3 31} 17 26_06.250 ₁₂₂_36.170 _{26.2 34.22 132 4.5 5.1 101} 19 25_42.000 ₁₂₃_00.260 _{27.7 33.98 461 3.2 1.5 43} 20 25_29.970 ₁₂₃_11.930 _{27.9 34.01 147 2.9 8.4 14} 30±50 m 1 25_17.880 ₁₂₁_48.670 _{23.0 34.04 91 12.7 32.4 345} 3 24_54.010 ₁₂₂_12.040 _{20.9 34.44 62 14.6 48.5 429} 4 24_42.310 ₁₂₂_24.210 _{23.0 34.57 82 6.6 6.7 48} 5 24_54.290 ₁₂₂_36.250 _{25.1 34.58 82 8.1 4.3 32} 6 25_06.280 ₁₂₂_24.300 _{22.8 34.37 82 38.7 39.1 504} 8 25_29.900 ₁₂₂_00.050 _{21.5 34.60 59 5.1 9.8 116} 9 25_41.880 ₁₂₂_12.040 _{22.2 34.66 124 5.5 9.4 90} 11 25_18.350 ₁₂₂_35.930 _{24.2 34.30 260 18.7 19.9 339} 12 25_06.460 ₁₂₂_48.450 _{25.6 34.49 83 4.4 0.9 9} 13 25_17.920 ₁₂₂_59.870 _{26.8 34.38 49 4.8 3.1 83} 14 25_29.990 ₁₂₂_48.090 _{25.2 34.64 280 8.0 1.5 16} 16 25_54.180 ₁₂₂_23.920 _{23.1 34.84 73 4.1 4.6 46} 17 26_06.250 ₁₂₂_36.170 _{24.7 34.61 258 3.3 1.5 19} 19 25_42.000 ₁₂₃_00.260 _{25.3 34.57 217 2.9 0.7 13} 20 25_29.970 ₁₂₃_11.930 _{26.8 34.35 517 4.6 2.6 13}

a _{Taken from Hsu et al. (1998).} b_{Taken from Hsu (1998).}

3.1. Pb concentrations

The concentrations of DPb and PPb in 30 surface seawater samples and the associated hydrographic parameters are summarized in Table 1. DPb and PPb con-centrations of all samples (n=30) range from 41 to 517 and 1.6 to 38.7 ng/l, and average 128‹117 and 6.8‹7.7 ng/l, respectively (Table 2). The PPb accounts for only a small fraction (<10% on average) of total Pb (assumed to be the sum of DPb and PPb), agreeing well with earlier studies (Balls, 1985a; Helmers, 1996; Schaule & Patterson, 1981). The DPb concentrations in the southern ECS surface seawaters are comparable with those (range 7±880 ng/l, average 235 ng/l, sampling in May; range 7±800 ng/l, average 301 ng/l, sampling in October) of the coastal seawater of southeastern China reported by Gao and Zou (1998). Their sampling area (25₁₅0_±

25₄₅0_{N, 119}₃₂0_±120₁₀0_{E) is approximately 250 km away from our study area.}

Also, their values and ours fall in the concentration range (0.03±3.26 mg/l with a

Fig. 2. Circulation pattern (current ®eld) at 16 m depth of the southern East China Sea (ECS) o€ north-ern Taiwan, especially for summer season. It is measured by Shipboard Acoustic Doppler Current Pro®ler (Sb-ADCP) simultaneously on the same cruise as seawater sampling in this study (Tang et al., 1999). KC, Kuroshio Current; KBC, the Kuroshio Branch Current; E, the cyclonic eddy; F, the ®lament; TS, Taiwan Strait. This map is after Hsu et al. (1998) which gave a detailed review for the entire circulation pattern of the study area.

mean of 0.55 mg/l) of the northwestern Paci®c Ocean surface seawater o€ southern Japan (28₃₀0_±36₃₀0_{N, 129}_±145_{E) (Wang, Zou, Lu & Jing, 1990). It has been}

noted that there were much higher DPb levels (0.9±2.9 mg/l) in the Changjiang River mouth water (Yu, 1995). The DPb concentrations in the southern ECS waters, however, are higher than those of many other oceanic regions. For example, in the Paci®c, the surface concentrations of DPb are usually smaller than 20 ng/l (Flegal & Patterson, 1983; Schaule & Patterson, 1981), while in the Atlantic, they fall in a range of 20±75 ng/l, averaging around 30 ng/l (Boyle, Chapnick, Shen & Bacon, 1986; Brugmann, Danielsson, Magnusson & Westerlund, 1985; Helmers, Mart, Schulz-Baldes & Ernst, 1990). Furthermore, the elevated DPb concentrations in surface waters from the study area are higher than those from the Mediterranean Sea but they have declined from 85 ng/l in 1986 to 32 ng/l in 1992 (Nicolas, Ruiz-Pino, Buat-Menard & Bethoux, 1994), and even about two times higher than those from the North Sea with an average concentration of about 60 ng/l (Balls, 1985b; Brugmann et al., 1985). Although surface concentratons of DPb are high, they are still reasonable, based on their relationships with atmospheric ¯uxes of Pb and the calculated DPb residence times comparable with literature data, as discussed below. Coupled with the above Pb data of the ECS, it can demonstrate that the DPb pool of the entire ECS may be considerably perturbed by anthropogenic Pb.

3.2. Spatial distributions

Spatial distributions of DPb in both 5 and 30±50 m water layers are displayed in Fig. 3; those of PPb, in Fig. 4. The concentrations of DPb tend to increase with distance from the Taiwan coast towards o€shore. The distributions of PPb show an opposite pattern to those of DPb, with decreasing concentrations seaward and with an `'-like shape. The pattern is similar to that of particulate Al (PAl) (Hsu et al., 1998) and particulate Mn (PMn) (Hsu, 1998). PPb concentrations are also strongly correlated with PAl and PMn concentrations: in 5 m water layer, PPb vs. PAl r=0.82 and PPb vs. PMn r=0.81; and in 30±50 m water layer, PPb vs. PAl r=0.74 and PPb vs. PMn r=0.86.

The seaward increases of the DPb concentrations suggest that the atmospheric Pb supply, predominantly from Mainland China, either by dry or wet deposition should

Table 2

Means and ranges of dissolved and particulate Pb (DPb and PPb) concentrations in the southern East China Sea surface seawater

DPb (ng/l) PPb (ng/1)

5 m (n=15) Range 41±461 1.6±23.2

Mean 137‹107 6.5‹5.9

30±50 m (n=15) Range 49±517 2.9±38.7

Mean 119‹129 7.1‹9.3

Overall samples (n=30) Range 41±517 1.6±38.7

be the major source of the seawater DPb. In fact, the total atmospheric Pb ¯uxes have been estimated to be 590 ng/cm2_{/year (Lin, unpublished data) based on a}

2-year aerosol measurement carried out on a small o€shore island (Pengchiayu

Fig. 3. Spatial distributions of dissolved Pb (DPb) in both (A) 5 m and (B) 30±50 m water layers in the southern East China Sea.

Island) located in the study area (Fig. 1). These ¯uxes are very high compared to many coastal or marginal seas (Injuk, Van Grienek & De Leeuw, 1998; Patterson & Settle, 1987). E€ective removal of DPb from overlying waters by particle scavenging

Fig. 4. Spatial distribution of particulate Pb (PPb) in both (A) 5 m and (B) 30±50 m water layers in the southern East China Sea.

by the high loads of suspended matter in coastal waters is another cause for the DPb variations observed (Fig. 3) (Brugmann et al., 1985; Helmers & Rutgers van der Loe€, 1993). In addition, an examination of the DPb contour patterns shown in Fig. 3 reveals that the isopleth of 200 ng/l curves around the eddy center (Station 11). It is speculated that the cyclonic eddy over the shelf-slope of the southern ECS o€ northeastern Taiwan a€ected the DPb distributions by introducing high DPb o€-shore waters toward coastal area of low DPb waters. The in¯uence of the eddy on the horizontal distributions of PAl (Al was taken as an indicator of lithogenic par-ticles) in the region has also been described (Hsu et al., 1998).

Maximum concentrations of PPb occur at Station 11 in 5 m water layer and at Station 6 in 30±50 m water layer (Fig. 4). The distribution patterns of PPb suggest that riverine input is the major source for crustal elements (like Al and Mn) and likely for anthropogenic elements (like Pb) and that the cyclonic eddy drives their distributions (Hsu et al., 1998). Mean Pb:Al ratios in seawater particulates (1.3610ÿ3_{) are high, compared to the mean Pb:Al ratio of shale (0.2510}ÿ3₎

(Turekian & Wedepohl, 1961). This indicates that Pb is enriched in the ECS surface seawater particulates. Coupled with the landward decrease of DPb concentrations, the results suggest that the PPb concentrations could be enhanced due to DPb transformation via particle scavenging processes. The aluminosilicates (or clay minerals) and manganese oxides (or manganese oxide coating onto various particles) are likely to act as an ecient scavenger to DPb (Li, 1981; Turekian, 1977; Whit®eld & Turner, 1987), as indicated by the strong correlation among PPb, PAl and PMn and concurrence of PPb, PAl and PMn maxima in the region. The horizontal dis-tributions of PPb are also a€ected by the cyclonic eddy, like those for PAl (or lithogenic particles) (Hsu et al., 1998) as indicated by the `'-like distribution. 3.3. Relationships between DPb concentrations in upper waters and magnitudes of atmospheric Pb input ¯uxes

Using the analytical results of this work coupled with the earlier results of other researchers, a positive correlation between surface concentrations of DPb and eolian Pb ¯uxes from other oceanic regions (the North Paci®c, the North Atlantic and the North Sea) (Balls, 1985a,b; Boyle & Huested, 1983; Brugmann et al., 1985; Flegal & Patterson, 1983; Helmers et al., 1990; Injuk et al., 1998; Maring & Duce, 1990; Maring, Patterson & Settle, 1989; Patterson & Settle, 1987) can be obtained (Fig. 5). This is further evidence that the sources of Pb pollution are eolian inputs principally from Mainland China. This contrasts to many oceans where the Pb concentrations are declining due to the phase-out of leaded-gasoline recently (Boyle et al., 1986; Helmers et al., 1990; Nriagu, 1996).

3.4. Residence times of DPb in the surface water

In general, the residence time () of an element in seawater can be estimated by the following equation:

 ˆ I=F

where t is the residence time of a given element; I, the inventory of a given element in the environmental reservoir of interest; and F, the input or output ¯uxes of a given element into or from the reservoir. Under steady-state conditions, the rates of input and output are equal.

Firstly, inventories of DPb in the top 50 m at each station were calculated, and then residence times at each station could be estimated if the atmospheric input ¯uxes of Pb are available. Here we assumed the atmospheric Pb input ¯uxes to be 295 ng/cm2_{/year based on the measurements that the total Pb atmospheric ¯ux is}

590 ng/cm2_{/year (Lin, unpublished data) and the Pb solubility of aerosols as}

depos-ited into seawater is 50% (Duce et al., 1991).

The calculated mean residence time of DPb in the surface water is 2.1 years, very comparable with those estimated from210_{Pb (1.8 years) in the same study region}

Fig. 5. Relationships between surface seawater dissolved Pb (DPb) concentration and total eolian Pb input ¯ux. Sources: for the North Paci®c easterlies (NPE), DPb from Flegal and Patterson (1983), eolian Pb from Flegal and Patterson (1983) and Maring and Duce (1990); for the North Paci®c westerlies (NPW), DPb from Flegal and Patterson (1983), eolian Pb from Flegal and Patterson (1983) and Maring et al. (1989); for the North Atlantic easterlies (NAE), DPb from Helmers et al. (1990), eolian Pb from Pat-terson and Settle (1987); for the North Atlantic westerlies (NAW), DPb from Helmers et al. (1990) and Boyle and Huested (1983), eolian Pb from Flegal and Patterson (1983); for the North Sea (NS), DPb from Balls (1985a,b) and Brugmann et al. (1985), eolian Pb from Injuk et al. (1998); for the East China Sea (ECS), data from this study.

(Chung & Wu, 1995) and also from literature data of common Pb or210_{Pb (1.7±5}

years) from many seas (Bacon, Spencer & Brewer, 1976; Nozaki, Thomson & Tur-ekian, 1976; Schaule & Patterson, 1981; Veron et al., 1987). It should be mentioned that the temporal variations in surface concentrations of DPb and atmospheric Pb ¯uxes were not taken into account in the calculations of residence times.

4. Summary

In the southern ECS, DPb concentrations in upper waters are several times higher than those in the North Paci®c; the pronounced DPb concentrations correspond to high atmospheric input ¯uxes of Pb. The results agree well with the suggestion by Flegal and Patterson (1983) that atmospheric inputs and seawater concentrations should be correlated. Also, the results would provide background information of Pb concentrations for monitoring future change in the ECS seawater compositions if Mainland China gradually changes production of pollutants owing to its rapid industrialization. Distributions suggest that DPb is a function of atmospheric sup-plies of Pb and particle scavenging processes. Whereas for PPb, distributions are controlled by riverine sources with an anthropogenic origin, scavenging and by eddy circulation. The DPb usually accounts for most (>90%) of total Pb; its residence times in the upper water (50 m) are estimated to be about 2 years.

Acknowledgements

We thank the technicians and crew of R/V Ocean Researcher I for help with sampling. Thanks are also extended to Mr. K. Huang for pre-treatment of samples. Comments by Dr. Samuel N. Luoma and two anonymous reviewers helped improve the manuscript. This work was supported by National Science Council (ROC) grants NSC 84-2611-M002A-007K2 and NSC 85-2611-M002A-026K2 to F.J.L. References

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