Summer Distribution and Geochemical Composition of ...svr4.terrapub.co.jp/journals/JO/pdf/6302/63020189.pdf · tal marginal regions (Hung and Chan, 1998; Zhang and Liu, 2002). Many

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Journal of Oceanography, Vol. 63, pp. 189 to 202, 2007

Keywords:⋅⋅⋅⋅⋅ Suspended-particulate matter,

⋅⋅⋅⋅⋅ distribution,⋅⋅⋅⋅⋅ geochemicalcomposition,

⋅⋅⋅⋅⋅ resuspension,⋅⋅⋅⋅⋅ lateral transport,⋅⋅⋅⋅⋅ Changjiang,⋅⋅⋅⋅⋅ Kuroshio,⋅⋅⋅⋅⋅ East China Sea.

* Corresponding author. E-mail: [email protected]

Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

Summer Distribution and Geochemical Composition ofSuspended-Particulate Matter in the East China Sea

JIA-JANG HUNG1*, CHIA-LIN CHAN1 and GONG-CHING GONG2

1Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan2Institute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, Keelung, Taiwan

(Received 24 April 2006; in revised form 14 September 2006; accepted 30 September 2006)

The distribution and geochemical composition of suspended-particulate matter (SPM)in the East China Sea (ECS) were investigated during the summer period of highcontinental runoff to elucidate SPM sources, distribution and cross-shelf transport.The spatial variability of SPM distribution (0.3–6.5 mg l–1) and geochemical compo-sition (POC, Al, Si, Fe, Mn, Ca, Mg and K) in the ECS was pronounced during sum-mer when the continental fluxes of freshwater and terrestrial materials were highestduring the year. Under the influences of Changjiang runoff, Kuroshio intrusion, sur-face production and bottom resuspension, the distribution generally showed stronggradients decreasing seaward for both biogenic and lithogenic materials. Particulateorganic carbon was enriched in surface water (mean ~18%) due to the influence ofbiological productivity, and was diluted by resuspended and/or laterally-transportedmaterials in bottom water (mean 9.4%). The abundance of lithogenic elements (Al,Si, Fe, Mn) increased toward the bottom, and the distribution correlations were highlysignificant. Particulate CaCO3 distribution provided evidence that the SPM of thebottom water in the northern part of the study area was likely mixed with sedimentsoriginally derived from Huanghe. A distinct benthic nepheloid layer (BNL) was presentin all seaward transects of the ECS shelf. Sediment resuspension may be caused bytidal fluctuation and other forcing and be regarded as the principal agent in the for-mation of BNL. This BNL was likely responsible for the transport of biogenic andlithogenic particles across or along the ECS shelf. Total inventories of SPM, POC andPN are 46, 2.8 and 0.4 Tg, respectively, measured over the total area of 0.45 ××××× 106 km2

of the ECS shelf. Their mean residence times are about 27, 13 and 11 days, respec-tively. The inventory of SPM in the water column was higher in the northernmostand southernmost transects and lower in the middle transects, reflecting the influ-ences of terrestrial inputs from Changjiang and/or resuspended materials fromHuanghe deposits in the north and perhaps from Minjiang and/or Taiwan’s rivers inthe south. The distribution and transport patterns of SPM and geochemical elementsstrongly indicate that continental sources and cross-shelf transport modulate ECSparticulate matter in summer.

gradient generally occurs in most estuarine and continen-tal marginal regions (Hung and Chan, 1998; Zhang andLiu, 2002). Many biogeochemical processes are conse-quently mediated by the formation, transformation andtransport of SPM over the continental margin. The EastChina Sea (ECS) comprises one of the largest continen-tal shelves in the world, receiving tremendous amountsof suspended sediments from Changjiang (Yangtze River)and Huanghe (Yellow River). The annual sediment loadsof Changjiang and Huanghe have previously been esti-

1. IntroductionSuspended-particulate matter (SPM) in the marginal

sea plays an important role in transport of materials fromland to the oceans (Walsh et al., 1988; Chester, 1990;Nittrouer and Wright, 1994). A strong SPM distribution

190 J.-J. Hung et al.

mated to be 4.90 × 108 ton.yr–1 and 9.00 × 108 ton.yr–1,respectively (Milliman and Meade, 1983), althoughHuanghe’s sediments discharged mainly into Bohai andthe Yellow Sea (Ren and Shi, 1986; Zhang et al., 1990;Chen et al., 2005). Despite the fact that sediment dis-charges from Huanghe have decreased steadily since the1950s and the average discharge was estimated as only8 × 108 ton.yr–1 in the 1980s (Walling and Fang, 2003)and about 3.9 × 108 ton.yr–1 in the 1990s (Yang et al.,1998; Wang et al., 2006), the sediment deposited in theHuanghe delta remains a potential source of suspendedsediment in the Yellow Sea and/or ECS. The suspendedsediments of riverine origin are likely subject to greatmodification in the ECS before emptying into the openocean.

During the last two decades, studies on SPM trans-port and distribution patterns of sediment in the ECS haverevealed that about 40% of suspended sediment trans-ported by Changjiang was deposited in the estuary, whilethe remaining sediment was deposited directly offshoreduring the summer season; however, much is resuspendedand carried southward by subsequent winter storms(Milliman et al., 1985a). DeMaster et al. (1985) estimatedthat about 40% of the sediment escaping the Changjiangestuary was deposited on the inner shelf north of 30°N.The remaining sediment may be moved farther south anddeposited in coastal areas (Milliman et al., 1985a). Thebroad middle shelf was dominated by sand, well-sortedfine sands and muddy sands with a very low accumula-tion rate of recent sediments (DeMaster et al., 1983; Huhand Su, 1999). However, a mud patch on the outer shelfsouth of Cheju Island is composed of sediments contain-ing calcite and appears to be derived from Huanghe ma-terials (Milliman et al., 1985b). In general, mostterrigenous suspended particles in the northern part of theECS water column may be derived from Huanghe, hav-ing been transported from the Yellow Sea during winterstorms (Yang and Milliman, 1983; Milliman et al., 1985b).Nevertheless, given the present dramatic reduction inHuanghe discharge, most Yellow Sea sediments wouldhave to come from the resuspension of seabed sediments.

Although studies on the transport of sediment in theECS have attracted international interest in the past twodecades, they have focused mainly on the Changjiangestuary and adjacent coastal areas (Xie et al., 1983;Milliman et al., 1985a, b; Xie and Li, 1988). Honjo et al.(1974), however, have studied the distribution of surfaceSPM in wide ranges of the ECS and Yellow Sea, and founda general decrease in concentration from the Changjiangriver mouth to the offshore area, while the non-combus-tible fraction of SPM also decreases seaward. Neverthe-less, surface distributions of SPM are sometimes mark-edly influenced by in-situ productivity and can hardlyelucidate the fate of river derived sediments in the mar-

ginal sea. The transport of SPM and its relationship tothe sedimentary record of surface sediment may not beclear if one cannot identify the distribution of SPM in thewater column. The composition of SPM not only revealsSPM sources but also affects elemental partitions betweendissolved and particulate phases in water columns. It isalso important to understand the inventory, geochemicalcharacteristics and transport of SPM during the summerseason, as both Changjiang and Huanghe deliver moresediment in summer than in winter (Milliman et al.,1985b; Zhang et al., 1990). More importantly, this studyprovides valuable data on the ECS prior to the construc-tion of the Three Gorges Dam, which may be necessaryto understand the impact of the Three Gorges Dam op-eration on the change of SPM distributions and particulategeochemistry in the ECS.

2. Methods

2.1 Cruises and hydrographic data acquisitionThis study was derived from the scientific results of

two cruises performed during two summer seasons in 1992(July 19–27) and 1993 (August 29–September 1). Duringthe 1992 summer season, the KEEP-MASS (KuroshioEdge Exchange Processes—Marginal Seas Study) Cruise(ROC-Russia joint cruise) was conducted aboard the R/V Vinogradov to occupy and explore most ECS stations(E1–E46a). During the 1993 summer season, a KEEPcruise (ORI-365) was conducted aboard the R/V OceanResearcher I to take samples from the southernmosttransect (Stations 1–16). The sampling locations areshown in Fig. 1a. During the KEEP-MASS Cruise,seawater samples were taken with Go-flo bottles (20 l,General Oceanics) attached to plastic-coated hydrowirefrom depths of 4 m, 50 m and 5 mab (5 meter above thebottom). The sampling depths were determined with deep-sea reversing thermometers attached to Go-flo bottles. ACTD (Guildline Inc., Canada) was also deployed to recordtemperature, salinity and light transmission. DuringCruise ORI-365, seawater samples were taken with Go-flo bottles (20 l) mounted on a CTD/Rosette (GeneralOceanic, USA) that also recorded the temperature, salin-ity and transmission profiles.

2.2 Seawater and particulate sample collectionThe recovery of particulate matter from ECS

seawater has been described in Hung et al. (1999). Briefly,particulate samples were collected by filtration of 10–20liter of seawater, depending on water turbidity. Seawaterin the Go-flo bottle was drained into the cleaned PE con-tainer (20 l) and forced by a peristaltic pump through ahome-made filter holder (Teflon) containing pre-weighedNuclepore filter (0.4 µm, 142 mm). After each filtration,the filter’s residue was washed with 500 ml of distilled-

Summer SPM Geochemistry in the East China Sea 191

deionized water (DDW) to remove the remaining sea salt,which did not cause the lysis of plankton cells as therewas no obvious DOC (dissolved organic carbon) releasefrom a test experiment. The filter was dried at 60°C in anoven for 24 hr. After cooling in a desiccator, the filterwas weighed with an electronic microbalance (MettlerAT20), and the concentration of SPM was calculated fromthe net weight of particulate and filtered volume ofseawater.

2.3 Chemical analysis of particulate samplesTo determine particulate Al, Si, Fe, Mn, Ca, Mg, K,

the particulate filter was placed in a Teflon bottle, digestedwith super-pure acids (HNO3:HCl:HF = 6:1:3 ml) andheated stepwise in a microwave oven (CEM 2000). Thedigested solution was diluted with Milli-Q water to 25ml and stored in a PE bottle. Aluminum and silicon weredetermined with N2O-C2H2 flame atomic absorption spec-trophotometry, whereas Fe, Mn, Ca, Mg and K were de-termined with air-C2H2 flame atomic absorption spec-trophotometry (Perkin-Elmer 5100PC). Recovery rates ofAl, Si, Fe, Mn, Ca, Mg, K from certified sediments (NBS-1646 (USA), MESS-1, BCSS-1 and PACS-1 (Canada))were generally greater than 95% (n = 9).

Another portion of seawater (2~3 l) was filteredthrough the pre-combusted GF/F filter to measureparticulate organic carbon (POC) and particulate nitro-gen (PN) contents. The residue was washed with 200 mlDDW to remove the remaining sea salt. The filter wasdried at 60°C in an oven for 24 hr. The dried filter wasplaced in a silver cup and a few drops of 2 M HCl wereadded to remove carbonate completely. The acidified sam-ple was dried again in an oven and POC and PN werethen measured with a C/N/S analyzer (Fisons NCS 1500)(Hung et al., 2000). The blank value attributed to GF/Ffilter and silver boat was deducted from the raw value ofa sample concentration. The sampling and measuringprecisions of POC and PN were ±0.2 µM C and ±0.3µM N (±1σ), respectively, as evaluated from eight rep-lica samples drawn from the same depth.

3. Results and Discussion

3.1 Hydrographic characteristicsThe ECS hydrological features are largely deter-

mined from the mixing of the Changjiang runoff, the Yel-low Sea Cold Water (YSCW), the Kuroshio Water (KW)and the Taiwan Current Warm Water (TCWW) (Fig. 1b).

CJCW

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Fig. 1. (a) Map of study area and sampling locations. Sampling locations are grouped as Transect K sampled from KEEP Cruiseand as B, C, D, E and F Transects sampled from KEEP-MASS Cruise (left panel). (b) Schematic representation of variouswaters in the East China Sea (right panel). CJCW, Changjiang Coastal Water; KW, Kuroshio Water; TCWW, Taiwan CurrentWarm Water; UW, Upwelling Water; YSCW, Yellow Sea Cold Water.


The Changjiang river plume and the Kuroshio upwellingnortheast of Taiwan are especially marked during the sum-mer season. Both regions were also characterized bystrong nutrient inputs, from the continent and Kuroshiosubsurface (Liu et al., 1992; Gong et al., 1996). TheChangjiang river plume was identified as the low-salin-ity water observed in the northern part of the study area(Fig. 2b). As the Changjiang runoff on the shelf bifur-cates near the river mouth in summer (Beardsley et al.,1985; Chao, 1991), the sampling stations may have onlycaught the northeast limb of the major plume and missedsome of the southward China Coastal Water (Gong et al.,

1996). However, the observations were still sufficient toreveal the influence of Changjiang plume on thegeochemistry of suspended matter, particularly at thosestations in the northwestern part of the study area.

The Kuroshio Water (KW) was indicated by thehigher salinity (S > 34.6) off the shelf (Fig. 2b) with verylow nutrients in the surface layer. The striking reversedS-shape in the T-S relationship (Fig. 3b) shows a maxi-mum salinity >34.7, which is also characteristics of theNorth Pacific Subtropical Water (Nitani, 1972). This KWflows northward along the east coast of Taiwan and re-sults in a persistent upwelling from the Kuroshio subsur-

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Fig. 2. Contour plots of temperature (a) and salinity (b) distri-butions in surface water (2 m) of ECS shelf.

Fig. 3. Water types defined by temperature-salinity relation-ships in the East China Sea. CDW, Changjiang DilutedWater; KW, Kuroshio Water; LOW, Low Oxygen Water;TCWW, Taiwan Current Warm Water; YSCW, Yellow SeaCold Water; YSMW, Yellow Sea Mixed Water.


face onto the ECS shelf (Wong et al., 1991; Liu et al.,1992). The Kuroshio upwelling was indicated by the lo-cal surface temperature minimum of 23.6°C around Sta-tion 11 of Transect K off northeast Taiwan (Fig. 2a). Theupwelled water is laden with nutrients and may also in-fluence carbon biogeochemistry (Wong et al., 1991; Liuet al., 1992; Gong et al., 1996).

The subsurface water (50 m) in the northern part ofthe study area had the lowest temperature, in contrast tothe overlying surface water with the highest temperature.The subsurface water showing minimum temperature cor-responded to the minimum of salinity and maximum SPMin the northernmost corner of the study area. This waterof low temperature, low salinity (Beardsley et al., 1985)and high turbidity (Milliman et al., 1985a) was similar tothe Yellow Sea Cold Water (YSCW), yet the minimumtemperature observed in this study (14.5°C, Fig. 3a) wasat the upper limit of the temperature range as defined forYSCW (Chen et al., 1994). This water, therefore, is re-ferred to as the Yellow Sea Mixed Water (YSMW, Fig.3a) because it might well have been a mixture of theYSCW and other shelf waters (Gong et al., 1996).

In the southern part of the study area the subsurfacewater on the shelf had a rather uniform temperature (23–25°C) and salinity (34.2–34.6), which characterized theTCWW (Taiwan Current Warm water, Beardsley et al.,1985) (Fig. 3a). To the southeast of the Changjiang rivermouth, a pool of low oxygen water (LOW, T > 23°C and33.7 psu < S < or =34.2 psu, Fig. 3a) was indicated by thehigh apparent oxygen utilization (AOU) reaching a maxi-mum of 100 µM, determined by Gong et al. (1996). Incontrast to the surface plume, this water was enriched inboth nitrate and phosphate (Gong et al., 1996).

The plume of Changjiang Diluted Water (CDW, Fig.3a) was defined as the water with a salinity lower than 31psu in the surface layer on Transects E and F. AlongTransect F, the surface plume was apparently confined ina thin layer (15–20 m). This plume extends out to theshelf break (Fig. 2b), consistent with previous observa-tions that the CDW can extend to just southwest of ChejuIsland (125°E) (Liu et al., 1992; Su and Weng, 1994).Hur et al. (1999) also mentioned that the Changjiang wateraffected the southeastern Yellow Sea and northern ECSduring summer when river discharge reached a maximum.On the other hand, the very salty subsurface water withsalinity greater than 34.7 psu characterizing the KW wasfarther from the shelf break on Transect K than on othertransects. This salty water occupied most of the middleand outer shelf on Transect B, indicating the intrusion ofthe upper layer of KW onto the shelf. Further north ofTransect B, this salty water retreated from the shelf as aresult of the seaward deflection of the intruding current(Chao, 1991; Tang et al., 1999). The specific water typeand its particulate geochemistry in the ECS are therefore

determined mainly by various contributions of majorsource waters with distinct differences in turbidity.

3.2 Distributions of SPM over the ECS shelfThe distributions of SPM in surface (4 m), mid-depth

(50 m) and bottom (5 mab) waters over the shelf are dis-played in Figs. 4a–c. Concentrations of SPM rangeroughly from ~0.3 mg l–1 near the shelf break to 1.2mg l–1 at Stn. E37 for surface water (Fig. 4a), from ~0.4mg l–1 near the shelf break to 1.3 mg l–1 at Stn. E40 formid-depth water (Fig. 4b), and from 0.5 mg l–1 near theshelf break to 6.5 mg l–1 at Stn. E36 for bottom water(Fig. 4c). The measured SPM concentrations in theChangjiang plume were similar to previous findings insummer 1983 by Milliman et al. (1985b) and in summer1994 by Yanagi et al. (1996). Both reported ~1 mg l–1 in

Fig. 4. Distributions of SPM concentrations (mg l–1) in surface(a, 4 m), middle (b, 50 m) and bottom (c, 5 m above bot-tom) waters over the ECS shelf.


the surface water and ~2 mg l–1 in the bottom water aroundthe location (30.5°N, 124.5°E). Milliman et al. (1985b)also measured summer SPM distribution in theChangjiang plume area and reported an increase of SPMfrom 126°E up to 50 (surface)–1000 (bottom) mg l–1 nearthe Changjiang Estuary. In our study area, an anomalouslyhigh concentration (~11 mg l–1) was measured at 5 mabat Stn. E33, which was apparently due to the trappingeffect of low topography. It should be noted that the mid-depth water covers only those stations with water depthgreater than 60 m. Despite the well-known fact that non-synoptic measurements of water and biogeochemicalproperties are subject to tidal impacts in most continen-tal shelves, the SPM concentrations in the ECS shelf gen-erally display seaward decreases for all three layers, al-though there are a few spots with anomalous distributions.

At the surface, the higher SPM concentration(0.8~1.2 mg l–1) is located in stations near the China con-tinent (Transects B & K) and in the Changjiang plume(Transects E&F). The higher SPM distributions appearto be caused by terrestrial inputs and higher local pro-duction, as inferred from higher particulate Al and Chl acontents at these stations. The higher SPM contents weregenerally found in the western edges of transects, coin-ciding with the higher Al concentrations, indicating theinfluence of terrestrial inputs. Consequently, a positivebut nonlinear correlation was found between SPM andAl contents (Fig. 7b). Although the Chl a distribution isgenerally reduced by SPM contents (turbidity) in mostcoastal zones, it continues to show an increasing trendfrom the shelf break to the western edge of study area inthe surface water (Gong et al., 1996), similar to the trendof surface SPM distribution. However, very high Chl acontents (2.6–8.5 mg m–3) at the depths of subsurfacemaximum were detected at several locations (StationsE33, E34, E35, E41 and E42), which may result in fairlyhigh SPM distributions. The SPM concentration in theupwelling center (Stn. 11–Transect K) deviated from thedecreasing trend, resulting mainly from enhanced localproductivity. The lowest temperature (16°C) and ratherhigh nitrate (11 µM) and Chl a (up to 1 mg m–3) wereobserved around this upwelling center (Gong et al., 1996).Elevated primary productivity was also reported by Gonget al. (2003) around the upwelling center, which appar-ently resulted from the uplifting of nutrient-laden sub-surface water. Although surface SPM concentrations werenot very difference between the northernmost Transect F(~0.3–1.2 mg l–1) and the southernmost Transect K (~0.3–0.8 mg l–1), the bottom SPM concentrations were muchgreater in the northernmost Transect F (~0.5–6.5 mg l–1)than in the southernmost Transect K (~ 0.5–2.8 mg l–1),despite the fact that Transect K is closer to the continentof China. The Changjiang runoff and Yellow-Sea derived

sediments were likely to be responsible for the north-southdifference of SPM distributions in the bottom water. Spa-tial distributions were not very different between surfaceand mid-depth water, but there was a significant differ-ence between surface and bottom (5 mab) waters. A sig-nificant increase in SPM contents was found in the bot-tom water, which apparently resulted from the formationof a benthic nepheloid layer (BNL). The response of SPMdecrease to the increase in salinity was more remarkablein the bottom water [SPM (mg l–1) = 93.4–2.67S, R =–0.60, p < 0.001] than in the surface water [SPM (mg l–1)= 3.04–0.073S, R = –0.45, p < 0.005], implying that SPMcontent in the bottom water may be largely derived fromresuspension/lateral transport of pre-deposited sediment.Thus, the SPM content in the bottom water decreasedsharply with increasing salinity, as resuspension gener-ally decreases significantly with the increase of waterdepth and distance from land.

Continuous transmission data (attenuation coeffi-cient, AC) provided a more precise profile distribution ofSPM because they were recorded as 1 AC/meter. The ACcontour data are shown in the profile of each transect (Fig.5). The attenuation coefficient is a true indicator of SPMcontent, as a highly significant correlation between thetwo was found in this study [SPM (mg l–1) = 1.933AC–0.563, R = 0.9269, p < 0.0001]. A distinct BNL layer wasclearly seen in the shelf with water depth <150 m in alltransects from north to south. Strong attenuation coeffi-cient gradients can be seen between surface and bottom,between inner and outer shelves, and between north andsouth transects. The very high AC found at 5 mab at Stn.E33 corresponds to an anomalously high value of mea-sured SPM (~11 mg l–1). Previous studies also reportedmuch higher SPM concentrations in the bottom water thanin the surface water in the coastal zone adjacent to theChangjiang estuary (Milliman et al., 1985b; Xie and Li,1988). The relative difference in SPM concentrations ap-pears to be larger in the inner shelf than in the outer one,corresponding to the variation of historical tidal ampli-tude. In addition, tidal currents were found to be respon-sible for the resuspension and transport of fine-grainedsediments from the inner shelf and from the eastern sideof the Yellow Sea (Milliman et al., 1985b). The simu-lated tidal current (Choi, 1984) displayed an increase inamplitude from the shelf break toward the seashore, rang-ing from 80 to 160 cm in the northern ECS and from 80to 200 cm in the southern ECS. In the absence of a stormwind effect during the study period, tidal fluctuation ratherthan wind/wave forcing appears to be the major drivingforce controlling the BNL formation, as various strengthsof thermal stratification developed, except for theupwelling area in Transect K during the summer season(Gong et al., 1996).


3.3 POC, PN and C/N ratioThe POC distributions shown in Figs. 6a–c range

from ~60 µg l–1 near the shelf break to 150 µg l–1 at sta-tions near the Changjiang plume for surface water, from~50 µg l–1 near the shelf break to 90 µg l–1 at mid-shelfstations, and from ~50 µg l–1 near the shelf break to 120

µg l–1 at Stn. E36 for bottom water. It should be notedthat the POC distributions of Transect K were not mea-sured for middle (50 m) and bottom waters. It is also worthnoting that a very high concentration (324 µg l–1) wasmeasured at the surface of Stn. E35 corresponding to arather high concentration of Chl a (2.8 mg m–3). ThosePOC distributions between 70 µg l–1 and 90 µg l–1 in themid-water correspond fairly well to distributions of Chla subsurface maximum. In general, the pattern of sea-ward decrease in POC abundance coincides with the pat-

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Fig. 5. (continued). Profiles of AC contours in Transects C, Band K.


tern of nutrient availability, which appears to control bio-logical productivity (Gong et al., 1996). Although POCabundance was inversely correlated with salinity for bothsurface water [POC (µM) = 40–1.02S, R = –0.534, p <0.001] and bottom water [POC (µM) = 86.4–2.35S, R =–0.645, p < 0.001], the slope (sensitivity) of POC distri-bution versus salinity change was greater in bottom wa-ter than in surface water, implying a larger bottom POCderived from terrestrial sources. In effect, the correlationbetween POC concentration and Chl a concentration ishighly significant in the surface water (R = 0.7453, p <0.0001) but very poor in the mid-depth (R = 0.05) andbottom waters (R = 0.174). Most surface POC were likelyderived from biological production but were subject todecay in the middle and bottom waters. The elevated abun-dance of POC in the bottom water must be due to addi-tion of resuspended detritus.

The elemental ratio of organic C and N usually pro-vides a clue to the predominant sources of organic mat-ter. The ratios were not very different among waters, rang-ing from 5.7 to 15.2 for the surface water, from 5.0 to13.8 for the mid-water and from 5.5 to 17.4 for the bot-tom water, likely resulting from the mixture of living andnon-living materials. Such C/N ratios are close to theranges (4.2–29.2) found in water columns of the PN linein the ECS during the fall season (Wu et al., 2003). How-ever, the ∆POC/∆PN ratio, an indicator of freshly pro-duced materials, is 6.18 (R = 0.8523, p < 0.0001), whichis close to the Redfield ratio (6.63) in the surface water(Fig. 7d), supporting the evidence of largely biologicalorigins during summer when primary production is high-est (Gong et al., 2003). A slight deviation of ∆POC/∆PNfrom the Redfield ratio may be the result of the inclusionof living zooplankton, which usually has a C/N ratio about5.1 (Anderson, 1992). The ∆POC/∆PN ratio may not besuitable to indicate the freshly produced C/N ratio in themid- and bottom waters as both waters contain largelyterrigenic and/or recycled detritus. This would also re-sult in larger scattering distributions (less fine correla-tion) between organic C and N and produce lower slopesin the mid- and bottom waters (Figs. 7e and f). In addi-tion, the influence of zooplankton, fecal pellets and at-tached bacteria on the relatively low ∆POC/∆PN ratio maynot be excluded for the mid- and bottom waters. Overall,POC contents (wt. %) are inversely proportional to SPM(Fig. 7a) and particulate Al contents (Fig. 7c), indicatingthe dilution of lithogenic materials as both SPM and Alcontents increase significantly in the bottom water (Fig.7b). In general, the average proportion of POC (µg l–1) inSPM (µg l–1) is ca. 18% (0.18 slope between POC andSPM, figure omitted for brevity) in the surface water andca. 9.4% in the bottom water, corresponding to ca. 36%and ca. 18.8% of organic matter in SPM in the surfaceand bottom water, respectively. The contents of organiccarbon are much higher in SPM, particularly those in thesurface water, than in in-situ surface sediments (0.11–0.80%) reported by Sheu et al. (1995). This is largely dueto the fact that surface sediments are generally coarserthan suspended particulates. In addition, the POC abun-dance in the bottom water is not necessarily determinedby in-situ resuspension of fine sediment, but may also beaffected by lateral transport processes associated withresuspension from distant locations. Overall, the in-situproduction and bottom resuspension and/or lateral trans-port result in the enrichment of POC in surface water andbottom waters, respectively. Owing to very low rates ofsediment and TOC (total organic carbon) accumulationon mid-outer shelves (Huh and Su, 1999; Lin et al., 2002),the POC enrichment in the BNL provides an importantimplication that the shelf POC may be transported later-ally into the ocean through the BNL.

Fig. 6. Distributions of POC concentrations (µg l–1) in surface(a, 4 m), middle (b, 50 m) and bottom (c, 5 m above bot-tom) waters over the ECS shelf.


3.4 Geochemical composition and distribution of SPMParticulate Al, Fe and Mn are generally regarded as

indicators of lithogenic particles. Distributions ofparticulate Al are shown in Figs. 8a–c. The abundance ofAl ranges from ca. 0.2 to 13 µg l–1 for the surface water(Fig. 8a), from ca. 0.3 to 5 µg l–1 for the mid-depth water(Fig. 8b), and from ca. 5 to 310 µg l–1 for the bottomwater (Fig. 8c). Similar to SPM distributions, the abun-dance of particulate Al was relatively low in the surfaceand mid-depth waters but increased significantly towardthe bottom water. The large difference of Al abundanceversus salinity (Al-salinity slope) between surface (0.05)and bottom waters (144) may imply that Al, as a proxy oflithogenic components, in the bottom water was likelyinvolved in the resuspension processes and is more sen-sitive to salinity (terrestrial sources) change in the bot-tom water than in the surface water. The spatial gradientis also strong, with a significant seaward decrease. Themean fraction of particulate Al in SPM is ca. 0.047% inthe surface water, 4.1% in the mid-depth water and 5% inthe bottom water (figures omitted for brevity). The verylow Al content in particles of surface water was due todilution by biogenic materials. The Al content in parti-cles of bottom water is also much lower than those inSPM of Changjiang (8.4–9.7%) and Huanghe (7.5–8.0%)(Li et al., 1984; Qu and Yan, 1990), also likely due to

biogenic dilution as POC content is much higher in thebottom water (ave. 9%) than in the Changjiang (0.9–1.2%,Cauwet and Mackenzie, 1993). Particulate Al correlatesclosely with particulate Si in the surface, mid- and bot-tom waters (Fig. 9). However, the ratio (slope) ofparticulate Si and Al differs markedly between the sur-face water and bottom water. The ratio is much greater inthe surface water (277) than in the bottom water (3.66),primarily due to enrichment of biogenic Si (opal) and di-lution of Al by biogenic components in the surface water.The Si/Al ratio in the bottom water (Si/Al = 3.66), how-ever, is quite similar to those in Changjiang (Si/Al = 2.8–3.6, Li et al., 1984; Qu and Yan, 1990) and Huanghe (3.4–4.08, Li et al., 1984; Qu and Yan, 1990). Although the Si/Al ratio in the bottom water was not clearly distinguishedfrom the mean ratio (3.42) of the continental crust (Taylor,1964), it provided evidence for the inference that the SPMin the bottom water was likely of terrestrial origin, de-rived from Changjiang and Huanghe, as both rivers arewidely recognized as major sources of SPM in the ECS(Milliman et al., 1985a; Xie and Li, 1988; Gao et al.,2000; Liu et al., 2007).

The abundance of particulate Fe ranges from 2.1 to48 µg l–1 for the surface water, from 2.9 to 72.1 µg l–1 forthe mid-depth water and from 2.4 to 284 µg l–1 for thebottom water (figures omitted for brevity). The spatial

0 2 4 6 8 10 12

0

10

20

30

40

a

POC(%

)SPM (mg l

-1)

0 2 4 6 8 10 12

0

2

4

6

8

b

Al(%

)

SPM (mg l-1)

0 2 4 6 8

0

10

20

30

40

50

0.0 0.5 1.0 1.5 2.0

2

4

6

8

10

12(5mab)

f

POC(µM)

PN (µM)

[POC]= 3.26 + 4.22[PN]

R= 0.783, p<0.0001

0.0 0.5 1.0 1.5

2

4

6

8

10(50m)

e

POC(µM)

PN (µM)

[POC]= 3.08 + 4.34[PN]

R= 0.553, p<0.005

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

5

10

15

20

25

30

d

PN (µM)

[POC]= 2.43 + 6.18[PN]

R=0.8523, p<0.0001

4m

POC(µM)

POC(%

)

Al (%)

c

R= -0.5259, p<0.0001

Fig. 7. Correlation plots of various pairs of parameters measured from the ECS shelf.


patterns of particulate Fe in the surface, mid-depth andbottom waters resemble those of particulate Al. The over-all correlation between Al and Fe is highly significant(R = 0.9502, p < 0.0001). The overall ratio of particulateFe/Al is 0.861, which is higher than that (0.684) in thecrust (Taylor, 1964) and in particulates from Changjiang(0.56–0.59) and Huanghe (0.4–0.47) (Li et al., 1984; Quand Yan, 1990), showing the enrichment of Fe in ECSSPM (especially for surface SPM) over crust and riverparticulates. The particulate (Fe/Al)surface/(Fe/Al)bottom isgenerally greater than 40, indicating highly biologicalenrichment of Fe in surface water. The abundance ofparticulate Mn ranges from 0.064 to 0.747 µg l–1 in thesurface water, from 0.059 to 1.41 µg l–1 and from 0.17 to11.0 µg l–1 in the bottom water (figures also omitted forbrevity). Particulate Mn also correlates significantly(R = 0.9710, p < 0.0001) with particulate Al. The overallratio of particulate Mn/Al is 0.0198, which is greater than

the ratio (0.0115) in the crust (Taylor, 1964) and the ratioin Changjiang (0.0099–0.0116) and Huanghe (0.01) (Liet al., 1984; Qu and Yan, 1990), also indicating the en-richment of Mn in ECS SPM over crust and riverparticulates. Biological enrichment may be principallyresponsible for Fe and Mn enrichments in ECS SPM. Suchdistribution patterns of particulate Al, Fe and Mn stronglyindicate the substantial influence of terrestrial inputs onparticle composition and transport.

Because particulate Ca is the major composition of

0 100 200 300 400 500 600

0

500

1000

1500

2000

4m (S= 277; R=0.8861, p<0.0001)

50m (S= 2.84; R=0.7964, p<0.0001)

5mab (S= 3.66; R=0.9896, p<0.0001)

Si(µgl-1)

Al (µg l-1)

Fig. 8. Distributions of Al concentrations (µg l–1) in surface(a, 4 m), middle (b, 50 m) and bottom (c, 5 m above bot-tom) waters over the ECS shelf.

Fig. 9. Correlation plot between particulate Si and particulateAl in surface, middle and bottom waters. S denotes the slopebetween two parameters.

Fig. 10. Distribution of particulate Ca (µg l–1) in the bottomwater over the ECS shelf.


0 20 40 60 80 100 120 140

0

20

40

60

80

100

120

E36-5mab

E38-5mab

a

R=0.8149, p<0.0001

Ca(µgl-1)

Mg (µg l-1)

0 20 40 60 80 100 120

0

20

40

60

80

100

120

140

K (µg l-1)

Mg(µgl-1)

c

R= 0.8213, p<0.0001

0 20 40 60 80 100 120

0

20

40

60

80

100

120

0 100 200 300 400 500 600

0

20

40

60

80

100

120

f

4m (R=0.223, p=0.204)

50m (R=0.684, p<0.001)

5mab (R=0.848, p<0.0001)

K(µgl-1)

Al (µg l-1)0 100 200 300 400 500 600

0

20

40

60

80

100

120

140

e

4m (R=0.094, p=0.581)

50m (R=0.114, p=0.614)

5mab (R=0.958, p<0.0001)

Mg(µgl-1)

Al (µg l-1)

0 100 200 300 400 500 600

0

20

40

60

80

100

120d

4m (R=0.038, p=0.823)

50m (R=0.407, p=0.066)

5mab (R=0.821, p<0.0001)

Ca(µgl-1)

Al (µg l-1)

R= 0.8160, p<0.0001

b

Ca(µgl-1)

K (µg l-1)

E36-5mab

Fig. 11. Correlation plots of various pairs of particulate elements measured from the ECS shelf.

3.5 SPM and POC inventories and implicationDistributions and contours of integrated SPM in the

water column over the ECS shelf are shown in Fig. 12.Data in the marked square area near the Changjiang Es-tuary have been adopted from the report of Milliman etal. (1985b), based on sampling conducted in August 1981.Such complementary data are included to show the entirepicture of SPM distributions and is used to assess the in-ventory of SPM over the ECS shelf, assuming that col-umn integrated SPM distributions are not very differentbetween August 1981 and August 1992, before the con-struction of the Three Gorges Dam. It may be arguablethat the annual discharge of Changjiang sediment up tonow has decreased significantly (>40%) since the 1950sand 1960s, and the second-phase reduction occurredmainly after 1985 due to the installation of numerous damsand the institution of soil conservation practices in theJialingjiang catchment area (Yang et al., 2006). However,the distribution of SPM in the ECS may not follow thedecreasing trend of Changjiang sediment loads as sedi-ment resuspension may critically determine the SPM dis-tribution and inventory. In fact, the SPM contents werenot much different around certain locations (e.g. 30.5°N,124.5°E) as between this study and other observationperiods (Milliman et al., 1985b; Yanagi et al., 1996). It istherefore reasonable to include the data of Milliman et

calcite, which is a significant characteristic of Huanghe-derived sediments, the particulate calcium contents maybe a good proxy to justify the influence of Huanghesediments on the SPM distributions in the ECS. Distribu-tions of particulate Ca in the bottom water are shown inFig. 10. It should be noted that distribution contours donot include Transect K as no data were obtained fromTransect K. The particulate Ca contents decrease obvi-ously from north to south transects, and decrease signifi-cantly near the shelf break (from 20 µg l–1 to 10 µg l–1).The 20 µg l–1contour may be regarded as the margin ofinfluence of the Huanghe sediments, as revealed from thesharp difference of spatial gradients between 22–20µg l–1 contours and 20–10 contours. In addition, this zoneis roughly coincident with the reported area of bottomsediment mixed with Huanghe sediments (Milliman et al.,1985b). Overall, particulate Ca, Mg and K correlateclosely with each other (Figs. 11a–c), showing similarsources. Some data deviate from the regression lines,which may be due to differential accumulation of differ-ent elements by biota. However, particulate Ca, Mg andK correlate significantly only with Al in the bottom wa-ter but not in the surface water (Figs. 11d and f). Appar-ently, particulate Ca, Mg and K are likely derived fromterrestrial sources but may also be largely involved inbiological accumulation in the surface water.


al. (1985b) in inventory calculation. To improve the pre-cision of estimation, the SPM inventory of each stationwas calculated from SPM contents converted from con-tinuous bean attenuation data [SPM (mg l–1) = 1.933AC–0.563, R = 0.9269, p < 0.0001], rather than from observedSPM contents for just three depths at one location. How-ever, the POC and PN inventories in a water column weretrapezoidally integrated from the measured data in threedepths. The POC and PN inventories in the Changjiangplume (marked square area in Fig. 12) were estimated byemploying total SPM inventory (Milliman et al., 1985b)and average POC contents and C/N ratio (Wu et al., 2003).It is worth noting that the left boundaries of SPM con-tours (80–100 kg/m2) are close to similar contours re-

ported in previous findings in the Changjiang plume byMilliman et al. (1985b) (Fig. 12). To a first approxima-tion, such estimates appear to be appropriate and providebetter precision for the total inventories of SPM, POCand PN in the ECS shelf. The total inventory of each pa-rameter in the ECS shelf is integrated over the shelf areabetween 25°N and 33°N at a water depth of 200 m. Theshelf area is estimated to be about 0.45 × 106 km2. Thedistribution pattern of seaward decrease can be clearlyseen and also displays a major pathway of seaward trans-port of SPM. The integrated SPM distributions appear tobe relatively higher in the northernmost and southernmosttransects than in the middle transects, also reflecting theinfluence of continental sources. The total inventories ofSPM, POC and PN are 46, 2.8 and 0.4 Tg (T = 1012),respectively (Table 1). Despite the lack of seasonal data,these summer inventories provide valuable backgroundinformation for future comparison with those distributionsafter the operation (June 2003) of Three Gorges Dam, asdischarge from Changjiang peaks in summer and mayprovide a pronounced signal of Three Gorges Dam ef-fects. In fact, a reduction of Changjiang sediment loadhas been recorded after water started to fill the reservoirbehind the Dam (Yang et al., 2006). After the ThreeGorges Dam came into operation, distributions of SPMand geochemical elements in the ECS would change fol-lowing significant decreases in water and SPM dischargesin summer. The residence times of SPM, POC and PNare estimated by dividing the total inventories by the sumsof river fluxes from China. The residence times of SPM,POC and PN are 27, 13 and 11 days (Table 1), respec-tively, which are much shorter than the residence time(~1 yr) of shelf water (Li, 1994; Peng et al., 1999). Inaddition to depositing on the shelf, POC and PN are sub-ject to microbial degradation and/or grazing in the watercolumn, resulting in a shorter residence time than that ofSPM. The residence time of PN is also slightly shorterthan that of POC due to the different lability between thetwo (Hung et al., 2003). Much SPM, POC and PN must

Tg = 1012 g.(a)Total discharge from Changjiang, Jiaojiang, Qiantangjiang and Minjiang (Zhang and Liu, 2002).(b)Milliman et al. (1984).(c)Based on primary productivity (145 g Cm–2yr–1) in the ECS shelf (Gong et al., 2003).(d)Derived from primary productivity using Redfield ratio (6.6).(e)Derived from primary productivity (org. c) assuming POM = 2 × POC.

Fig. 12. Contours of integrated SPM in the water column overthe ECS shelf. Data in the marked square area near theChangjiang Estuary were adopted from the report ofMilliman et al. (1985b) of sampling done in August 1981.

Inventory(Tg)

River discharge(Tg yr–1)

ECS primary productivity(Tg yr–1)

Residence time(day)

SPM 46 485(a) 130(e) 27POC 2.8 12(b) 65(c) 13PN 0.4 1.3(b) 12(d) 11

Table 1. Inventory and residence time of SPM, POC and PN in the East China Sea shelf.


deposit and/or recycle quickly before a parcel ofChangjiang freshwater leaves the ECS shelf.

4. ConclusionSummer distributions of suspended-particulate mat-

ter and geochemical compositions in the East China Seawere significantly influenced by large river inputs(Changjiang), in addition to physical mixing of variouswater types and in-situ biological productivity. Horizon-tal decreases and vertical increases of SPM andgeochemical distributions strongly indicate the impor-tance of BNL in seaward transport of particulate materi-als. The tidal fluctuation appears to be responsible forthe BNL formation and material transports in the bottomlayer of the ECS shelf. Distributions of SPM and POC(N)contents and inventories over the shelf reflect the influ-ence of large river inputs, resuspension, local upwellingand enhanced productivity. These data provide valuableinformation for evaluating the future impacts of the ThreeGorges Dam on particulate geochemistry in the ECS infuture, as Changjiang sediment load has been significantlyreduced after the Three Gorges Dam came into opera-tion.

AcknowledgementsThe authors would like to thank the National Sci-

ence Council, Republic of China, for financial support tothis study through Contract Nos. NSC 83-0209-M110-009k and NSC 93-2611-M110-012. We are grateful forpartial support from the “Aim for the Top University Plan”of the National Sun Yat-sen University and Ministry ofEducation, Taiwan, R.O.C. (C030214) during preparationof the manuscript. This paper represents Contribution No.5 of Kuroshio Research Group, Asia-Pacific Ocean Re-search Center.

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Summer Distribution and Geochemical Composition of ...svr4.terrapub.co.jp/journals/JO/pdf/6302/63020189.pdf · tal marginal regions (Hung and Chan, 1998; Zhang and Liu, 2002). Many