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(2007) 367–377www.elsevier.com/locate/marchem

Marine Chemistry 107

Tracing suspended organic nitrogen from the Yangtze Rivercatchment into the East China Sea

Ying Wu a,⁎, Thorsten Dittmar b, Kai-Uwe Ludwichowski c, Gerhard Kattner c,Jing Zhang a, Zhuo Y. Zhu a, Boris P. Koch c

a State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North,Shanghai 200062, PR China

b Florida State University, Department of Oceanography, Tallahassee, Florida 32306-4320, USAc Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany

Received 3 August 2006; received in revised form 30 January 2007; accepted 31 January 2007Available online 21 February 2007

Abstract

Total suspended matter was collected along the Yangtze River (Changjiang) and in the East China Sea in April to May and inSeptember 2003, respectively, to study origin and fate of particulate organic nitrogen. Concentrations of particulate organic carbon(POC), nitrogen (PN) and hydrolyzable particulate amino acids (PAA; D- and L-enantiomers) were higher in the Yangtze Estuarythan in the river and decreased offshore towards the shelf edge. In the coastal area, higher values of PAA were observed in thesurface layer than in the bottom water. Stable carbon isotope ratios (δ13C) of POC increased from −24.4‰ in the river to valuesaround −21‰ on the East China Sea Shelf. Dominant amino acids were aspartic acid+aspartine (Asx), glutamic acid+glutamine(Glx), glycine, alanine and serine. The proportions of Asx, Glx and isoleucine were higher in the marine than in the riverinesamples contrary to the distributions of glycine, alanine, threonine and arginine. The proportions of D-amino acids were highest inthe riverine suspended organic matter (6% of PAA) decreasing towards the shelf edge (1.5% of PAA). D-arginine, not reported innatural aquatic samples so far, was the most abundant D-amino acid in the river. The amino acid composition of the particulateorganic matter (POM) in the Yangtze River indicates an advanced stage of degradation of POM. Highly degraded organic matterfrom soils is probably a main source of POM in the Yangtze River, but the relatively high δ13C values and low C/N ratios (7.7±1.6) also indicate contribution from anthropogenic sources. The degraded riverine material was a dominant organic matter source inthe estuary, where aquatic primary production had only a small overall contribution. In the East China Sea, gradual settling ofriverine organic matter and the addition of fresher phytoplankton impacted the amino acid composition and δ13C values, and on theouter shelf relatively fresh phytoplankton-derived organic matter dominated.© 2007 Elsevier B.V. All rights reserved.

Keywords: Yangtze River; East China Sea; Suspended organic matter; Amino acids; Stable isotopes; Diagenesis

⁎ Corresponding author. Tel.: +86 2162232073; fax: +86 2162546441.E-mail address: wuying@sklec.ecnu.edu.cn (Y. Wu).

0304-4203/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.marchem.2007.01.022

1. Introduction

Continental margins act as a filter and/or sink of bothnatural and anthropogenic material that is transportedfrom land into the oceanic provinces (Wollast, 1991),where at least 80% of the suspended matter reaching the

368 Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

ocean is deposited. The continental flux of nutrientsfuels a high biological production in coastal environ-ments, making them an important player in the globalcarbon cycle (Biscaye et al., 1994; Hedges and Keil,1995; Wollast, 1998). Nitrogen is a principal limitingplant nutrient in marine production. Particulate nitrogenis transported mainly in organic form from land to ocean(Dittmar et al., 2001a and references therein). Itschemical identity and origin determines the impact ofthe continental fluxes on coastal mass-balance andecosystem function.

The East China Sea is one of the largest continentalshelves in the world. The Yangtze River (Changjiang),which discharges into the East China Sea, drains amajor part of the Chinese landmass (ca. 20%) and isamong the largest world rivers in terms of water andsuspended sediment loads (Yang et al., 2005). Theenormous amounts of sediments and nutrients carriedby the Yangtze River sustain coastal ecosystems withrelative high primary production. It is of particularinterest to examine the change in riverine flux at theland–sea continuum as defined by the Yangtze River–East China Sea because of the recent construction of theThree Gorges Dam, the world largest hydrographicproject (Zhang et al., 1999). These changes are expectedto affect coastal circulation, nutrient availability andmarine production in the East China Sea, which is stillpoorly understood. On the eastern boundary of the EastChina Sea, the Kuroshio Current interacts actively withshelf waters through frontal and upwelling processes(cf. Su, 2001).

While there have been several efforts of tracing thesuspended load of the Yangtze River in the estuary andfurther onto the adjacent East China Sea Shelf (e.g.Milliman et al., 1984; Cauwet and Mackenzie, 1993;Wu et al., 2003), the dynamics of organic nitrogen inthis area remain largely unknown. Bulk organic nitrogenmeasurements were made to identify the Yangtze Riveras an important source of particulate organic nitrogen(PON) to the East China Sea (Milliman et al., 1984), butthe fate of PON and the potential influence of the land-derived flux on marine production remains speculative.

To understand the fate of terrigenous organic matterin the oceanic waters, thus far lignin is the onlyunambiguous molecular tracer that has been success-fully applied (Meyers-Schulte and Hedges, 1986;Dittmar et al., 2001b; Hernes and Benner, 2006). Themolar C/N ratios of terrestrial organic matter vary over awide range (ca. 12–400) and usually show a decreaseduring diagenesis, whereas C/N ratios of phytoplanktonare relatively constant (ca. 6–8) (e.g., Hedges et al.,1986; Onstad et al., 2000). Stable isotopes (i.e., δ13C

and δ15N) are widely used to distinguish the sources ofPOM in rivers and estuaries (Peterson et al., 1985; Saliotet al., 1988; Andrews et al., 1998; Middleburg andNieuwenhuize, 1998). For organic nitrogen, the applica-tion of lignin and stable carbon isotopes as tracers islimited, because organic nitrogen may cycle uncoupledfrom organic carbon or bulk organic matter (e.g.,Kattner and Becker, 1991).

Amino acids are the building blocks of proteins, thelargest reservoir of organic nitrogen in most organisms.Their natural occurrence and geochemical behavior inmarine plankton (e.g. Cowie and Hedges, 1996),suspended and sinking particles (e.g. Ittekkot et al.,1984; Cowie and Hedges, 1992) and sediments (e.g.Gupta, 2001) have been intensively studied. Aminoacids are useful indicators of decomposition andtransport in the marine environment. During diagenesisin marine sediments, the proportion of the individualamino acids changes in a characteristic way and cantherefore be used as a proxy for diagenetic processes(Dauwe et al., 1999). Dauwe and Middelburg (1998)introduced an empirical degradation index for proteinamino acids applied on a set of sedimentary amino aciddata. The index ranges from +1 for phytoplankton andbacteria to −1.5 for highly degraded oxic sediments.Highly degraded, refractory organic matter of marineand terrestrial origin exhibits significantly differentamino acid patterns, which reflect the contrastingenvironmental conditions (Dittmar et al., 2001a; Ditt-mar, 2004). D-enantiomers of amino acids are otheruseful tracers for identifying sources and degree ofdegradation of organic nitrogen. Bacterial biomass isrich in D-amino acids, whereas phytoplankton and mostother primary producers contain almost exclusively L-enantiomers (e.g., Jørgensen et al., 1999). D-enantio-mers of some amino acids are present in significantamounts in seawater (McCarthy et al., 1998; Dittmar etal., 2001a), indicating a major bacterial contribution tomarine organic nitrogen.

The objective of this study was to understand thecomposition and transportation of suspended organicnitrogen from the Yangtze River catchments, across theestuary and further into the outer East China Sea. Wesought to identify the major sources of PON and itsdegree of degradation by applying a combined tracerapproach of stable isotopes and amino acid enantiomers.

2. Materials and methods

Sampling in the East China Sea was conducted fromSeptember 4 to 26, 2003, during the R/V “Dong FangHong 2” cruise along three representative transects

369Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

(Fig. 1): PN from the Yangtze River Estuary towardssoutheast across the East China Sea Shelf, YT from theinner to the outer shelf northeastward along the mainaxis of summer plumes from the Yangtze River, andAS along the Okinawa Trough at the shelf edge. Allstations were sampled twice within 20 days. Tempera-ture, salinity, turbidity, epi-fluorescence and dissolvedoxygen were recorded in the water column using a Sea-Bird 911plus CTD-Rosette assembly. Seawater wassampled with 10-L Niskin bottles at different waterdepths designed from the CTD profiles.

The main stream of the Yangtze River was sampledin April to May, 2003, from the river mouth upstreamover a distance of 3500–4000 km (Fig. 1). Watersamples of 100–200 L were collected from small boatsacross the main stream using submersible pumps at 1–2 m below the surface. All water samples were filteredimmediately after sampling through pre-combusted(500 °C, 5 h) Whatman GF/F filters (nominal poresize: 0.7 μm). Depending on the in situ river turbidity,variable amounts of 0.5 to 5 L of water were filtered.The filters were kept frozen (−30 °C) until determina-tion of POC and particulate nitrogen (PN), stableisotopes and amino acid enantiomers. For nutrients,water was filtered through precleaned Millipore filters(pore size: 0.45 μm), and the filtrate was poisoned byHgCl2 and stored at room temperature.

In the laboratory, nutrients were analyzed color-imetrically with a precision of 5–10% at <1–10 μMlevel and 1–5% at about 10–100 μM (Zhang et al.,1999). GF/F filters were freeze-dried and analyzed forPOC and PNwith a CHNOS analyzer (Vario EL III) afterremoving the inorganic carbon by reaction with HClafter Zhang et al. (1997). The detection limit of carbonwas 7.5 μg and of nitrogen 8.0 μg, with a precision of

Fig. 1. Map of the East China Sea and the Yangtze River catchments withdifferentiated on the East China Sea shelf: the relatively fresh, cool and nutriensaline Taiwan Warm Current (TWC), the cold and nutrient-rich Kuroshio W

better than 6%, based on repeated determinations. Stableisotope analyses were performed with a Finnigan Delta-plus XP analyzer. Results are expressed as δ13C andδ15N, normalized to PDB and air standard, respectively.

For the determination of particulate amino acids(PAA), filters were hydrolyzed with HCl (16%, 20 mL)in the presence of ascorbic acid at 110 °C (24 h) in pre-combusted sealed glass ampoules according to Fitznar etal. (1999). After precolumn derivatization with o-phthaldialdehyde and N-isobutyryl-L/D-cysteine, gly-cine (Gly) and the D- and L-enantiomers of theindividual amino acids aspartic acid (Asp), glutamicacid (Glu), serine (Ser), threonine (Thr), arginine (Arg),alanine (Ala), gamma-amino butyric acid (GABA),valine (Val), phenylalanine (Phe), isoleucine (Ile), andleucine (Leu) were determined in the hydrolyzates byhigh-performance liquid chromatography equipped withan intelligent autosampler for automatic derivatizationand fluorescence detection (Merck, LaChrom). Gluta-mic acid (Glu) and aspartic acid (Asp) are also formedduring hydrolysis from glutamine (Gln) and asparagine(Asn), respectively. Consequently, results of the sum ofGln+Glu (Glx) and Asp+Asn (Asx) are reported, andin the following the terms glutamic acid and asparticacid are used for Glx and Asx, respectively. For the EastChina Sea samples a reversed phase column (Super-spher RP 18, 4 μm particle diameter, 125 mm length,4 mm inner diameter) and a multi-step gradient systemwas used (Fitznar et al., 1999). River samples wereanalyzed with a slightly modified method using aPhenomenex™ Hyperclone column (5 μm particlediameter, BDS C18, 250 mm length, 4 mm innerdiameter) with guard column (4×3 mm). Eluent A was125 mM sodium acetate with 2% methanol set to pH 6.8with acetic acid and eluent B was 100% methanol.

sampling stations. Different water masses which can be commonlyt-rich Yangtze River DilutedWater (CDW), the oligotrophic and highlyater (KW).

370 Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

Gradient started with 1% eluent B increasing to 12% Bafter 10 min. Eluent B reached 47% after 90 min and100% after 110 min. External standards of the aminoacid enantiomers (Fluka, Switzerland; Aldrich, USA;Sigma, USA) were used for calibration. Each samplewas analyzed in duplicate with two derivatization agents(N-isobutyryl-L-cysteine and N-isobutyryl-D-cysteine).We only report amino acid concentrations that werereproducibly quantified after derivatization with bothreagents and only those D-enantiomers that weresignificantly different from the racemization blank.The coefficients of variation between the duplicateswere 1 to 8%; the relative standard deviation for theindividual amino acids and each run was <3.5%. Thedetection limits for each amino acid were in the lowerpicomol range. The contribution of racemization duringhydrolysis compared to the naturally occurring D-aminoacid content was small as discussed in a previous study(Dittmar et al., 2001a).

3. Results

3.1. Hydrographic and bulk chemical parameters

Water column profiles of hydrographic parameterswere discussed in detail in previous publications (Pan etal., 2005; Zhu et al., 2006). Briefly, low salinities (<30)were measured in the Yangtze River Estuary owing tothe river effluent (Table 1). The water over the shelf wasstratified in terms of temperature and salinity with apycnocline at 20–50 m depth. Temperature and salinityshowed similar distributions at all transects. The watertemperature decreased steadily with depth to <5 °C atthe deepest stations (section A), and the salinityincreased steadily with depth.

Table 1Mean concentrations (±standard deviations) of bulk parameters and particul

River (12) Estuary (8)

Salinity 0 27.2±3.6TSM (mg L−1) 68±33 139±187POC (μM) 56±25 147±176PN (μM) 7.7±4.0 13.6±13.1POC/PN 7.7±1.6 8.7±3.0δ13C (‰) −24.4±0.37 −21.2±1.3δ15N (‰) 1.6±1.5 5.0±0.6NO3

− (μM) 94.1±34.4 26.1±25.6POC (mass-% of TMS) 1.1±0.4 1.1±0.3PAA (μM) 2.2±0.8 4.4±2.3PAA (mole-C% of POC) 5.8±2.2 7.9±5.8PAA (mole-N% of PN) 43±16 61±30D-AA (mole-% of PAA) 6.0±0.3 3.3±0.9

In addition, proportions of amino acids to various parameters are presented.

The continental margin was horizontally dividedinto four areas based on the water mass structure:estuary (3 stations; 8 samples at different depth),coastal (4 stations; 11 samples), shelf areas (5 stations;12 samples), and shelf edge (3 stations; 4 samples).Highest concentrations of total suspended matter(TSM), particulate organic carbon (POC) and nitrogen(PN) were observed in the estuary, exceeding thevalues in the river and on the shelf (Table 1). Theirconcentrations decreased during the transition from theestuary to the outer shelf. The organic carbon contentof TSM was low in all samples but showed a verysimilar trend as TSM and POC concentrations. Themass percentage of organic carbon in TSMwas about 1%in the river and estuary and decreased across the shelf to<0.5%. δ13C of POC increased from −24.4‰ onaverage in the river to values around −21‰ on theshelf. δ15N values were also higher on the shelf (4.0‰)than in the river (1.6‰) with highest values at near-shorestations (6.4‰). Highly variable concentrations werefound for nitrate being highest in the riverine samples(94.1 μM), decreasing sharply from the estuary(26.1 μM) to shelf (3.7 μM). At the shelf edge, nitrateincreased again to 7.5 μM due to contribution ofKuroshio waters (Table 1).

3.2. Hydrolyzable particulate amino acids

Similar to TSM, POC and PN, highest particulateamino acid (PAA) concentrations were found in theestuary (Table 1; Fig. 2). From the upper to the middlestream of the Yangtze River, the concentrations of PAAsteadily decreased from 2.3 to 1.3 μM, but increasedfrom the middle to the lower stream to reach maximumconcentrations in the estuary (4.4 μM on average).

ate amino acids from the different study sites

Near-shore (11) Shelf (12) Shelf edge (4)

31.8±2.5 33.3±1.3 33.8±0.518±10 17±11 7.6±2.1

11.7±4.4 6.9±3.1 2.4±1.01.6±0.7 1.0±0.5 0.36±0.187.2±0.8 7.0±0.7 8.2±1.3

−20.5±1.3 −21.9±1. 0 −22.8±1.76.4±1.1 4.0±1.7 –5.9±6.1 3.7±6.2 7.5±13.90.9±0.5 0.5±0.3 0.4±0.21.7±1.6 0.67±0.43 0.23±0.1310.0±2.8 8.6±3.1 4.9±2.066±18 61±26 46±323.3±1.5 1.5±0.5 1.5±0.3

Number of samples (n).

Fig. 2. Distribution of particulate amino acids from the river to the sea.

371Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

Across the shelf, the PAA concentration steadilydecreased to 0.23 μM at the shelf edge (Fig. 2).

Vertical profiles of PAA along the two cross-shelfsections are shown in Fig. 3a. At the innermost stationPAA increased with water depth. Contrary to the coastalstations, on most parts of the shelf slightly higher PAAconcentrations were observed at upper water depth.Along the northern transect (YT) the typically highernearshore PAA concentrations spread further offshorethan along the southern transect which could beassociated with the Yangtze River effluent plume. The

Fig. 3. Vertical distribution of particulate amino acids and t

contribution of amino acid carbon to POC increasedfrom 5.8% on average in the river to 10% nearshore, anddecreased again across the shelf to values of 4.9% at theshelf edge. The contribution of amino acid nitrogen toPON was high with about 50% in the river and shelfedge samples and >60% in the estuary and on the innershelf (Table 1).

The molecular composition of PAA differed sig-nificantly between riverine and oceanic samples (Fig.4). The most abundant amino acids were the acidic andneutral amino acids aspartic acid, glutamic acid, glycine,alanine and serine. Higher proportions of aspartic acid,glutamic acid and isoleucine were observed in marinethan in riverine samples, while the opposite trend wasfound for glycine, alanine, threonine and arginine. Theother amino acids did not exhibit a consistent orsignificant trend between river and marine samples.The non-protein amino acid, gamma-amino butyric acid,was close to the quantification limit and contributedvariably to PAA (0–5%). Isoleucine was undetectable inriver samples.

The D-enantiomers of aspartic acid, glutamic acid,serine, alanine and arginine were found in significantamounts in all particulate samples (Fig. 5). Other D-amino acids were not present in reliably detectableconcentrations. The percentage of D-amino acids to PAA

he degradation index along the transects PN and YT.

Fig. 4. Average mol percentage of individual particulate amino acids in the different sampling areas with confidence intervals (p<0.05).

372 Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

was highest in the river (6.0% on average). In theestuary and nearshore, the D-amino acid percentage waslower (3.3%) and decreased across the shelf to 1.5% atthe shelf edge. The most abundant D-enantiomer inriverine PAAwas D-arginine (21% of total arginine) andin the shelf edge PAA D-alanine (5.9% of total alanine).The proportions of D-aspartic acid, D-glutamic acid andD-arginine were significantly higher in riverine than inoceanic PAA and increased slightly with depth on theshelf.

We calculated the degradation index (DI) for ourPAA samples after Dauwe et al. (1999). The DI valuesranged between −1.2 and +0.9 (Fig. 6), indicating awide range in the degree of degradation from freshplanktonic biomass (positive values) to highly degradedorganic matter (negative values). The DI was constantlylow and negative in the river (−1.1±0.1) and increasedsteadily on the shelf with distance offshore to +0.8±0.1at the shelf edge (Fig. 6). On the shelf, the DI was mostlylower for samples collected near-bottom than in thesurface waters (Fig. 3b) indicating an advanced degreeof degradation at the bottom.

Fig. 5. Molar D-enantiomer fractions of the individual particulateamino acids with confidence intervals (p<0.05), calculated as D /D+L

amino acids.

4. Discussion

4.1. Sources of suspended organic nitrogen in theYangtze River

The concentrations of TSM and POC were variablewithout obvious trends along the course of the YangtzeRiver. The δ13C values (−24.4‰ on average) indicate asubstantial contribution of isotopically heavy organicmatter. In most world rivers, POC is composed of C3

plants and associated soil organic matter whose δ13Cvalues usually range between −28 and −26‰, whereasC4 plants have values of −15‰ and higher (e.g.Cifuentes, 1991). Riverine phytoplankton can cover awide range of δ13C, but because of the turbidity of theYangtze, phytoplankton abundance is low (Chla: 0.7±0.5 mg m−3 on average), and thus, cannot explain therelatively high δ13C values (Wu et al., 2007). Contribu-tions from corn plantations (C4) in the catchment areamay contribute to riverine suspended matter and explainthe high δ13C values in POC (Zhang et al., 1997), whichis, however, in contrast to the low C/N ratios (7.7 on

Fig. 6. Distribution of the degradation index (DI) from the river to thesea.

Fig. 7. Percentage of D-alanine of total alanine versus degradationindex from the river to the sea (probability for marine samplesp<0.005, for all samples p<0.001).

373Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

average). These ratios are much lower than C/N ratios ofhigher plant detritus and most soils from the YangtzeDrainage Basin (Wu et al., 2007). The C/N ratios of oursamples are also lower than those reported for POM inmajor world rivers (8.1 to 12.9; Ittekkot, 1988).Milliman et al. (1984) attributed similarly low C/Nratios to aquatic primary production in the YangtzeRiver, but speculated on the possible contribution ofammonium that might adsorb onto clay minerals. Amajor contribution of inorganic nitrogen to PN is veryunlikely because almost half of the particulate nitrogenin the Yangtze River was composed of amino acids(Table 1) as also reported for other world rivers (e.g.Dittmar et al., 2001a). Anthropogenic, organic inputsassociated with big urban centers at the Yangtze (mainlydomestic sewage and industrial inputs) are characterizedby relatively high δ13C (−23‰) and low C/N values(7.5) (Wu et al., 2007). A major contribution ofanthropogenic inputs could explain the δ13C and C/Nsignatures of our samples.

Stable nitrogen isotopes (δ15N) can also be usefultracers of organic matter sources in river and estuarysystems (e.g. Middleburg and Nieuwenhuize, 1998). Inthe Yangtze, however, the differences between the majorPON sources are small (Wu et al., 2007). The observedδ15N of PON in the river samples (1.6‰ on average) iswithin the range of soils, plants or anthropogenic inputsin the Yangtze catchment (Wu et al., 2007).

The composition of the particulate amino acids (Fig.4) provides information mainly on the degree ofdegradation of organic matter (e.g. Ittekkot, 1988; Keilet al., 2000; Ingalls et al., 2003). In POM of the YangtzeRiver, the cell wall constituent glycine, for instance, wasrelatively enriched, as was arginine, which might beselectively protected by mineral surfaces (Hedges andKeil, 1999). The degradation index (Dauwe et al., 1999)of −1.1 on average also indicates a uniformly highlyadvanced stage of degradation of POM in the entireYangtze River with little variations along the course ofthe mainstream (Fig. 6). The degradation index does notclearly distinguish between the influences of sourcesand degradation (Ingalls et al., 2003), and the multiplesources and fates of amino acids in organic matter aredifficult to fully establish, requiring a careful applicationof this index (Yamashita and Tanoue, 2003). In ourstudy, the results from the degradation index areconsistent with significant amounts of D-amino acidswhich are indicative for microbial metabolites.

D-amino acids are a major component of bacterialcell walls (as peptidoglycan) and not produced byalgae or vascular plants. Cell-wall constituents arepresumably less accessible to biodegradation than bulk

organic matter (Tanoue et al., 1995; Nagata et al.,1998) so that the respective D-amino acids mayaccumulate during diagenesis. Surprisingly, the mostabundant D-amino acid of the POM in the YangtzeRiver was D-arginine (Fig. 5). The presence of thisamino acid has not been reported before in naturalaquatic samples. It is a known bacterial metabolite(KEGG, 2006), but at this point we cannot reach aconclusion on the sources of this amino acid in theYangtze River. D-enantiomers of aspartic acid, glutamicacid, serine and alanine have been found in soil humicacids (Kimber et al., 1990), and riverine and marineorganic matter (Dittmar et al., 2001a). D-aspartic acid isthe most abundant D-amino acid in soil-derived organicmatter (Kimber et al., 1990; Dittmar et al., 2001a),whereas aquatic production seems to produce mainlyD-alanine (Amon et al., 2001; Dittmar et al., 2001a). Inthe Yangtze River POM similar amounts of D-alanineand D-aspartic acid were found indicating the contribu-tion of both, aquatic and terrigenous processes in thedegradation of organic matter.

Samples with an advanced degree of degradation(i.e., low degradation index) had on average signifi-cantly higher D-enantiomer ratios and a higher propor-tion of gamma-amino butyric acid (GABA), thansamples with a low degree of degradation. This trendwas most pronounced for D-alanine, which showed asignificant inverse correlation with the degradationindex (Fig. 7). This connection between D-aminoacids, GABA and the degradation index confirms thesignificance of independent approaches for assessing thedegree of degradation of suspended organic nitrogen.Besides heterotrophs, autotrophic bacteria might con-tribute another potential source of D-amino acids. Theconcentration of Synechococcus and other cyanobac-

374 Y. Wu et al. / Marine Chemistry 107 (2007) 367–377

teria in the East China Sea (Pan et al., 2005), however,was unrelated to the D-amino acid concentrations in thesame water samples. The lack of a positive correlationindicates that heterotrophs, and not autotrophs, are theprincipal source of D-amino acids in our samples.

Anthropogenic inputs (sewage) are rich in proteinsand would probably yield a positive degradation index.The highly advanced degree of degradation in theYangtze points towards a mixed source of riverinePOM: highly degraded plant detritus (high C/N and lowδ13C), relatively undegraded anthropogenic sources(very low C/N and high δ13C) and probably smallcontribution of phytoplankton.

4.2. Sources and transformation of suspended organicnitrogen in the Yangtze River Estuary and the EastChina Sea

The concentrations of TSM sharply increased in theYangtze River Estuary and decreased along the dispersalof the Yangtze River effluent plumes in the open EastChina Sea, which is in accordance to earlier observa-tions (e.g., Tan et al., 1991; Chen et al., 1999).Compared to these sharp changes in concentration, theamino acid pattern changed only gradually during thetransition from the river to the open ocean. Thesechanges are consistent with diagenetic trends observedin marine sediments (e.g., Dauwe et al., 1999). Thedegradation index indicated a gradual change to fresherPOM at the shelf edge. A deviation from the amino acidtrends described by Dauwe et al. (1999) is the sharpdecrease of aspartic acid from the outer shelf towards theestuary and river. Aspartic acid is abundant in proteinsand because of the highly hydrophilic character it tendsto be found on the surface of proteins and rather insolution than protected by mineral surfaces (Nunn andKeil, 2006). These properties may facilitate decomposi-tion and explain the observed trend of aspartic acid inthe East China Sea. The general decrease of the mostabundant riverine D-amino acids (Asp, Glu, Ala, Arg)along the transect offshore is consistent with the changesof the degradation index.

The gradual shift from highly degraded POM in theriver towards relatively fresh POM at the shelf edgeindicates that the sharp increase of PAA concentrationsin the estuary is probably not caused by a phytoplanktonbloom but by resuspension or trapping of riverine POM.A phytoplankton bloom in the magnitude of this sharpPAA increase would cause a distinct signal in the aminoacid composition, which was not observed. Themoderate changes in δ15N from 1.6‰ in the river to5.0‰ in the estuary is consistent with the explanation of

a minor phytoplankton contribution to PON in theestuary, because the δ15N of phytoplankton is usuallyenriched by 8–10% relative to terrestrial plants(O'Donnell et al., 2003). The effect of increasedchlorophyll and biomass concentrations in the YangtzeRiver Estuary (Chen et al., 1999) are probably maskedby the dominating hydrographic processes that inducealternative sediment resuspension and accumulation.The slightly elevated PAA concentrations and morenegative degradation indices at greater water depths alsoindicate resuspension of degraded riverine sediments onthe inner shelf.

From the view of sediment dynamics, the YangtzeRiver Estuary historically exhibits a tide-dominatedsediment transport regime (Shen et al., 1982). Thedynamic character of river discharge and tidal currentresults in an estuarine turbidity maximum zone, wheresuspended particles are trapped and accumulated (Li andChen, 1998; Chen et al., 1999). The Yangtze RiverEstuary is well known for its high turbidity character,and the turbidity maximum exists persistently causingthe high TSM concentrations. It acts as a barrier forsuspended particulates which flocculate, aggregate andsettle (Chen et al., 1999). Terrestrial sediments arefurther dispersed over the broad shelf through turbidbottom layers.

For the East China Sea, Hoshika et al. (2003)reported a well-developed turbid bottom layer insummer and fall spreading almost to the shelf edge,probably associated to the high discharge of suspendedparticles from the Yangtze River. The turbid bottomlayer may be composed from resuspended sediments,primary settling particles or from Yangtze Riverparticulates (Walsh et al., 1988; Iseki et al., 2003). Ourdata indicate an advanced degree of degradation near thebottom of the shelf, which points towards a primarilyriverine source of these particles. Our observation of afar reaching contribution of riverine particles onto thecentral shelf matches earlier studies that found asignificant contribution of riverine suspended matteras far as 250 km offshore (Zhang, 1999; Wu et al., 2003;Ren et al., 2006). Salinity and temperature profilesindicate upwelling on the central shelf probably inducedby the exchange of shelf mixed water with water massesfrom the Yellow Sea. Upwelling facilitates verticalmixing and brings up nutrients to support local primaryproduction in the East China Sea (Pan et al., 2005).According to the profiles of epi-fluorescence data, achlorophyll a maximum (0.1–0.8 mg m−3) occurred insubsurface waters (20–50 m) on the outer shelf (Zhu etal., 2006). This is consistent with the amino acidcomposition, which indicates diagenetically younger

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POM on the outer shelf and shelf edge, in contrast to thehighly degraded riverine POM in the estuary and on theinner shelf.

5. Conclusions

The amino acid composition shows an advancedstage of degradation of POM in the Yangtze River.One of the main sources of POM in the river isprobably soil-derived organic matter, but owing to thelow C/N ratios and the relatively high δ13C values amajor anthropogenic input (sewage) is likely. Theamino acid signature in the East China Sea reflectsthe gradual settling of decomposed riverine POM onthe inner shelf, and the production of fresh planktondetritus on the central and outer shelf. Estuarineproduction was masked by the large riverine inputand did not significantly impact the amino acidsignature of suspended particles. However, thephytoplankton fraction in riverine POM may con-tribute to benthic and pelagic food webs, because itmay be more bioavailable than the soil (humic)fractions. Considering the big building projects in thecatchment, sewage inputs may further increase whichwill change the transportation and fate of dissolvedand particulate matter and consequently the dynamicsof the riverine and coastal biogeochemistry andecology.

Although the overall amino acid composition issimilar to other world rivers and shelf regions, there aretwo exceptions: The high proportions of aspartic acidin the shelf POM and in particular the high proportionsof D-arginine in the riverine POM have not beenreported in natural aquatic matter so far. Future studieson the source of these amino acids may elucidateimportant general features of the biogeochemistry ofthe region.

Acknowledgements

The authors appreciate the support of the EducationMinistry of China and Deutsche Forschungsge-meinschaft (DFG) for the cooperation betweenSKLEC and AWI. We thank captain, officers and crewof the R/V “Dong Fang Hong 2” and colleagues inSKLEC for the great help during the cruise. We are alsograteful to the reviewers for their constructive com-ments. The study was supported by Ministry of Scienceand Technology of P. R. China (No. 2006CB400601,2004CB720505), the Ministry of Education of P.R.China (No. NCET-04-0424 and PCSIRT0427), theRising Star Program for Youth in Shanghai Science

and Technology (No. 04QMX1420) and NSFC (No.40476037, 40406017).

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