Geochemical records of decadal variations in terrestrial input and recent anthropogenic eutrophication in the Changjiang Estuary and its adjacent waters

Applied Geochemistry 27 (2012) 1556–1566

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Geochemical records of decadal variations in terrestrial input and recent anthropogenic eutrophication in the Changjiang Estuary and its adjacent waters Yu Yu a,b, Song Jinming a,⇑, Li Xuegang a, Duan Liqin a,b a b

Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanography, Chinese Academy of Sciences, Qingdao 266071, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 14 September 2011 Accepted 4 May 2012 Available online 14 May 2012 Editorial handling by G. Filippelli

a b s t r a c t Increasing eutrophication and seasonal anoxia in bottom water in the Changjiang Estuary and its adjacent waters has progressed in recent decades, caused by elevated anthropogenic N and P input. Sedimentary biogenic elements were investigated to determine whether the biogenic proxies could be used in paleoenvironmental studies in an energetic estuary, as well as to reconstruct the histories of environmental changes in the East China Sea (ECS). Two 210Pb-dated cores from the coastal and offshore waters were analyzed for organic C (TOC) and its stable isotope (d13C), total N (TN), biogenic Si (BSi), total P (TP) and P species. In coastal sediment, the variations of P species, especially Fe-P, Al-P and detrital apatite P (Det-P), reflected the dry–wet oscillations in the Changjiang River for the past century, which has influenced the sediment grain size and terrestrial material input. Much lower BSi content (0.756%) at 16–22 cm likely recorded the pronounced decrease in silicate flux in the Changjiang River and its lower flow in the late 1980s. In offshore sediment, higher concentrations of TOC, TN, BSi, Ex-P, Fe-P and Lea-OP indicated higher primary productivity in response to the strong winter monsoons during the 1960s– 1980s, and their 20-a fluctuations were in agreement with the decadal variations of the winter monsoon. Low contents and little variations of Al-P and Det-P indicated the slight influence of the terrestrial sediment input in offshore waters. The influence of human activities on the environment in recent decades has also been recorded in coastal sediment. Grain-size normalized concentrations of TOC, TN, TP, Ex-P, FeP and Lea-OP increased by 24%, 23%, 15%, 13% and 51% in the upper 16 cm of coastal sediment, indicating elevated P and N load and primary productivity since the 1990s. Elevated TN/TP ratios and decreased BSi/ TOC recorded the changed nutrient structure and the decrease in the proportion of the diatom to phytoplankton community. However, the sediment record indicated that the eutrophication might actually have started from the end of the 20th century rather than the reported middle of 20th century. In contrast, biogenic elements in offshore sediment did not reflect disturbance by human activities. This study revealed that multi-nutrient proxies in sediment in the ECS could indicate natural environmental changes including runoff and the winter monsoon over the past century, as well as the influence of human activities in recent decades. Phosphorus species with distinct origins and biogeochemical behaviors could effectively reflect different aspects of past environmental conditions. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Biogenic elements in sediments can provide information that helps in reconstructing past environmental conditions, evaluating climate change, and assessing the impacts of human activities on aquatic ecosystems. The accumulation of total organic C (TOC), N, biogenic Si (BSi), and P in sediments provides information about the deposition and preservation of these materials, while the stable isotopes of C (13C) and N (15N), and molar C/N ratios could be used to assess the origins of organic matter (Herczeg et al., 2001; Lamb et al., 2006; Rosenbauer et al., 2009). Thus environmental conditions including phytoplankton productivity, material sources, nutrient ⇑ Corresponding author. Tel./fax: +86 532 82898583. E-mail address: [email protected] (J. Song). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.05.002

loadings, eutrophication and hypoxia in waters could be reconstructed using these biogenic proxies (Hodell and Schelske, 1998; Benitez-Nelson, 2000; Chmura et al., 2004; Rosenbauer et al., 2009). Fractionation of sedimentary P by sequential extraction is an effective method to investigate the cycling and ecological significance of P in waters (Jensen et al., 1998; Andrieux-Loyer and Aminot, 2001). Generally, P species are operationally defined as exchangeable P (Ex-P), Fe-bound P (Fe-P), Al-bound P (Al-P) and leachable organic P (Lea-OP), authigenic carbonate fluorapatite (CFA-P), and detrital apatite (Det-P). These P species have distinct origins and biogeochemical behaviors in sediment. Exchangeable P is loosely adsorbed P in sediments which is in equilibrium with dissolved P and readily released into water (Zhang et al., 2004). Iron-bound P is mainly composed of the redox-sensitive fraction of P bound to hydrated oxides, mainly those of Fe. Aluminum-bound P is molybdate

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reactive P in NaOH extractant, primarily P bound to Al oxides and non-reducible Fe. Leachable OP is the non-reactive P extracted by NaOH, majorly fresh organic P bound to humic and fulvic acid (Jensen and Thamdrup, 1993). Authigenic carbonate fluorapatite is mainly CaCO3-bound P and authigenic carbonate fluorapatite which is deemed as an oceanic sink for reactive P (Ruttenberg, 1992; Cha et al., 2005). Detrital-P is apatite minerals, mainly the weathering product of terrestrial rocks (Lü et al., 2007). Non-apatite inorganic P including exchangeable P, Fe-bound P and Al-bound P are relatively active and could be released from sediment under certain conditions, such as the influence of biological processes and changed Eh and pH values (Christophoridis and Fytianos, 2006; Jiang et al., 2008). Detrital apatite is stable and is usually deemed a permanent sink of P in sediment (Ruttenberg, 1992). Different P species in sediment could reflect distinct environmental conditions, and might be promising proxies in reconstructing the paleoenvironment in aquatic ecosystems. The Changjiang Estuary is a dynamic estuarine system, annually receiving 8.9  1011 m3 of fresh water and 3.97  1011 kg sediment from the Changjiang River, the third largest river in the world in length and runoff. The estuary and adjacent waters have experienced environmental deterioration influenced by anthropogenic activity since the middle 20th century. According to monitoring data from Datong Station (625 km upstream of the estuary mouth), N and P inputs to the ECS have increased sharply since the 1960s, majorly caused by application of chemical fertilizer in the watershed. The concentration of NO 3 in the estuary has increased by nearly three times, and dissolved P concentration has increased by almost 30% in last four decades (Zhou et al., 2008). Elevated nutrient input has lead to increasing eutrophication, hazardous blooms, and seasonal hypoxia in bottom water in the estuary (Li et al., 2002; Chai et al., 2006). Furthermore, due to the construction of dams and reservoirs along the river, dissolved silicate and sediment load in the Changjiang River decreased from the 1960s, with accelerated decrease after the 1980s (Li et al., 2007). Decrease in the diatom proportion of the whole phytoplankton community has been observed in the ECS, probably related to the decreased silicate input and increased N/P ratios (Zhou et al., 2008). These environmental changes may be recorded in biogenic elements in the sediments, and reconstructing the history of the environmental change using these biogenic proxies is important for environmental assessment and management. Sedimentary biogenic elements have been proved to be reliable proxies of past environmental conditions in lakes, which generally have weaker hydrodynamic conditions and are more able to preserve these sediment records (Hodell and Schelske, 1998; BenitezNelson, 2000; Herczeg et al., 2001). In contrast, the application of these nutrient proxies has been limited in dynamic estuaries, in which the strong diagenesis and material transport might alter or erase the sediment records of biogenic elements (Ellegaard et al., 2006; Vaalgamaa and Korhola, 2007). Therefore, it is valuable to explore whether these biogenic elements could be used as tracers of paleoenvironmental conditions in dynamic estuaries. Based on measured C, N, biogenic Si, P species, and stable C isotopes in core sediments, this study aimed at exploring the application and effectiveness of these nutrient proxies, especially P species, in indicating environmental conditions, and to reconstruct the histories of environmental change in coastal and offshore ECS.

Asia Monsoon (Liu et al., 2007). The Changjiang River is the major sediment and nutrient source in the ECS. The Changjiang dilute water (CDW) is divided into two branches when it emerges from the estuary mouth, with one branch extending southward along the coast line of Zhejiang and the other northeastward toward Cheju Island. Influenced by the northward KC in the shelf break, a southward coastal current system is formed on the west coast of the ECS, including the Jiangsu Coastal Current (JCC) and the Zhejiang–Fujian Coastal Current (ZFCC). A strong NE winter monsoon prevails from September to April and a weaker SW summer monsoon from May to August. The stronger winter monsoon intensifies the coastal currents and thus increases terrestrial input by the JCC to the offshore ECS (Zhu et al., 2011). Since the Changjiang is the major source of material to the ECS, two sample sites were used along the southward and northeastward branch of the CDW to study the influence of terrestrial input on the environment of the ECS. The sample sites were located on the coast of Zhejiang Province (G1: 29.5°N, 122.5°E) and the offshore waters SW of Cheju Island (A6: 32.5°N, 125°E) (Fig. 1). Core sediments were obtained in May 2009, during the cruise of the ‘‘Kexue 1’’, using a gravity corer. Due to the more rapid sedimentation rate in coastal than offshore waters, the coastal core G1 was longer (92 cm) than the offshore core A6 (48 cm). Core sediments were sub-sampled at 2 cm intervals, and the sub-samples were then frozen until further analysis. 2.2.

210

Pb activity and grain size analysis

Samples selected at 10 cm intervals in cores G1 and A6 were analyzed for 210Pb activity by measuring its granddaughter 210Po, which was assumed to be in secular equilibrium with 210Pb. Spiking with 208Po and self-plating were performed by the method of Zaborska et al. (2007). After deposition, disks were analyzed for 210 Po and 208Po using a spectroscopy (YQ-14). The activity of 210 Po in the sample was determined based on chemical recovery by comparing the measured and spiked activities of 208Po. The supported 210Pb activity was calculated as the average of several 210Pb activities in deep layers (below the zone of 210Pb exponential decline). As 210Pb activity in core G1 had not reached its exponential decline bottom, the average level of supported 210Pb in the ECS was

N

YSWC

Yellow Sea Jia n

gsu

JCC Pro vi

China

Cheju Island nce

Ch a ngjia ng R

KC iver

CDW

Zhejiang Rrovince

ZFCC

TWC

East China Sea

2. Material and methods 2.1. Study sites and sample collection The material delivery pattern to the ECS is primarily governed by the Changjiang River, the Korushio Current (KC) and the East

E Fig. 1. Sampling sites of cores G1 and A6, regional circulation model (arrows), and the upwelling areas (shaded) in the ECS (CDW: Changjiang Dilute Water; JCC: Jiangsu Coastal Current; ZFCC: Zhejiang–Fujian Coastal current; TWC: Taiwan Warm Current; KC: Kuroshio Current; YSWC: Yellow Sea Warm Current).

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Table 1 Sequential extraction method for P species in sediments and their environmental significances. Step

P species

Extractant

P component extracted

Biogeochemistry and environmental significances

I II

Ex-P Fe-P

1 mol/L MgCl2 (1 h) 0.11 M BD (1 h)

III

Al-P Lea-OP

0.1 mol/L NaOH (18 h)

Exchangeable or loosely adsorbed P P bound to reducible hydrated oxides (mainly Fe-bound P) P bound to non-reducible Fe and Al oxides Non-reactive organic P (leachable organic P)

Readily released from sediment; related to primary productivity in water Released under reducing conditions; related to terrestrial input and primary productivity in water Governed by hydrological sorting; tracers for terrestrial sediment input Related to biogenic matter; tracers for terrestrial and marine organic matter input

IV

CFA-P

V

Det-P

Acetate buffer solution (pH 5) (3 h) 0.5 M HCl (1 h)

CaCO3-bound P + authigenic carbonate fluorapatite Apatite-bound P

Mainly of terrestrial lithogenic source; governed by hydrological sorting

The sequential extraction method is modified after Ruttenberg (1992), Jensen et al. (1998) and Lukkari et al. (2007). BD is the abbreviation of bicarbonate–dithionite.

adopted as 210Pbsupp in core G1. The relative standard deviations of 210 Pb activity were less than 10%. Parallel wet sediment samples were used for grain size analysis. Hydrogen peroxide (10%) and HCl (1 mol/L) were sequential added to remove organic matter and carbonate (Zhu et al., 2011). Neutralized slurry washed with distilled water was then disaggregated ultrasonically, and measured for grain size with a Laser Particle Size Analyzer (Cilas 940L). 2.3. Elemental and stable isotope analysis The 13C content, together with total organic C (TOC) and total N (TN) were determined using a mass spectrometer (Delta V Advantage). Results for d13C were reported in ‰ notation relative to the VPDB standard, and in weight% for TOC and TN, with relative standard deviations of less than 0.15% for d13C and 0.8% for TOC and TN. Biogenic Si (BSi) in sediments was determined by the wet-alkaline digestion technique using weak Na2CO3 (Mortlock and Froelich, 1989). Homogenized and disaggregated sediments were extracted with 40.0 mL 2 mol/L Na2CO3 at 85 °C for 5 h. Biogenic Si in the extractant was determined using the molybdenum blue method (Mullin and Riley, 1955). For TP determination, ground sediments were digested with a HNO3–HF–HClO4 mixture at 180 °C and the soluble reactive P was determined using the molybdenum blue method (Murphy and Riley, 1962). Based on duplicate analyses, relative standard deviations were less than 10% for BSi and 7% for TP. 2.4. Phosphorus fractionation A modified sequential extraction procedure, after Ruttenberg (1992), Jensen et al. (1998) and Lukkari et al. (2007), was applied to extract P fractions from the sediments (Table 1). Homogenized sediments were sequentially extracted with 1 mol/L MgCl2, 0.11 mol/L bicarbonate-dithionite (BD), 0.1 mol/L NaOH, Acetate buffer solution (pH 5), and 0.5 M HCl. After each step, the residue was rinsed with 0.5 mol/L NaCl once or twice to minimize adsorption. The P species in each step were defined as exchangeable P (Ex-P), Fe-bound P (Fe-P), Al-bound P (Al-P) and leachable organic P (Lea-OP), carbonate fluorapatite (CFA-P) and detrital apatite (Det-P), respectively. All the P species except Lea-OP were mainly inorganic P, and were measured by the molybdenum-blue method (Murphy and Riley, 1962). Lea-OP was determined as the difference of total P and the inorganic form in the NaOH extractant, with total P being determined following digestion with potassium persulfate (Thien and Myers, 1992). 3. Results and discussion 3.1. Sediment chronology The CIC (constant initial concentration) model and CRS (constant rate of supply) model are two commonly used models in

210 Pb chronology to estimate the sediment accumulation rate (Zaborska et al., 2007). The CIC model presumes that 210Pb activity and mass of deposited materials are constant, in which 210Pb activity attenuates exponentially without the influence of sedimentation rate. The CRS model presumes that the supply of 210Pb is constant while the sedimentation rate is variable, and it requires that there is no mixing of surface sediment. In core A6, 210Pb activities decreased exponentially between 4–20 cm (R2 = 0.737), which is consistent with the typical 210Pb profiles in the CIC model (Fig. 2). The CRS model is not appropriate for core A6 because a mixed layer of surface sediment existed at 0– 4 cm. Thus the CIC model was used to calculate the accumulation rate of core A6, yielding 0.18 cm/a in the upper 20 cm with the chronology of about 1909–2009 AD. The 210Pb profile of core G1 displayed different declining trends between the upper 42 cm and the lower section (Fig. 2). The 210Pb activities in both parts decreased exponentially, with R2 0.778 and 0.961. Thus the CIC model was applied to calculate the accumulation rates of the two sections, yielding 0.92 cm/a in the upper section (0–42 cm) and 0.39 cm/a in the lower section (42–83 cm), with the corresponding chronology of approximately 1966–2009 and 1858–1960 AD.

3.2. Origins of the sedimentary organic matter in the ECS indicated by d13C The isotopic composition of organic C (d13C) is a reliable proxy to distinguish organic matter (OM) sources in sediments (Lamb et al., 2006; Zhang et al., 2007; Rosenbauer et al., 2009). It is reported that the d13C of phytoplankton from the ECS varies from 21‰ to 19‰, much higher than that in terrestrial organic matter (27‰ to 25‰) in the Changjiang River (Wu et al., 2003, 2007b; Zhang et al., 2007). In this study, d13C in core G1 and A6 varied from 23.45‰ to 22.06‰ and 22.05‰ to 21.25‰, respectively, suggesting mixed sources of OM in the sediment (Fig. 3). A two end member model was applied to estimate the proportion of terrestrial and marine OM in sediment (Eadie et al., 1994):

d13 Cmeasured ¼ x  d13 Cterrestrial þ ð1  xÞ  d13 Cmarine where x and 1  x represent the proportion of terrestrial OM and marine OM to total OM in sediments. Values of 26‰ and 20‰ as d13Cterrestrial and d13Cmarine, respectively, were adopted based on the above reported d13C of terrestrial and marine OM in the ECS. The results showed that terrestrial input accounted for 34–58% of total OM in core G1, while it was only 21–34% in core A6. This was consistent with the coastal waters receiving more terrestrial material, mainly from the Changjiang River, than offshore waters. It was shown that though the primary productivity in coastal waters was much higher than offshore waters due to nutrient abundance (Luo et al., 2007), average TOC, TN and BSi concentrations in core A6 (0.512%, 0.068%, and

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0.01 0

Pb activity (dmp/g) 0.1

210

1

10

0

Pb activity (dmp/g)

0.1

1

10

10 20

Depth (cm)

Depth (cm)

20 40

60

30

40

80 50 210

210

Pb total 210 excess Pb 100

Fig. 2. Profiles of total

total Pb 210 excess Pb

G1 60

210

0.1

Pb activity (dmp/g) and excess

0.969%, respectively) were comparable to those in core G1 (0.509%, 0.062%, 0.923%, respectively). This likely resulted from higher terrestrial sediment input into coastal waters, which mainly contained minerals thus counteracting the higher content of biogenic material in marine particles (Wu et al., 2007a). The C/N ratio could also reflect information about OM sources if its original record is well preserved (Meyers, 2003; Lamb et al., 2006). The TOC/TN ratios in cores G1 and A6 ranged from 5.46– 16.07 and 6.53–11.99, respectively, between the C/N ratios in the terrestrial OM in the river (15–20) and marine OM in the ECS (5–10) (Zhang et al., 2007). The TOC/TN ratios in core A6 had significant negative correlations with d13C (R = 0.463, P = 0.083, n = 15), suggesting that the variations in the C/N ratios were consistent with d13C and that C/N ratios could indicate the OM sources in core A6. In contrast, the correlations between C/N and d13C in core G1 were positive and weak with R = 0.194. Since d13C in sediment is relatively stable during diagenesis and reliably indicates OM sources (Hodell and Schelske, 1998; Meyers, 2003), this weak correlation might suggest that C/N records in sediments have been altered by early diagenesis or hydrological sorting in the dynamic coastal environment. The C/N in core G1 could not be used to indicate sources of OM. 3.3. Sediment record of terrestrial input from the Changjiang River and the Jiangsu Coastal Current (JCC) 3.3.1. Sediment record in core G1 The Changjiang River is the major sediment source along the coast of Zhejiang Province (Liu et al., 2007; Zhu et al., 2011). The runoff from the Changjiang River exhibits periodic wet and dry variations after the 1950s (Fig. 4), with wet periods in 1953– 1955, 1980–1984 and 1989–1999, and dry periods in 1956–1960, 1971–1972, 1978–1979, 1984–1987, and after 2004. In general, the 1980s–1990s were very wet decades for the river while the 1960s-, 1970s and 2000s were dry decades. Consistent with the 70-a variation of dry-wet oscillation, another wet period occurred from the 1920s to 1940s (Qian et al., 2003). The river runoff level greatly influences the nutrient and sediment input into the ECS (Li et al., 2007), and significant correlation exists between annual sediment load and water discharge from 1953 to 2009 (R = 0.566, P = 0.028, n = 15).

210

A6

1

10

Pb activity in cores G1 and A6.

The average concentrations of biogenic elements below 24 cm in core G1 and from 6 to 20 cm in core A6 are given in the depth profiles of biogenic elements for comparison (Figs. 3 and 5). In view of the lesser influence of human activities before the 1980s and smaller variations in biogenic elements in the lower core, the average down core values might represent the natural background concentrations of biogenic elements in sediment. Much lower concentrations of BSi (0.756%) than the average down core value (0.973%) were found at 16–22 cm (ca. late 1980s) (Fig. 3). This might result from the decreased diatom production or lower BSi input from the river. During this period, the sediment had lighter d13C values (23.13‰) and lower TP and Lea-OP concentrations (4% and 19% lower) than average down core values, implying decreased deposition of marine organic matter. The Changjiang River is one of the major sources of silicate and particulate BSi in the ECS (Liu et al., 2005; Chen, 2008). Fig. 4 shows that the dissolved silicate (DSi) flux in the Changjiang was very low at the end of 1980s, which might be related to the dry climate in the Changjiang River at late 1980s and the construction of Gezhouba Dam on the river. Decreased silicate input might have limited diatom production in the coastal ECS and thus led to lower BSi concentrations in sediments. In addition, the input of riverine particulate BSi likely decreased with lower runoff, which might also contribute to lower BSi in sediment. The Ex-P, Fe-P, and Al-P and Lea-OP concentrations at 16–22 cm were 32%, 19%, 6% and 19% lower than their average down core values, whereas Det-P reached a peak of 321 lg/g (Fig. 5). This was likely not the result of changes in P input, since the dissolved inorganic P (DIP) flux was high in this period shown in Fig. 4. This distinct variation between the non-apatite P and Det-P likely was related to the changes in grain size resulting from lower runoff. Non-apatite inorganic P (including Ex-P, Fe-P, Al-P, CFA-P) and organic P are closely related to fine sediment, while detrital apatite is enriched in coarse sediment (Andrieux-Loyer and Aminot, 2001; Yu et al., 2011). As shown in Fig. 6, the median grain size at 16– 22 cm (PHI 6.94) was much lower than the average of PHI 7.07 throughout the core, implying coarser particles in this layer. Thus the low content of Ex-P, Fe-P, Al-P, Lea-OP and high Det-P were likely caused by coarser sediment in this period. There were significant linear correlations between grain size and P species in core G1, especially for Al-P and Det-P (RFe-P = 0.428, P = 0.077, n = 18;

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Fig. 3. Depth profiles of TOC (%), TN (%), BSi (%), TP (lg/g), d13C (‰) and C/N ratios in cores G1 and A6 from the ECS. Dashed lines represent the average down core concentrations of biogenic elements below 24 cm in core G1 and below 6 cm in core A6, respectively.

RAl-P = 0.827, P = 0.000, n = 18; RLea-OP = 0.443, P = 0.066, n = 18; RDet-P = 0.752, P = 0.000, n = 18), indicating the strong influence of grain size on the distributions of P species. Decreased sediment grain size was probably related to lower runoff of the Changjiang River. Successive drought in the river likely led to decreased sediment deposition in the Zhejiang coastal areas. Fine sediments in the coastal waters were continuously resuspended by upwelling currents and transported away by the southward ZFCC, whereas the input of particles were not sufficient to replace them when the input of riverine sediment decreased. Thus sediment particles in the coast of Zhejiang became coarser with continuously low river runoff. Phosphorus species in core G1 also recorded other dry and wet periods in the Changjiang River. At 60–68 cm (ca. the 1900s– 1910s), Fe-P, Al-P and Lea-OP concentrations showed decreasing trend upwards, and Al-P and Lea-OP reached their lowest concentrations, while Det-P increased and reached its maximum concentration. Consistently, median grain size showed minimum values of PHI 6.24 at 60–62 cm. These sediment records indicate the dry period around the 1900s–1910s, which are consistent with the 70-a variation of dry-wet oscillation in the Changjiang (Qian

et al., 2003). Above 8 cm, Det-P showed an increasing trend with median grain size notably decreased. As Det-P was stable and would hardly be decomposed by early diagenesis, its increasing trend most likely indicated the dry period of the Changjiang in 2000s. The Fe-P, Al-P, Lea-OP and CFA-P concentrations at 10– 16 cm (ca. the end of the 1990s) were 5%, 23%, 68% and 6% higher than average values, while Det-P was 11% lower than average concentrations during this period. This was in agreement with the finer particles of sediment at 8–10 cm (PHI 7.27). These sediment records reflect the wet period of the Changjiang at the end of 1990s. The TN was higher at 48–54 cm where d13C was lighter, implying a higher input of terrestrial organic matter. The Al-P, Lea-OP and CFA-P at 44–58 cm were 24%, 35% and 3% higher than average down core values while Det-P was 13% lower and reached its minimum concentration. This might correspond to the higher sediment grain size (PHI 7.69) at 44–46 cm. These sediment records indicate the wet period in the Changjiang around the 1920s–1940s. In conclusion, the deposition and distribution of P species in sediment, especially Fe-P, Al-P and Det-P, were greatly influenced by the hydrological sorting and Changjiang River input. Accordingly,

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runoff flow average flow from 1953 to 2009 5-point moving average

14000

runoff flow (×108 m3/yr)

13000 12000 11000 10000 9000 8000 7000 6000 1950

1960

1970

1980

1990

2000

2010

Year

(A) 25

3500

1800 1600

20

6

15

6

2500

1400

DIN flux (10 kg/y)

DIP flux (10 kg/y)

6

DSi flux (10 kg/y)

3000

2000

10

1000 800 600 400

5

200

1500 0 1960

1200

1970

1980

1990

2000

1960

0 1970

1980

1990

1960

1970

1980

Year

Year

Year

(B)

(C)

(D)

1990

2000

Fig. 4. Inter-annual variations in runoff (108 m3) and dissolved nutrient fluxes of the Changjiang River. (A) Runoff; (B) dissolved Si (DSi) flux; (C) dissolved inorganic P (DIP) flux; (D) dissolved inorganic N (DIN) flux. Runoff data are from the records of the Datong Hydrological Station, 625 km upstream of the estuary mouth. Nutrient flux data are from Li et al. (2007).

P species in part reflected the dry/wet oscillations in the Changjiang River in the past 100 years. Apart from the grain size effect, another reason why Fe-P and Al-P could reflect the Changjiang River input probably was that the river was one of the major sources of Fe-P and Al-P in marine sediment (Yu et al., 2011). Iron and Al are known terrestrial elements and mainly input into coastal waters by rivers, thus Fe-P and Al-P in sediment were closely related to riverine sediment input. This was also confirmed by higher contents of Fe-P and Al-P (70%, 320% higher, respectively) in coastal core G1 than in the offshore core A6 (Fig. 5). Sedimentary BSi was related to diatom production and particulate BSi input from the river, and likely recorded the pronounced decrease in silicate flux in the Changjiang River and its lower flow in the late 1980s. TOC and TN concentrations showed little variation below 18 cm, and could hardly reflect the variations in terrestrial input and grain size. It is suggested that TOC and TN (mainly organic matter) in the lower sediment core might have been altered by mineralization in dynamic conditions (Vaalgamaa and Korhola, 2007).

3.3.2. Sediment record in core A6 The offshore water SW of Cheju Island is mainly influenced by the NE branch of the CDW and the JCC (Hu and Yang, 2001; Zhu et al., 2011). Marine organic matter plays a bigger role in total organic matter deposition in offshore site A6 than in coastal waters as indicated by d13C. The strength of the JCC and NE branch of the CDW which determines the delivery of terrestrial matter was influenced by the NE winter monsoon, which strengthened JCC but weakened the NE CDW when the winter monsoon was stronger (Wang et al., 2003; Zhu et al., 2011). The strength of the winter monsoon grew stronger from the 1920s to 1960s, reached its maximum strength during the 1960s–1980s, and then waned after the late 1980s (Xu et al., 1999; Shi et al., 2007). Grain size in core A6 deviated little in the upper 20 cm with a CV (coefficient of variation) of 2.3%, except for 12–14 cm (ca. the 1940s) with notably low PHI (7.24), implying relatively stable hydrological conditions in the 100 years. TOC and TN had obviously higher concentrations (16% and 22% higher than average down core value) at 6–10 cm (ca. the

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Ex-P 10

15

Fe-P 20

0

60

80

Al-P 100

0

Lea-OP

40 60 80 100

5

0

10

CFA-P 15

0

50

Det-P 60

0

200

300

400

0

2009 1998

20

20

20

20

20

1987

20

40

40

40

40

40

40

1966 1940

60

60

60

60

60

60

Year

Depth (cm)

1976

1914 1889

80

80

80

80

80

80

1863

G1

Ex-P 10

15

Fe-P 20

40

60

80

Al-P 100

Lea-OP

20 40 60 80 100

5

10

CFA-P 15

50

60

Det-P 70

200

0

0

0

0

0

0

2

2

2

2

2

2

4

4

4

4

4

4

6

6

6

6

6

6

8

8

8

8

8

8

10

10

10

10

10

10

12

12

12

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1960s–1970s) for TOC and 4–8 cm (ca. late the 1970s–1980s) for TN. High concentrations of BSi occurred at 6–8 cm approximately

in the late 1970s. Concentrations of Ex-P, Fe-P and Lea-OP at 4– 10 cm (ca. the 1960s–1980s) were 21%, 16% and 30% higher than average down core values. High concentrations of TOC, TN, BSi and non-apatite P, combined with pronounced heavier d13C (21.28‰) in sediment at 4–8 cm, likely implied the increased marine organic matter and higher phytoplankton productivity during the 1960s to 1980s. It seems that this elevated sedimentary biogenic elements and phytoplankton production was not caused by the variation in Changjiang input, since the Changjiang was largely in its dry period from the 1960s to 1980s. In addition, the pronounced wet period of the runoff during the 1980s–1990s is virtually unreflected in these sedimentary proxies. In contrast, it corresponded to the stronger winter monsoon during the 1960s– 1980s, which might be the major cause of the elevated phytoplankton production. With the strengthened JCC during this period, more P and N nutrient from the Jiangsu coast might be delivered into the northeastern regions of the ECS and promote phytoplankton production (Zhu et al., 2011). In addition, throughout the upper 20 cm, TOC, TN, BSi, d13C, Ex-P, Fe-P and Lea-OP generally exhibited 20-a fluctuations, with higher values at around 4–10 cm (ca. 1960s–1980s) as discussed above, and much lower values at around 2–6 cm (ca. the late 1980s–1990s) and 8–14 cm (ca. the 1940s–1960s). This may be in agreement with the decadal variations in the winter monsoon, which were strongest during the

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with TOC in core A6 (R = 0.690, P = 0.000, n = 27). Thus, like TOC, Lea-OP could also indicate phytoplankton production. The JCC influenced the dissolved nutrient input into the offshore waters in the northern ECS and thus the primary productivity. Therefore, these biogenic proxies including TOC, TN, BSi, Ex-P, Fe-P and LeaOP could reflect the variation in primary productivity and the strength of the winter monsoon.

1960s to 1980s but weaker during 1920–1960 and after the late 1980s. Decreased BSi at 2–6 cm (in the late 1980s and 1990s) was also likely related to the marked decrease in the DSi flux in the Changjiang River after the 1980s (Fig. 4). The Al-P and Det-P exhibited little variation in core A6 with CVs of 8% and 4% in the upper 20 cm, consistent with the less varied sediment grain size. These proxies of Changjiang River input as suggested by core G1 did not reflect the variations in the Changjiang runoff and the winter monsoon. It is suggested that terrestrial sediment, mainly from the Changjiang, exerted only slight influence on the offshore depositional environment, at least compared to other influencing factors such as hydrological sorting. This is consistent with the sediment delivered from the Changjiang River majorly being deposited in the coastal waters east to 123°E in the ECS (Hu and Yang, 2001). The reason why the variations of winter monsoon were recorded in TOC, TN, BSi, Ex-P, Fe-P and Lea-OP proxies likely is related to their distinct origins and geochemical behaviors. Organic matter in the offshore ECS is primarily from phytoplankton, and thus TOC, TN and Lea-OP are mainly related to phytoplankton production in the offshore. BSi is related to the diatom production in water. The Ex-P and Fe-P are the most active P forms in sediment, and could be released into the overlying water under certain conditions (Jensen et al., 1998; Zhang et al., 2004). They are potential sources of bioavailable P and might be related to the dissolved P pool in water and phytoplankton growth (Zhang et al., 2004). The Lea-OP is organic P combined with humic or fulvic acids, and might contain live algae and bacteria (Jensen and Thamdrup, 1993) (Table 1). The Lea-OP was significantly correlated

0

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3.4. Sediment record of anthropogenic eutrophication in the ECS 3.4.1. Sediment record in Core G1 The Changjiang Estuary and adjacent waters including the coast of Zhejiang have been reported to have undergone severe eutrophication and elevated primary productivity since the 1960s, mainly 3 resulting from the marked increase in terrestrial NO 3 and PO4 input into the water, especially the former (Fig. 4) (Li et al., 2002; Zhou et al., 2008). Pronounced increase in the occurrence of the red tide had been observed in the coastal ECS since the 1980s, and a large proportion of them occurred in coastal water south of the estuary (Zhou et al., 2008). The grain-size normalized concentrations of sedimentary biogenic elements were used to determine the influence of human activities on sedimentary biogenic elements without grain size effects (Fig. 7). The Det-P was not considered because that it is negatively correlated with grain size and is mainly of natural lithogenic origin. TOC/U, TN/U, TP/U, Ex-P/U, Fe-P/U and Lea-OP/U showed pronounced increasing trends in the upper 16 cm (ca. from the 1990s–2000s), with average increases of 24%, 23%, 15%, 13% and

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Fig. 7 (continued)

51% compared to their average down core values. It seems that the increased concentrations were neither the result of mineralization of organic matter, in view of the abrupt elevation of TN and LeaOP in the upper core, nor the result of the Changjiang input since the river was in the dry period in 2000s. This persistent and marked change in these biogenic proxies might be majorly caused by eutrophication and elevated primary productivity in coastal waters in recent decades. The d13C is heavier in the upper 14 cm with an average of 22.56‰, suggesting elevated deposition of marine organic matter. It is worth noting that the marked increase of Ex-P and Fe-P in the upper core might also result from redox cycling of Fe in sediments (Cha et al., 2005). That is, Fe-P is remobilized under reducing conditions in deeper sediment, diffuses upward and then re-precipitates in oxides in surface sediment. As reflected by biogenic proxies in core G1, the eutrophication and elevated primary productivity started around the 1990s, and there were no continuous increases in sedimentary biogenic elements before the 1990s. This disagrees with some reports that eutrophication had begun in the middle of the century (1960s) when the application of chemical fertilizers began (Li et al.,

2007; Zhou et al., 2008). The DIP and DIN flux in the Changjiang River (Fig. 4) showed that they actually started to increase markedly in the 1980s. Therefore, in agreement with the sedimentary record, it seems that the eutrophication and elevated primary productivity in the coastal ECS started at the end of 20th century (1980s or 1990s) rather than the 1960s. This is consistent with the period when the reform and opening-up policy were implemented (in 1978) and the economy of China started to grow rap3 idly. Due to the more rapid increase of NO 3 input than PO4 from the Changjiang River, N/P ratios in the coastal waters have increased notably in recent decades (Li et al., 2007; Zhou et al., 2008). These elevated N/P ratios were also recorded in sediments, with TN/TP ratios above 14 cm (after the 1990s) 12% higher than the average down core value (Fig. 8). The Si/C ratios could be used to reflect the condition of diatom growth and the structure of the phytoplankton assemblage (Liu et al., 2005). The BSi/TOC molar ratios in core G1 showed a slightly upward decreasing trend below 20 cm (Fig. 8). This resulted from a slight increase of TOC with a generally constant BSi in sediment (Fig. 3). The increase of TOC probably resulted from

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the mineralization of organic matter, which most likely happened in view of the minimal fluctuation of TOC and TN below 20 cm. In the upper 20 cm, BSi/TOC remained much lower (21% lower than the average down core value). Because of the pronounced increase in TOC and constant BSi in the upper core, decreased BSi/TOC most likely suggests a decreased proportion of diatoms in the whole phytoplankton community after the 1980s. This is in agreement with the report that the proportion of diatoms decreased from 33% in the 1980s to 24% in the 2000s and red tide causative species had apparently shifted from diatoms to dinoflagellates in the coastal ECS (Li et al., 2007; Zhou et al., 2008). Many processes might contribute to the retarded growth of diatoms in eutrophic waters, such as the marked decrease in silicate input in late 1980s, and elevated N/P ratios in recent decades which made the ecosystem P-limited, since diatoms are more easily affected by P limitation than flagellates (Egge, 1998). Therefore, limitation of diatom growth was most likely occurring in the ECS, and declined BSi/TOC ratios in coastal sediment recorded the decrease in the proportion of diatoms in the phytoplankton community. 3.4.2. Sedimentary record in core A6 It was shown in Section 3.3.2 that the variations of nutrient proxies in core A6 were in good agreement with the variations of winter monsoon strength, and thus they seemed to be majorly governed by climatic conditions. Vertical variations of grain-size normalized elements resembled that of their absolute concentrations since the grain size varied little above 20 cm in core A6. As stated in 3.3.2, high concentrations of TOC, TN, BSi, Ex-P, Fe-P and Lea-OP and heavier d13C at 4–10 cm indicated higher phytoplankton productivity during the 1960s to 1980s. If eutrophication in coastal waters started from the 1960s as reported (Zhou et al., 2008), the offshore waters might also be influenced by elevated terrestrial P and N input, though the long-term monitoring data on nutrient concentrations in the offshore waters were lacking. Therefore, in addition to the strong winter monsoon, eutrophication might also contribute to the marked increase in primary productivity during the 1960s–1980s. However, if the eutrophication started in the late 20th century as suggested by core G1, the elevated productivity then was majorly caused by the strong winter monsoon. In addition, TOC, TN, Ex-P, Fe-P and Lea-OP concentrations in core A6 decreased at 2–4 cm (ca. the 1990s) when eutrophication was more serious in the coastal waters (Zhou et al., 2008). Therefore, the biogenic proxies in offshore core A6 mainly reflected the influence of the winter monsoon rather than human activities.

4. Conclusions This study investigated the sediment record of terrestrial input from the Changjiang River and the JCC over the past century based on nutrient proxies in sediments from the ECS, especially P species. In response to changes in grain size, variations of P species, especially Fe-P, Al-P and Det-P, in coastal sediment reflected in part the dry-wet oscillations in the Changjiang River flow over the last century. Much lower BSi in sediment recorded the low flux of dissolved silicate in the Changjiang River and the lower runoff in the late 1980s, which influenced diatom growth and riverine BSi input. In the offshore core, low contents and little variations of Al-P and Det-P reflected only the slight influence of terrestrial sediment input. Biogenic proxies including TOC, TN, d13C, BSi, Ex-P, Fe-P and Lea-OP indicated the variations of primary productivity mainly caused by the varied strength of the winter monsoon and the JCC. Decreased BSi in the late 1980s–1990s in offshore sediment might also be related to decreased silicate flux in the Changjiang River after the 1980s. Due to human activities, the coastal ECS has undergone severe eutrophication and environmental deterioration in recent decades. The histories of eutrophication and elevated primary productivity have been recorded in the coastal sediment, with increased absolute and grain-size normalized concentrations of TOC, TN, TP, Ex-P, Fe-P and Lea-OP since the 1990s. Increased TN/TP ratios and decreased BSi/TOC ratios in sediment recorded the changed nutrient structure and decrease in the proportion of diatoms in the phytoplankton community in coastal eutrophic waters. The sediment record indicated that eutrophication might actually have started from the end of the 20th century rather than the reported middle of 20th century. This might help to reassess the histories of anthropogenic eutrophication in the coastal ECS, especially when the records of eutrophication in the early period are lacking. In contrast, biogenic elements in offshore sediment reflected only slight disturbance by human activities. It was demonstrated that the nutrient records were influenced by many factors in the dynamic estuary, such as the allochthonous and autochthonous input, hydrological sorting, and diagenesis. However, the nutrient proxies, especially d13C, BSi and P species could still provide useful information about paleoenvironmental conditions if their records were properly read. In addition, this study showed that different P species had distinct origins and biogeochemical behaviors, and could effectively indicate different aspects of past environmental conditions.

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Acknowledgements This research was supported by the Natural Science Foundation of China for Creative Research Groups (No. 41121064), the National Basic Research Program (973) of China (Nos. 2011CB403602 and 2010CB951802), and the National Natural Science Foundation of China (No. 40906056). We are very grateful to the editor Prof. Ron Fuge and Dr. Gabe Filippelli, and anonymous reviewers for their constructive comments, suggestions and corrections which helped improve the manuscript. References Andrieux-Loyer, F., Aminot, A., 2001. Phosphorus forms related to sediment grain size and geochemical characteristics in French coastal areas. Estuar. Coast. Shelf Sci. 52, 617–629. Benitez-Nelson, C.R., 2000. The biogeochemical cycling of phosphorus in marine systems. Earth-Sci. Rev. 51, 109–135. Cha, H.J., Lee, C.B., Kim, B.S., Choi, M.S., Ruttenberg, K.C., 2005. Early diagenetic redistribution and burial of phosphorus in the sediments of the southwestern East Sea (Japan Sea). Mar. Geol. 216, 127–143. Chai, C., Yu, Z.M., Song, X.X., Cao, X.H., 2006. The status and characteristics of eutrophication in the Yangtze River (Changjiang) estuary and the adjacent East China Sea, China. Hydrobiologia 563, 313–328. Chen, C.T., 2008. Distributions of nutrients in the East China Sea and the South China Sea connection. J. Oceanogr. 64, 737–751. Chmura, G.L., Santos, A., Pospelova, V., Spasojevic, Z., Lam, R., Latimer, J.S., 2004. Response of three paleo-primary production proxy measures to development of an urban estuary. Sci. Total Environ. 320, 225–243. Christophoridis, C., Fytianos, K., 2006. Conditions affecting the release of phosphorus from surface lake sediments. J. Environ. Qual. 35, 1181. Eadie, B.J., Mckee, B.A., Lansing, M.B., Robbins, J.A., Metz, S., Trefry, J.H., 1994. Records of nutrient-enhanced coastal ocean productivity in sediments from the Louisiana continental-shelf. Estuaries 17, 754–765. Egge, J.K., 1998. Are diatoms poor competitors at low phosphate concentrations? J. Mar. Syst. 16, 191–198. Ellegaard, M., Clarke, A.L., Reuss, N., Drew, S., Weckstrom, K., Juggins, S., Anderson, N.J., Conley, D.J., 2006. Multi-proxy evidence of long-term changes in ecosystem structure in a Danish marine estuary, linked to increased nutrient loading. Estuar. Coast. Shelf Sci. 68, 567–578. Herczeg, A.L., Smith, A.K., Dighton, J.C., 2001. A 120 year record of changes in nitrogen and carbon cycling in Lake Alexandrina, South Australia: C/N, d15N and d13C in sediments. Appl. Geochem. 16, 73–84. Hodell, D.A., Schelske, C.L., 1998. Production, sedimentation, and isotopic composition of organic matter in Lake Ontario. Limnol. Oceanogr. 43, 200–214. Hu, D.X., Yang, Z.S., 2001. The Key Processes of the Ocean Flux in the East China Sea. China Ocean Press, Beijing. Jensen, H.S., Thamdrup, B., 1993. Iron-bound phosphorus in marine-sediments as measured by bicarbonate–dithionite extraction. Hydrobiologia 253, 47–59. Jensen, H.S., McGlathery, K.J., Marino, R., Howarth, R.W., 1998. Forms and availability of sediment phosphorus in carbonate sand of Bermuda seagrass beds. Limnol. Oceanogr. 43, 799–810. Jiang, X., Jin, X., Yao, Y., Li, L., Wu, F., 2008. Effects of biological activity, light, temperature and oxygen on phosphorus release processes at the sediment and water interface of Taihu Lake, China. Water Res. 42, 2251–2259. Lamb, A.L., Wilson, G.P., Leng, M.J., 2006. A review of coastal palaeoclimate and relative sea-level reconstructions using delta C-13 and C/N ratios in organic material. Earth-Sci. Rev. 75, 29–57. Li, D.J., Zhang, J., Huang, D.J., Wu, Y., Liang, J., 2002. Oxygen depletion off the Changjiang (Yangtze River) Estuary. Sci. China Ser. D – Earth Sci. 45, 1137–1146. Li, M.T., Xu, K.Q., Watanabe, M., Chen, Z.Y., 2007. Long-term variations in dissolved silicate, nitrogen, and phosphorus flux from the Yangtze River into the East China Sea and impacts on estuarine ecosystem. Estuar. Coast. Shelf Sci. 71, 3–12. Liu, J.P., Xu, K.H., Li, A.C., Milliman, J.D., Velozzi, D.M., Xiao, S.B., Yang, Z.S., 2007. Flux and fate of Yangtze River sediment delivered to the East China Sea. Geomorphology 85, 208–224.

Liu, S.M., Zhang, J., Li, R.X., 2005. Ecological significance of biogenic silica in the East China Sea. Mar. Ecol. Prog. Ser. 290, 15–26. Lü, C., He, J., Sun, H., Xue, H., Liang, Y., Bai, S., Sun, Y., Shen, L., Fan, Q., 2007. Application of allochthonous organic carbon and phosphorus forms in the interpretation of past environmental conditions. Environ. Geol. 55, 1279–1289. Lukkari, K., Hartikainen, H., Leivuori, M., 2007. Fractionation of sediment phosphorus revisited. I: Fractionation steps and their biogeochemical basis. Limnol. Oceanogr.-Methods 5, 433–444. Luo, M.B., Lu, J.J., Wang, Y.L., Shen, X.Q., Chao, M., 2007. Horizontal distribution and dominant species of phytoplankton in the East China Sea. Acta Ecol. Sin. 27, 5076–5085 (in Chinese). Meyers, P.A., 2003. Applications of organic geochemistry to paleolimnological reconstructions: a summary of examples from the Laurentian Great Lakes. Org. Geochem. 34, 261–289. Mortlock, R.A., Froelich, P.N., 1989. A simple method for the rapid-determination of biogenic opal in pelagic marine-sediments. Deep-Sea Res. Part A – Oceanogr. Res. Pap. 36, 1415–1426. Mullin, J.B., Riley, J.P., 1955. The colorimetric determination of silicate with special reference to sea and natural waters. Anal. Chim. Acta 12, 162–176. Murphy, J., Riley, J.P., 1962. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 26, 31–36. Qian, W., Hu, Q., Zhu, Y., Lee, D.K., 2003. Centennial-scale dry–wet variations in East Asia. Clim. Dynam. 21, 77–89. Rosenbauer, R.J., Swarzenski, P.W., Kendall, C., Orem, W.H., Hostettler, F.D., Rollog, M.E., 2009. A carbon, nitrogen, and sulfur elemental and isotopic study in dated sediment cores from the Louisiana shelf. Geo-Mar. Lett. 29, 415–429. Ruttenberg, K.C., 1992. Development of a sequential extraction method for different forms of phosphorus in marine sediments. Limnol. Oceanogr. 37, 1460–1482. Shi, Y.S., Chi, J.C., Kong, F.C., 2007. Influence of East-Asia Monsoon intensity change on climate in Hebei Province. Meteorol. Sci. Technol. 35 (1), 49–52 (in Chinese). Thien, S.J., Myers, R., 1992. Determination of bioavailable phosphorus in soil. Soil Sci. Soc. Am. J. 56, 814–818. Vaalgamaa, S., Korhola, A., 2007. Geochemical signatures of two different coastal depositional environments within the same catchment. J. Paleolimnol. 38, 241– 260. Wang, B.D., Wang, X.L., Zhan, R., 2003. Nutrient conditions in the Yellow Sea and the East China Sea. Estuar. Coast. Shelf Sci. 58, 127–136. Wu, Y., Dittmar, T., Ludwichowski, K.U., Kattner, G., Zhang, J., Zhu, Z.Y., Koch, B.P., 2007a. Tracing suspended organic nitrogen from the Yangtze River catchment into the East China Sea. Mar. Chem. 107, 367–377. Wu, Y., Zhang, J., Li, D.J., Wei, H., Lu, R.X., 2003. Isotope variability of particulate organic matter at the PN section in the East China Sea. Biogeochemistry 65, 31– 49. Wu, Y., Zhang, J., Liu, S.M., Zhang, Z.F., Yao, Q.Z., Hong, G.H., Cooper, L., 2007b. Sources and distribution of carbon within the Yangtze River system. Estuar. Coast. Shelf Sci. 71, 13–25. Xu, J.Z., Zhu, Q.G., Zhou, T.H., 1999. Sudden and periodic changes of East Asian Winter Monsoon in the past century. Quart. J. Appl. Meteorol. 10 (1), 1–8 (in Chinese). Yu, Y., Song, J.M., Li, X.G., Yuan, H.M., Li, N., Duan, L.Q., 2011. Distributions and environmental implications of the phosphorus forms in the surface sediments from the Changjiang Estuary. Adv. Earth Sci. 26, 870–880. Zaborska, A., Carroll, J., Papucci, C., Pempkowiak, J., 2007. Intercomparison of alpha and gamma spectrometry techniques used in 210Pb geochronology. J. Environ. Radioact. 93, 38–50. Zhang, J., Wu, Y., Jennejahn, T.C., Ittekkot, V., He, Q., 2007. Distribution of organic matter in the Changjiang (Yangtze River) Estuary and their stable carbon and nitrogen isotopic ratios: implications for source discrimination and sedimentary dynamics. Mar. Chem. 106, 111–126. Zhang, J.Z., Fischer, C.J., Ortner, P.B., 2004. Potential availability of sedimentary phosphorus to sediment resuspension in Florida Bay. Global Biogeochem. Cycles 18. Zhou, M.J., Shen, Z.L., Yu, R.C., 2008. Responses of a coastal phytoplankton community to increased nutrient input from the Changjiang (Yangtze) River. Continental Shelf Res. 28, 1483–1489. Zhu, C., Wang, Z., Xue, B., Yu, P., Pan, J., Wagner, T., Pancost, R.D., 2011. Characterizing the depositional settings for sedimentary organic matter distributions in the Lower Yangtze River-East China Sea Shelf system. Estuar. Coast. Shelf Sci. 93, 182–191.