Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

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JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X

Journal of Environmental Sciences 2013, 25(11) 2185–2195

www.jesc.ac.cn

Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas Yu Yu1,2 , Jinming Song1,∗, Xuegang Li1 , Huamao Yuan1 , Ning Li1 , Liqin Duan1 1. Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. E-mail: [email protected] 2. Marine Science and Engineering College, Qingdao Agricultural University, Qingdao 266109, China Received 11 January 2013; revised 03 May 2013; accepted 07 May 2013

Abstract Biogenic elements and six phosphorus (P) fractions in surface sediments from the Changjiang Estuary and adjacent waters were determined to investigate the governing factors of these elements, and further to discuss their potential uses as paleo-environment proxies and risks of P release from sediment. Total organic carbon (TOC) and leachable organic P (Lea-OP) showed high concentrations in the estuary, Zhejiang coast and offshore upwelling area. They came from both the Changjiang River and marine biological input. Biogenic silicon (BSi) exhibited a high concentration band between 123 and 124˚E. BSi mainly came from diatom production and its concentration in the inshore area was diluted by river sediment. Total nitrogen (TN) was primarily of marine biogenic origin. Seaward decreasing trends of Fe-bound P and Al-bound P revealed their terrestrial origins. Influenced by old Huanghe sediment delivered by the Jiangsu coastal current, the maximum concentration of detrital P (Det-P) was observed in the area north of the estuary. Similar high concentrations of carbonate fluorapatite (CFA-P) and CaCO3 in the southern study area suggested marine calcium-organism sources of CFA-P. TOC, TN and non-apatite P were enriched in fine sediment, and Det-P partially exhibited coarse-grain enrichment, but BSi had no correlation with sediment grain size. Different sources and governing factors made biogenic elements and P species have distinct potential uses in indicating environmental conditions. Transferable P accounted for 14%–46% of total P. In an aerobic environment, there was low risk of P release from sediment, attributed to excess Fe oxides in sediments. Key words: biogenic elements; phosphorus fractions; sediment; source; grain size effect; Changjiang Estuary and adjacent waters DOI: 10.1016/S1001-0742(12)60302-7

Introduction Carbon, N, P and Si are essential nutrients for phytoplankton growth in water. Sediment is the largest marine sink of these biogenic elements. In addition to coming from marine organisms and detritus, biogenic elements in sediments could also be input from runoff, the atmosphere and redistribution induced by sediment transport and diagenesis (Nixon et al., 1996; Song et al., 2006; Zhang et al., 2003). Additionally, sediment grain size and environmental conditions such as salinity, pH and Eh are important factors determining the distributions of biogenic elements in sediments (Andrieux-Loyer and Aminot, 2001; Giblin et al., 2010; Lin et al., 2002). Owing to the close correlations of biogenic elements with phytoplankton productivity, terrestrial input and other environmental conditions, the records of sedimentary biogenic elements * Corresponding author. E-mail: [email protected]

could indicate past environmental conditions, especially the human-induced environmental changes in recent decades (Meyers, 2003; Schelske et al., 1988). To use these indicators in paleo-environmental studies, the biogeochemical behaviors of these elements, mainly the sources and the influencing factors, should be studied thoroughly. Surface sediment is an ideal site to investigate the behavior of biogenic elements and their relationships with environmental conditions. Sediment is an important internal source of nutrients in water (Fisher et al., 1982). Carbon, N, P and Si in surface sediments could be released into overlying water under certain conditions, such as biological activity and changes in pH, Eh and salinity (Rozan et al., 2002; Tyler et al., 2003). Nitrogen and P release from sediments provides necessary nutrition for phytoplankton, and might also pose a threat of eutrophication to waters (Rozan et al., 2002). Excess terrestrial loading of N and P has caused eu-

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Journal of Environmental Sciences 2013, 25(11) 2185–2195 / Yu Yu et al.

trophication, frequent occurrence of harmful algal blooms and seasonal bottom hypoxia in the Changjiang Estuary and its adjacent waters (Li et al., 2002; Zhou et al., 2008). Apart from monitoring the nutrient fluxes of the Changjiang River, it is important to assess the potential risks of nutrient release from sediment, especially for P, which is the limiting nutrient in the inshore of the East China Sea (ECS) (Wang et al., 2003). Sequential extraction is an effective method to estimate the mobility of P in sediment (Jensen et al., 1995). Generally, labile P fractions in sediments are releasable and bioavailable P pools, while detrital and refractory P are immobile pools (Christophoridis and Fytianos, 2006; Jensen et al., 1998; Jensen and Thamdrup, 1993). Besides, P species have distinct origins and behavior, and this makes P fractions promising indicators of environmental conditions (L¨u et al., 2008; Ruttenberg, 1992). In previous studies, Zhu et al. (2011) reported that total organic carbon (TOC) in sediments of the ECS is associated with fine-grained sediment, and undergoes degradation in the estuary. Kao et al. (2003) found that the distribution of TN in the ECS resembles that of fine sediment. There are few studies on the distribution of biogenic silicon (BSi) in sediments of the ECS. Liu et al. (2005) reported that a large proportion (3%–64%) of BSi in sediments is regenerated. The distributions of total phosphorus (TP) and P species are governed by sediment grain size (He et al., 2009). The burial efficiency of P in the ECS is over 90% due to the large proportion of detrital P (Det-P) and high sedimentation rate (Fang et al., 2007). Although we have gained some knowledge about the geochemistry of biogenic elements in the ECS, studies on the sources and influencing factors of these elements, especially for BSi and P species, are relatively few. This study aimed to systematically investigate the major sources and governing factors of TOC, total nitrogen (TN), BSi, TP and P species in sediments of the Changjiang Estuary and its adjacent waters through their surface distributions and correlations with hydrological and ecological conditions, and further to understand their environmental significance including their potential use as environmental indicators and the ecological risk of P release from sediment.

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including the Jiangsu coastal current and the ZhejiangFujian coastal current (Liu et al., 2007). The southeastward Jiangsu coastal current annually delivers 1.0×1011 kg particles into the ECS, which might be another important source of sediment to the ECS shelf (Hu and Yang, 2001). Three upwelling areas exist in the ECS: one cyclonic eddydriven offshore upwelling area to the southwest of Cheju Island, and two coastal upwelling areas off the estuary and in the Zhejiang coast driven by the Taiwan warm current and the coastal currents (Zhu et al., 2011). A total of 37 surface sediment samples were collected using a box sampler in the Changjiang Estuary and adjacent area during a cruise in May 2009 (Fig. 1). Sediment samples were kept in pre-cleaned polyethylene bags and frozen until lab analysis. Dried sediments were ground into fine particles or just disaggregated for homogenization (natural grain-sized) for different analytical purposes (Song, 2000). 1.2 Grain size, carbonate and total Fe analysis H2 O2 (10%) and HCl (1 mol/L) were successively added to wet sediment to remove organic matter and carbonate. The slurry was washed with Milli-Q water to neutral pH and then disaggregated by ultrasonication. The grain size of sediment was measured with a Laser Particle Size Analyzer (Cilas 940L). Carbonate was measured by shaking a mixture of natural grain-sized dried sediment and 10% acetic acid (HAc) at 100˚C for 30 min, filtering the slurry and then titrating the solution with EDTA to determine the calcium concentration (Moss, 1961). Ground sediment was digested with a mixture of HF-HNO3 -HClO4 at 150°C for 48 hr. Total Fe concentration in the solution was determined with a 5-sulfosalicylic acid spectrophotometry method (Karamanev et al., 2002). Based on duplicate analyses, the relative standard deviations (RSD) for carbonate and Fe measurement were less than 5% and 2% 34°N

33°N

Jia

ng s

JCC Cheju Island

u A0

32°N

Chan gjian g Riv

A1

A2

B1

er

1.1 Study site and sample collection The ECS continental shelf is the largest marginal sea in the western Pacific Ocean, with a vast continental area of 0.5×1012 m2 . The Changjiang River, the third largest river in the world in its length and runoff, is the major material source to the ECS. It annually delivers 8.9 × 1011 m3 of fresh water and 3.97 × 1011 kg sediment into the ECS. The current system in the ECS primarily consists of the Changjiang dilute water, the Kuroshio current flowing northward along the shelf break, and the coastal currents

31°N

Shanghai

A6

B4

C1

1 Materials and methods

A4

B6

C3

C6

CDWD1 D2 D3

D5

E2

E3

E4

F1

F2

F3

G1

G2

G3

G4

H1

H2

H3

H4

D6 E6

Hangzhou Bay 30°N Zhejiang H0

29°N

ZFCC 28°N 119°E

120°E

121°E

122°E

F5

TWC 123°E

F6 G6

H5

H6

KC

East China Sea 124°E

125°E

126°E

127°E

Fig. 1 Sampling sites of surface sediments (dots), regional circulation model (arrows), and the upwelling areas (shaded) in the Changjiang Estuary and its adjacent waters. CDW: Changjiang Dilute Water; JCC: Jiangsu Coastal Current; ZFCC: Zhejiang–Fujian Coastal current; TWC: Taiwan Warm Current; KC: Kuroshio Current.

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Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

respectively. 1.3 Biogenic element analysis Sedimentary TOC was measured with the modified Walkley-Black method as described by Mebius (1960). TN was measured by first digesting the natural grain-sized sediment with alkaline potassium persulfate (K2 S2 O8 ) at 124°C for 1 hr (Bronk et al., 2000) and then determining the NO3 − concentration in the solution using the zinc-cadmium reduction method (Wood et al., 1967). Sedimentary BSi was determined with the wet-alkaline digestion method (Mortlock and Froelich, 1989). Natural grain-sized sediments were extracted with 2 mol/L Na2 CO3 at 85°C for 5 hr. SiO4 4− concentrations in the leachates were 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 in each solution was determined using the molybdenum blue method (Murphy and Riley, 1962). Based on duplicate analyses, the RSD of determination were less than 5% for TOC, 7% for TN, 10% for BSi and 7% for TP. The 13 C contents in sediments at three stations (D1, G1 and A6) were determined using an isotope mass spectrometer (Delta V Advantage). Results for δ13 C were reported in notation relative to the VPDB standard, with the RSD less than 0.15%. 1.4 Sequential extraction of phosphorus A modified five-step sequential extraction procedure was applied to fractionate P in sediments (Table 1) (Jensen et al., 1998; Ruttenberg, 1992). Natural grain-sized sediments were sequentially extracted with 1 mol/L MgCl2 , 0.11 mol/L bicarbonate-dithionite, 0.1 mol/L NaOH, ac-

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etate buffer solution (pH 5) and 0.5 mol/L HCl. After each step, the mixtures were centrifuged for 15 min at 4000 r/min. The residues were washed with 0.5 mol/L NaCl solution once or twice to minimize adsorption of dissolved P, and the supernatants were added to the extracting solutions. The extracting solutions were measured for PO4 3− by the molybdenum-blue method (Murphy and Riley, 1962). The extracted inorganic P species were exchangeable P (Ex-P), Fe-bound P (Fe-P), Al-bound P (Al-P), carbonate fluorapatite (CFA-P) and Det-P, respectively. MgCl2 , NaOH and HAc solutions were also measured for total P after K2 S2 O8 digestion (Thien and Myers, 1992). Organic P was determined as the difference between total and inorganic P. The results showed that organic P only existed in the NaOH extract, which was defined as leachable organic P (Lea-OP).

2 Results and discussion 2.1 Terrestrial source of biogenic elements from the Changjiang River The Changjiang River is the major source of material to the ECS. River sediments deposit along the inner shelf of the ECS, resulting in the formation of an elongated subaqueous mud wedge extending from the river mouth southward off the Zhejiang and Fujian coast (Liu et al., 2007). Sediment grain size revealed the dispersal of terrestrial sediment in the ECS. Median grain size and mud content (clay and silt) displayed similar band-type distributions along the coast, decreasing rapidly from 6.5 Φ and 97% in the estuary to 1.7–4.2 Φ and 2%–54% at 123.5˚E, respectively (Fig. 2). This illustrated that fine terrestrial sediment was mainly deposited in the inner shelf west to 123.5˚E (Shen and Pan, 2001). In the middle shelf,

Table 1 Sequential extraction method for P species in sediments Step

Phosphorus species

Extractant

Phosphorus component extracted

I II III

Ex-P Fe-P Al-P Lea-OP CFA-P Det-P

1 mol/L MgCl2 (1 hr) 0.11 mol/L bicarbonate-dithionite (1 hr) 0.1 mol/L NaOH (18 hr)

Exchangeable or loosely adsorbed P P bound to reducible hydroxides (mainly Fe-bound P) P bound to non-reducible Fe and Al oxides leachable organic P CaCO3 -bound P+ authigenic carbonate fluorapatite Detrital apatite

IV V

Acetate buffer solution (pH 5, 3 hr) 0.5 mol/L HCl (1 hr)

33°N

33°N

33°N

Jiangsu

Jiangsu 32°N

31°N

Cha ngji ang Riv er

32°N

Shanghai

31°N

Hangzhou Bay

Shanghai

31°N

Hangzhou Bay

Median 122°E

123°E

124°E

125°E

126°E

Zhejiang

29°N 120°E

iver

Shanghai

30°N Zhejiang

Zhejiang

29°N

ngji ang R

Hangzhou Bay

30°N

121°E

Cha

iver

30°N

120°E

Jiangsu 32°N

Cha ngji ang R

29°N

Mud 121°E

122°E

123°E

124°E

125°E

Carbonate 126°E

120°E

121°E

122°E

123°E

124°E

125°E

126°E

Fig. 2 Distribution of median grain size (Φ), mud content (%) and carbonate content (%) of surface sediments in the Changjiang Estuary and its adjacent waters. PHI = –log2 d, where d is the grain size in mm.

Journal of Environmental Sciences 2013, 25(11) 2185–2195 / Yu Yu et al.

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Table 2 Ranges and averages of biogenic element concentrations in surface sediments from the Changjiang Estuary and its adjacent area

Range Average ± σ

Range Average ± σ

TOC (%)

TN (%)

BSi (%)

TP (µg/g)

Ex-P (µg/g)

0.09–0.60 0.32 ± 0.13

0.005–0.55 0.029 ± 0.012

0.27–1.93 0.77 ± 0.38

444.2–672.4 552.9 ± 85.9

6.8–20.7 13.6 ± 3.7

Fe-P (µg/g)

Al-P (µg/g)

Lea-OP (µg/g)

CFA-P (µg/g)

Det-P (µg/g)

31.9–103.9 58.4 ± 20.2

8.6–59.0 22.9 ± 13.3

B–11.6 3.9 ± 3.8

12.0–76.0 46.2 ± 16.4

216.0–426.0 316.4 ± 49.2

σ: standard deviation; B: below detection limit.

mud content was less than 20% in the northern middle shelf while higher than 40% in the southern area. This depositional pattern resulted from higher river sediment input into the southern area as opposed to relict sand in the northern middle shelf (Shen and Pan, 2001; Zhu et al., 2011). In the offshore upwelling area, though sediment grain size and mud content reached up to 7.7% and 100%, the low sedimentation rate (< 0.2 ∼ 0.5 cm/yr ) revealed that the supply of terrestrial sediment was limited and hydrodynamic sorting was the major cause of high median grain size and mud content (Lim et al., 2007). The ranges of biogenic element concentrations in surface sediments are presented in Table 2. In agreement with the dispersal of river sediment, elements of terrestrial source exhibited similar band-type distributions as finegrained sediment. TOC showed high concentrations off the estuary (0.38% on average), off the Zhejiang coast (0.42% on average) and in the offshore upwelling area (0.40% 33°N

Jia

on average) (Fig. 3). TOC also exhibited significant correlations with median grain size with R = 0.784 (p < 0.001) (Table 3). TOC contents in the SPM (suspended particulate matter) of the Changjiang River reach up to 0.71%–1.71%, far higher than TOC contents in sediments of the estuary (Duan et al., 2008). The huge amount of river sediment input likely contributed to high concentrations of TOC in sediments off the estuary and the Zhejiang coast. δ13 C in sediments at stations D1 and G1 varied between –24.22‰ ∼ –22.54‰ and –23.44‰ ∼ –22.05‰, respectively. Using a two end member model of δ13 C, we estimated that terrestrial organic matter accounted for 42%–70% and 34%–58% of total organic matter at stations D1 and G1, respectively (Yu et al., 2012a). Total phosphorus concentrations decreased rapidly from land toward sea with contours nearly parallel with the coastline (Fig. 3). In addition, significant correlations between TP and sediment median grain size were observed 33°N

ng su

Jia ng su P

Pr o.

the C ha

the C hang ji

ngjia ng R .

31°N

31°N

Shanghai

ang R . Shanghai

Hangzhou Bay

Hangzhou Bay 30°N

30°N Zhejiang Pro.

Zhejiang Pro.

29°N

29°N

TOC

120°E 33°N

ro.

32°N

32°N

121°E

122°E

123°E

124°E

125°E

126°E

TN

120°E 33°N

Jia ng su Pr o.

121°E

122°E

123°E

124°E

125°E

123°E

124°E

125°E

126°E

Jia ng su Pr o.

32°N

32°N

t he C hang 31°N

the C ha

jiang R. 31°N

Shanghai

ngjia ng R . Shanghai

Hangzhou Bay

Hangzhou Bay 30°N

30°N Zhejiang Pro.

Zhejiang Pro.

29°N 120°E

29°N

BSi 121°E

122°E

123°E

124°E

125°E

126°E

120°E

TP 121°E

122°E

126°E

Fig. 3 Distribution of TOC, TN, BSi and TP concentrations in surface sediments from the Changjiang Estuary and its adjacent area (Unit: % for TOC, TN and BSi, and µg/g for TP).

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Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

(Table 3). TP in sediment of the Changjiang Estuary and adjacent waters was mainly of terrestrial origin. It was reported that TP in the SPM of the Changjiang River reaches 682 µg/g on average (Yan and Zhang, 2003), which is higher than the maximum concentration of sedimentary TP (672 µg/g) in the Changjiang estuary and adjacent waters. River input of particulate P resulted in the seawarddecreasing distribution of TP. Fe-P and Al-P were the first and third largest nonapatite P pools in sediments, accounting for 6.2%–16.0 % and 1.7%–8.9 % of TP, respectively. Similar to TP and sediment grain size, Fe-P and Al-P exhibited band-type and seaward-decreasing distributions in the Changjiang estuary and adjacent waters (Fig. 4). In addition, Fe-P and Al-P showed significant linear correlations with TP 33°N

Jia

(RFeP-TP = 0.806, RAlP-TP = 0.716, p < 0.001, n = 27). Thus, Fe-P and Al-P were also mainly from terrestrial input. This is reasonable because Fe and Al are wellacknowledged lithogenic elements. Fe-P and Al-P also correlated significantly with total Fe in sediments, with R = 0.760 and 0.635 (p < 0.005, n = 25), respectively. High concentrations of Fe-P and Al-P were observed in the inner shelf, while Fe-P and Al-P concentrations were much lower in a vast area east to 123.5˚E, including the offshore upwelling area. The distributions of Fe-P and AlP well reflected the dispersal of river sediment. Lea-OP extracted by NaOH is mainly fresh organic P bound to humic acid and fulvic acid, and probably includes algae and live bacteria (Jensen and Thamdrup, 1993). Sedimentary Lea-OP in the Changjiang Estuary 33°N

Jia

ng su Pr o.

ng

su

Pr o.

32°N

32°N

the C ha n g jiang 31°N

the C hang

R. 31°N

Shanghai

jiang

R.

Shanghai

Hangzhou Bay

Hangzhou Bay 30°N

30°N Zhejiang Pro.

Zhejiang Pro. 29°N

29°N

Ex-P 120°E

33°N

121°E

Jia

122°E

123°E

124°E

125°E

126°E

Fe-P 120°E

33°N

ng s

121°E

Jia ng

uP ro.

122°E

123°E

124°E

125°E

126°E

su Pr o.

32°N

32°N

the C hang jiang R. 31°N

the C ha n g jiang 31°N

Shanghai

R.

Shanghai

Hangzhou Bay

Hangzhou Bay 30°N

30°N Zhejiang Pro.

Zhejiang Pro.

29°N

Al-P

120°E

121°E

33°N

Jia ng

122°E

123°E

124°E

125°E

126°E

29°N

Lea-OP

120°E

33°N

su

Pr

121°E

Ji a ng su P

o.

122°E

123°E

124°E

125°E

126°E

ro.

32°N

32°N

the C hang jiang

the C hang jiang R. 31°N

31°N

Shanghai

R.

Shanghai Hangzhou Bay

Hangzhou Bay 30°N

30°N Zhejiang Pro.

Zhejiang Pro. 29°N

29°N

CFA-P 120°E

121°E

122°E

123°E

124°E

125°E

126°E

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Det-P 120°E

121°E

122°E

123°E

124°E

125°E

Fig. 4 Distribution of P fractions in surface sediments of the Changjiang Estuary and its adjacent area (µg/g).

126°E

Journal of Environmental Sciences 2013, 25(11) 2185–2195 / Yu Yu et al.

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Table 3 Correlation coefficients (R) of TOC, TN, BSi, TP and P fractions with median grain size, clay, silt and sand contents of sediments

Median Clay Silt Sand

Median Clay Silt Sand

TOC

TN

BSi

TP

Ex-P

0.784** 0.734** 0.602** –0.751**

0.684** 0.670** 0.498** –0.642**

–0.231 0.071 0.329 0.248

0.546** 0.521** 0.627** –0.654**

0.762** 0.829** 0.607** –0.704**

Fe-P

Al-P

Lea-OP

CFA-P

Det-P

0.659** 0.537** 0.539** –0.593**

0.538** 0.537** 0.539** -0.593**

0.780** 0.801** 0.647** –0.765**

0.597** 0.710** 0.424* –0.564**

0.263 –0.466* 0.043 0.126

** p < 0.01, *p < 0.05.

and adjacent waters exhibited similar distributions to TOC, which exhibited higher concentrations in the estuary, off the Zhejiang coast and the offshore upwelling area (Fig. 4). In addition, Lea-OP correlated significantly to TOC with R = 0.598 (p < 0.005, n = 25). These results suggested that the sources and behavior of Lea-OP were similar to those of TOC, and Lea-OP was also influenced by Changjiang input. 2.2 Terrestrial source of biogenic elements-the old Huanghe delta Sediment from the abandoned Huanghe submarine delta transported by the JCC is an important source of sediment to the northern ECS (Hu and Yang, 2001). The influence of the JCC sediment was illustrated by the distribution of sediment grain size. North of the estuary, sediment grain size and mud contents decreased southeastward from 5.5 Φ and 98% off the Jiangsu coast to 1.7 Φ and 2% in the northern middle shelf, respectively. Fine sediment off the Jiangsu coast was modern terrestrial sediment mostly from the old Huanghe delta. In agreement with the distribution of fine sediment, TP concentrations displayed high values in the northern study area and gradually decreased southeastward (Fig. 3). This profile of TP resulted from a similar distribution of DetP, the major P species accounting for 42.6%–75.4% of TP. The Det-P concentration reached the maximum in the northern study area and also exhibited a southeastwarddecreasing trend (Fig. 4). This high concentration of Det-P in sediments to the north of the estuary was also reported by Zhu (2009). High contents of TP and Det-P in the northern ECS were likely related to the delivery of old Huanghe sediment. Det-P is mainly of terrestrial origin, coming from the weathering of rocks in the drainage basin (Ruttenberg, 1992). It was reported that apatite is the most common heavy mineral in the Huanghe sediment (Sun, 1990). The Huanghe sediment is characterized by high contents of apatite and Ca (0.18% and 4.01%, respectively) compared with the Changjiang sediment (0.15% and 3.18%, respectively) (Yang et al., 2004). Therefore, the delivery of old Huanghe delta sediment rich in apatite

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probably contributed to high concentrations of Det-P in the northern ECS. Det-P concentrations also showed higher values in the relict area of the northern middle shelf than in the coastal and offshore waters (Fig. 4). This was likely related to the relict sand deposited in the late Pleistocene from the Huanghe River and the coarse grain size of sediment (Hu and Yang, 2001). CFA-P and Fe-P concentrations also exhibited southeastward-decreasing trends north of the estuary (Fig. 4). P species in the HAc extract are mainly CaCO3 bound P and authigenic carbonate apatite (CFA-P) (Ruttenberg, 1992). Carbonate contents in sediments off the Jiangsu coast were higher than its adjacent waters, and gradually decreased southeastward (Fig. 2). This distribution revealed the impact of Huanghe delta sediment rich in carbonate on carbonate content in the northern ECS. Therefore, sedimentary CaCO3 -bound P and CFA-P in the northern ECS were also impacted by sediment input from the JCC. The distribution of Fe-P also illustrated the JCC source of Fe-P. 2.3 Marine source of biogenic elements Marine biological input is an important source of organic matter in sediments, especially in waters where less terrestrial material is supplied. The northwest ECS shelf has high primary productivity due to the input of large amounts of dissolved nutrients by the Changjiang River (Gong et al., 2003; Hama et al., 1997). High productivity promotes the deposition of marine organic matter to sediment. δ13 C in sediments at stations D1 and G1 showed that 30%–58 % and 42%–66% of total organic matter were of phytoplankton source off the estuary and the Zhejiang coast, respectively. The proportion of marine organic matter was even larger (66%–79%) in the offshore upwelling area based on δ13 C at station A6 (Yu et al., 2012a). Although phytoplankton productivity varied between seasons, Zhejiang coastal waters and the offshore upwelling area are two productive areas in the ECS shelf related to the upwelling currents (Gong et al., 2003; Yu et al., 2012b). Correspondingly, high concentrations of TOC, TN and Lea-OP were observed in these two areas (Figs. 3 and 4). These organic forms of C, N and P were greatly impacted by the biological input in these high productivity areas. In contrast with TOC and Lea-OP, whose concentrations were also high in the estuary, TN exhibited relatively low concentration in the estuary (0.026% on average) (Fig. 3). This suggested that Changjiang sediment had relatively low content of nitrogen. Although the Changjiang River is rich in dissolved nitrogen, the concentration of particulate N is relatively low. The influx of particulate N only accounts for 8.7% of total N flux from the Changjiang River (Zhang et al., 2003). Therefore, nitrogen in sediment of the Changjiang Estuary and adjacent waters was mainly of marine origin. This is consistent with previous studies

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Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

showing that nitrogen in particles and sediments in the ECS and Yellow Sea are primarily from marine biological input (Song et al., 2006; Yu et al., 2012b). Sedimentary BSi is amorphous silicon co-deposited with the detritus of siliceous organisms, and thus is closely related to the production of diatoms in water (Bern´ardez et al., 2005; Schelske et al., 2006). BSi concentrations in the sediments of the Changjiang Estuary and adjacent waters varied greatly, with a coefficient of variation of 50%. The most striking feature of the distribution of BSi was its patchy distribution and the presence of a high concentration band located between 123 and 124˚E (Fig. 3). From this band, BSi concentration decreased toward land and offshore, and was lower than 0.8% along the coast and in the offshore upwelling area. The distribution of BSi was related to the diatom production in water. It was reported that diatom density reaches maximum values in localized regions, but generally in the fresh water plume at salinity between 25–30 in summer, as a combined result of nutrient supply and the turbidity of water (Furuya et al., 1996; Ning et al., 1988). The location of the river plume and maximum diatom production was consistent with that of the highconcentration band of BSi. As for the high content of BSi around 125˚E/31˚N, it was probably attributable to the upwelling of Kuroshio subsurface water, which enhanced diatom density (Chiang et al., 1999, 2004). The coastal water has low diatom density due to the light limitation in turbid water (Ning et al., 1988; Yang et al., 2010), thus leading to low BSi concentration in sediment. In addition, terrestrial input of particulate BSi from the Changjiang River is much less significant than its biological input. The flux of particulate BSi from the Changjiang River is only 3.5% of the dissolved silicate flux (Shen et al., 2008). The maximum concentration of CFA-P appeared in the southern study area, with patchy distribution (Fig. 4). The location of maximum CFA-P was highly consistent with that of carbonate (Fig. 2). The carbonate concentration reached the maximum (20%) at 29˚N/124˚E and gradually decreased northward. This high carbonate content in the southern middle shelf is likely caused by high marine biological input under the influence of the Taiwan Warm Current-a branch of Kuroshio (Lin et al., 2002). Significant correlations between CFA-P and carbonate were observed (R = 0.448, p < 0.05, n = 25). The consistency between the distributions of CFA-P and carbonate suggested that CFAP in the southern study area was impacted by authigenic input from calcium-bearing organisms. It should be noted that C, N, and P in surface sediments might be disturbed by anthropogenic input (Schelske et al., 1988). Excess input of N, P and organic matter induced by human activities could promote the growth of phytoplankton and increase the amount of these elements adsorbed onto sediment, and accordingly elevate the contents of TOC, TN and TP in sediment. Sediment records off the Zhejiang coast showed that grain size-normalized TOC,

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TN, Ex-P, Fe-P and Lea-OP gradually increased upward in the upper 16 cm of the core, with average increases of 24%, 18%, 15%, 13% and 51%, respectively (Yu et al., 2012a). Correspondingly, δ13 Corg in sediments increased, suggesting elevated primary productivity in water and marine organic matter deposition. Sediment records revealed that C, N and P in coastal sediments had been influenced by human activities in the past three decades. However, the anthropogenic disturbance was likely less significant compared with the baseline biological input of these elements based on the following two observations. One was that the increases of grain size-normalized element values were relatively small; the other was that the disturbance was only confined to coastal waters according to core-sediment records and surface distributions of biogenic elements (Yu et al., 2012a). 2.4 Grain size effect Sediment grain size is also a major factor determining the contents and distributions of biogenic elements. The distributions of TOC, TN, TP and non-apatite P fractions (Ex-P, Fe-P, Al-P, Lea-OP and CFA-P) resembled those of median grain size and mud content. Linear regression analysis also showed that the concentrations of TOC, TN, TP, Ex-P, Fe-P, Al-P, Lea-OP and CFA-P were correlated positively with clay and silt content but negatively with sand content (Table 3). This indicated that TOC, TN and non-apatite P were strongly enriched in fine grain-sized sediments. It is probably because fine particles have larger specific surface areas to adsorb organic matter, Fe/Mn hydroxide, clay minerals, etc, which also have strong affinity for iron species including phosphate and ammonium (Mayer, 1994; Jensen and Thamdrup, 1993). It was reported that most opal, principally diatoms, is retained in mud sediment (silt and clay) (Bern´ardez et al., 2005). BSi content in sediment increases with the sediment grain size growing finer (Emelyanov, 2001). However, in this study, high concentrations of BSi did not occur in the coastal muddy area, and there was no linear correlation between BSi and sediment grain size (Table 3). The reason why BSi was not associated with fine sediment might be the different origins between BSi and fine sediment. Fine sediment in the inner shelf was mainly from the Changjiang River. The inner shelf sediments had low BSi concentrations due to low diatom production in the turbid water and the dilution effect of river sediment (Ning et al., 1988). In contrast, high BSi concentrations occurred in the middle shelf owing to higher diatom production. Therefore, fine sediment and BSi in the Changjiang Estuary and adjacent waters had distinct sources and deposited in different regions. This was likely the major cause of the de-coupling between BSi and fine sediment. As for TOC, Fe-P, Al-P, Lea-OP and CFA-P, large proportions of these species have a common source with fine sediment – the Changjiang River. In addition, influ-

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enced by marine biological input, higher concentrations of TOC, Lea-OP and TN occurred in the Zhejiang coast and offshore upwelling area, where the sediments are also fine under the rework of upwelling currents. These conditions led to the uniform distributions of TOC, TN, non-apatite P fractions and fine sediment. Our previous study revealed that Det-P is mainly enriched in coarse sediment in the Changjiang estuary (Yu et al., 2011). In the ECS shelf, the distribution of Det-P basically obeyed the rule of coarse-sediment enrichment. Det-P exhibited low concentration in the estuary, off the Zhejiang coast and in the offshore upwelling area where sediment mud content was higher than 60%. In contrast, Det-P concentration showed a tongue-shaped high value distribution from the northern middle shelf towards the area south of the estuary, where mud contents ranged from 2%–40% (Fig. 4). Due to limited terrestrial sediment input, the Det-P concentration was low in the relict sand area east to 123.5˚E. However, the linear correlation between Det-P and sediment grain size was weak (Table 3). This likely resulted from the low Det-P concentration in the relict sand area as well as the influence of sediment input from the JCC. Probably impacted by the Huanghe sediment rich in apatite, the whole northern study area had a high concentration of Det-P, even in the inshore area where mud content was higher than 60%. Therefore, as in the case of BSi, material source is an important factor disturbing the grain size effect on Det-P. 2.5 Environmental significance of biogenic elements 2.5.1 Potential application of biogenic elements to indicate past environmental conditions Affected by human-induced excess N and P input, lakes, estuaries and coastal waters have experienced eutrophication, elevated productivity and seasonal hypoxia in bottom water in recent decades (Foley et al., 2012; Zhou et al., 2008). These environmental changes in water might leave footprints in sediment. Biogenic elements in sediment are important indicators of environmental changes in water (Meyers, 2003; Schelske et al., 1988, 2006). Sources and governing factors have determined the potential use of biogenic elements as proxies in paleoenvironmental studies. Because of river and authigenic sources of TOC and Lea-OP in the Changjiang Estuary and its adjacent waters, variations in sediment TOC and Lea-OP concentrations could reflect both varied terrestrial input of organic matter and primary productivity in water. In many lakes where the terrestrial supply of organic matter is insignificant or remains stable, sedimentary TOC records have been used to indicate the changes of primary productivity in water (Meyers, 2003; Schelske et al., 1988). However, in waters with significant terrestrial input, discriminating between terrestrial and authigenic organic matter is necessary. For this purpose, stable isotopes of carbon (13 C) and nitrogen (15 N) and OC/ON ratio are

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commonly used (Kao et al., 2003; Schelske and Hodell, 1995). If the sediment records were preserved well, TN in sediment which is mainly of marine biogenic source could be used as the proxy of marine productivity. BSi might be a reliable indicator of diatom production in view of the lower number of influencing factors and relative stability of opal during burial (Nelson et al., 1995). The applications of P species to indicate past environmental conditions are rarely studied compared with TOC, TN, BSi and TP proxies. Fe-P and Al-P were mainly of terrestrial origin, and their distributions were consistent with those of the fine terrestrial sediment. Therefore, sedimentary Fe-P and Al-P might be good indicators of terrestrial P and sediment input, which had been verified by our study (Yu et al., 2012a). The record of Det-P except in relict areas might be also used to indicate the changes in terrestrial input (L¨u et al., 2008; Yu et al., 2012a). However, its negative relation with sediment median grain size should be kept in mind. CFA-P could reflect the conditions of terrestrial input and authigenic input from calcium organisms. Previous study found the relations between CFA-P content in sediment and the productivity and biomass in lake water (L¨u et al., 2008). Although TP and non-apatite inorganic P have been used to indicate increased P input and eutrophication in water (Hodell and Schelske, 1998; Schelske and Hodell, 1995), it is obvious that P species in sediment provide more environmental information than TP does, and thus are more promising paleo-environment proxies. 2.5.2 Ecological risk of P release from sediment P and N in sediment might be released into overlying water and pose the threat of eutrophication to the ecological environment of water. The inshore water of the Changjiang Estuary and its adjacent area is limited by P due to excess dissolved N input from the Changjiang River (Wang et al., 2003). Thus internal supply of P from sediment has larger ecological risk than the release of N. A great deal of studies revealed that Ex-P, Fe-P, Al-P, Lea-OP and CFA-P could be released from sediment and recycled (Christophoridis and Fytianos, 2006; Jensen et al., 1995), the sum of which was defined as transferable P. Transferable P in sediments of the Changjiang Estuary and adjacent area ranged from 68–250 µg/g, accounting for 14%–46% of TP. Fe-P is the largest contributor, accounting for 32%– 51% of transferable P. The contributions of other P species were in the order of CFA-P > Al-P > Ex-P > Lea-OP. The distributions of transferable P resembled that of mud content (Fig. 5). Sediments in the estuary, the Zhejiang coast and the offshore upwelling area retained higher content of transferable P. Additionally, hydrological conditions in these areas are dynamic, resulting from the mixture of fresh and saline water and/or upwelling currents. Dynamic conditions could strengthen the resuspension of sediment and increase the exchange of P between particles and water (Zhang et al., 2004). In addition, due to the halocline

No. 11

33°N

Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas J ia ng

su

32°N Chan gjian g

31°N

Shanghai Hangzhou Bay

30°N Zhejiang

29°N Transferable P 120°E 121°E 122°E 123°E 124°E 125°E 126°E Fig. 5 Distribution of transferable P in surface sediments of the Changjiang Estuary and its adjacent waters (µg/g).

and high sedimentation flux of organic matter, bottom hypoxia (O2 6 2 mg/L) appears during summer at 30– 32˚N/122.5–123.5˚E off the Changjiang estuary (Li et al., 2002). Low oxygen concentration in the sediment-water interface facilitates the reduction of Fe/Mn oxides and the release of Fe-P from sediment (Jensen and Thamdrup, 1993; Rozan et al., 2002). However, when the surface sediment is oxidized and the molar ratio of total iron to total phosphorus (TFe:TP) in sediment is above 15, there are sufficient sorption sites for P on iron hydroxides in sediment to control P release (Jensen et al., 1992). The ratios of TFe:TP in surface sediments of the estuary and adjacent waters ranged from 29 to 54, and molar ratios of iron to P in bicarbonate-dithionite leachate (BD-Fe:BD-P) were between 12 and 45. High ratios of iron to P revealed that sediments in the Changjiang Estuary and adjacent waters had higher ability and potential to retain P in the aerobic environment.

3 Conclusions The source and sediment grain size are the major factors determining the distribution of biogenic elements in sediments of the Changjiang Estuary and adjacent waters. The major sources of biogenic elements included the Changjiang River, the JCC and marine biological input. TOC and Lea-OP showed similar distributions, and they originated from both the river and marine biological input. TN and BSi were mainly of marine biogenic origin, relating to primary productivity and diatom biomass, respectively. Fe-P, Al-P and Det-P primarily originated from terrestrial input including the Changjiang and the JCC. CFA-P was impacted by the JCC and calcium organism input. TOC, TN, TP and non-apatite P fractions were closely associated with fine sediment. Det-P was generally enriched in coarse sediments, excepting its high concentrations in the whole northern study area. BSi had no correlation with sediment grain size, likely attributed to the disturbance of river sediment. This study revealed that

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whether elements and sediment have the same origins and the disturbance of terrestrial sediment input also influence the grain-size effect on biogenic elements. Sources and governing factors determined the potential uses of biogenic elements as environmental indicators. TOC and Lea-OP might indicate both terrestrial organic matter input and marine productivity. Fe-P, Al-P and DetP probably are reliable proxies of terrestrial input. TN, BSi and CFA-P could reflect the marine productivity, diatom production and calcium organism biomass, respectively. P species in sediments are promising proxies of environmental changes in water. Sediments in the estuary, Zhejiang coast and the offshore upwelling area retained higher contents of transferable P, and the dynamic hydrological conditions and seasonal bottom hypoxia off the estuary favored the release of P from sediments in these areas. However, in an aerobic environment, sediments in the Changjiang Estuary and adjacent waters had low risks of P release. Acknowledgments This work was supported by the Natural Science Foundation of China for Creative Research Groups (No. 41121064), the National Basic Research Program (973) of China (No. 2011CB403602, 2010CB951802), and the National Natural Science Foundation of China (No. 41306070).

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