Marine Chemistry 157 (2013) 93–105
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Distribution, partitioning and mixing behavior of phosphorus species in the Jiulong River estuary Peng Lin a,b, Laodong Guo b,⁎, Min Chen a, Yihua Cai a a b
College of Ocean and Earth Sciences, State Key Lab of Marine Environmental Science, Xiamen University, Xiamen 361005, China School of Freshwater Sciences, University of Wisconsin—Milwaukee, 600 East Greenfield Avenue, Milwaukee WI 53204, USA
a r t i c l e
i n f o
Article history: Received 8 February 2013 Received in revised form 26 August 2013 Accepted 5 September 2013 Available online 13 September 2013 Keywords: Phosphorus Nutrients Estuarine mixing Chemical speciation Jiulong River estuary
a b s t r a c t Surface and bottom water samples were collected along a salinity gradient from the Jiulong River estuary during May 2010 to examine the distribution and mixing behavior of dissolved and particulate phosphorus (P) species. Particulate P was further fractionated into five operationally defined phases through a sequential extraction method. In addition to field studies, laboratory experiments were carried out to examine key processes regulating the distribution and partitioning of P during estuarine mixing. Dissolved inorganic P (DIP) was the main species in the total dissolved P (TDP) pool, comprising up to 83 ± 3% at river end-member station and 32 ± 21% at coastal stations. While the concentration of dissolved organic P (DOP) changed little with salinity, DIP concentrations varied dramatically between stations, especially in the low-salinity (S b 3) and high-salinity regions, indicating intensive removal/addition and transformation between P species in the water column. The predominance of DIP and low abundance of DOP reflected a profound influence of anthropogenic inputs from the Jiulong River. The total particulate P (TPP) contributed up to 70 ± 10% of the total phosphorus (TP = TDP + TPP) in the lowsalinity area, showing a quasi-negative correlation with DIP during early estuarine mixing (S b 3), but the TPP decreased sharply with salinity, comprising 39 ± 19% of the TP pool in middle and higher salinity regions. River inputs, biological production (mostly for biogenic apatite and organic P), and repartitioning of P between dissolved and particulate (labile P) phases were major factors responsible for the dynamic variations in P species in the estuary. Deviation in results of laboratory mixing experiments from those of the field investigation indicated that, in addition to physicochemical and biological processes, additional end-member waters and sediment resuspension also play a role in controlling the mixing behavior and biogeochemical cycling of P in the Jiulong River estuary. Values of the distribution coefficient of P (in terms of logKd) were consistently high, but they were similar among stations with different salinities and had a poor correlation with suspended particulate matter concentration, likely due to the relatively long flushing time, elevated DIP, and lower colloidal effect in the Jiulong River estuary. Compared with results from early studies, elevated DIP from anthropogenic sources seemed to have altered the mixing behavior of P species in the estuarine environment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Phosphorus (P), one of the most important nutrient elements needed by all living organisms, can control primary productivity in estuarine and marine environments (Benitez-Nelson, 2000; Karl and Björkman, 2001; Paytan and McLaughlin, 2007). Knowledge about the dynamics of P species in estuarine systems is needed to better understand the biogeochemical cycling of P and its role in regulating water and environmental quality in coastal marine environments (Rabalais et al., 2002; Dagg et al., 2007; Boesch et al., 2009). Particulate and organic P can be the dominant species exported from rivers, especially in more pristine and undammed rivers (Howarth et al., 1995; Guo et al., 2004; Cai et al., 2008; Lin et al., 2012a). Therefore, the transformation and mixing behavior of particulate P may play an important role in ⁎ Corresponding author. Tel.: +1 414 382 1742. E-mail address:
[email protected] (L. Guo). 0304-4203/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.marchem.2013.09.002
controlling the distribution, speciation, and biological availability of dissolved P species in estuarine systems (Mayer et al., 1998; Cai et al., 2012; Lin et al., 2012a). However, nutrient studies considering all P species, including dissolved, particulate, inorganic and organic forms, are still few. Phosphorus in aquatic environments can be actively partitioned between particulate and dissolved phases and could exist in organic and inorganic forms. Therefore, in addition to the abundance of P, knowledge of its chemical speciation is needed. Recent evidence has shown possible utilization of dissolved organic phosphorus (DOP) by organisms and exchange of DOP with particles in aquatic environments (Mortazavi et al., 2000; Kolowith et al., 2001; Sylvan et al., 2006; Huang and Zhang, 2010). Furthermore, P in different particulate species may play different roles in the biogeochemical cycling of P and other trace elements. Thus, in addition to dissolved inorganic phosphorus (DIP), the fate and transport of DOP, as well as the transformation of these forms with various particulate P species in estuarine environments
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need to be better understood. It is also important to understand the linkage between increasing nutrient inputs and biogeochemical behavior of P in estuarine environments (Conley et al., 1995; van der Zee et al., 2007; Shen et al., 2008). Many estuaries are suffering from eutrophication and metal pollution due to increasing human impacts in Chinese coastal regions (Duan et al., 2008; Pan and Wang, 2012; Yan et al., 2012). The Jiulong River estuary is a shallow estuarine system on the coast of Fujian Province, China. Previous studies have demonstrated both conservative and non-conservative behaviors for different nutrients including P in the Jiulong River estuary (e.g., Zhang et al., 1999; Yan et al., 2012). However, the mixing behavior of DOP and various particulate P species, as well as their relationship with DIP, remain poorly understood. In addition, more baseline studies are needed to examine changes in the biogeochemical cycling of P species due to increasing population and human activities in the Jiulong River basin and its surrounding areas. Our goals were to examine the variation and partitioning of P between particulate and dissolved phases during estuarine mixing in the Jiulong River estuary, an anthropogenically influenced estuary. Both field study and laboratory mixing experiments were carried out to evaluate the mixing behavior of P species and the relative importance of different processes in the biogeochemical cycling of P in the estuarine environment. 2. Materials and methods 2.1. Study site The Jiulong River estuary, located on the southeast coast of Fujian, China, is a shallow estuary connecting to Xiamen Bay and the Taiwan Strait (Fig. 1). It receives freshwater from the Jiulong River, the second largest river in Fujian Province, with a total drainage basin area of 14,700 km2, and annual freshwater discharges of 8.2 × 109 m3 and 3.9 × 109 m3 from the North and West branches, respectively (Huang, 2008). After decades of rapid economic growth, the Jiulong River has been heavily influenced by human activities, resulting in high nutrient concentrations and eutrophication which have affected environmental quality in estuarine and coastal waters (Cao et al., 2005; Pan and Wang, 2012; Yan et al., 2012). However, it is not clear what changes have occurred in nutrient speciation and biogeochemical behavior, and what their relation is to the health of the Jiulong River estuary.
2.2. Sampling Water samples were collected along a salinity gradient in the Jiulong River estuary during May 2010, from the river mouth to the coastal region near the Xiamen Island (Fig. 1). Sampling locations, water salinity and other hydrographic data are listed in Table 1. The salinity ranged from 0.02 at station S1 to 26.7 at station S9 in the north of the Xiamen Island (Fig. 1). Both surface and bottom water samples were collected using precleaned HDPE bottles and stored in a cooler with ice. Samples were filtered in the laboratory through a 0.45 μm Nuclepore filter (Whatman, 47 mm) within 4 h of sampling. Both filtrate and filter samples were kept frozen for the measurements of total dissolved phosphorus (TDP), dissolved inorganic phosphorus (DIP), total particulate phosphorus (TPP), and particulate inorganic phosphorus (PIP). In addition, 0.4–1.0 L of water samples were filtered through a 0.45 μm Nuclepore filter to collect suspended particles for the determination of chemical speciation of particulate P. Aliquots of samples were also filtered through pre-combusted GF/F filters (Whatman, 47 mm) for the measurements of particulate organic carbon (POC) and particulate nitrogen (PN). 2.3. Laboratory mixing experiments Two end-member mixing experiments were carried out in the laboratory using unfiltered river water and seawater to simulate estuarine mixing processes and to examine the mixing behavior of P species in the Jiulong River estuary. The river water end-member was collected from station S1 in the Jiulong River and the seawater end-member was collected from station S9. In the laboratory, the two end-member waters were mixed in different proportions to result in different salinities ranging from 0.1 to 26.7 and a final volume of 500 mL. The samples were well mixed and then statically stored in the dark at 4 °C for 2 h. Finally, the mixed samples were filtered through 0.45 μm Nuclepore filters (Whatman, 47 mm) for the measurement of different P species. 2.4. Measurements of dissolved and particulate phosphorus species Measured P species included DIP, dissolved organic phosphorus (DOP), PIP and particulate organic phosphorus (POP). Concentrations of TDP (=DIP + DOP) were measured using an oven-assisted acid persulfate method (Koroleff, 1983; Cai and Guo, 2009) with some
Fig. 1. Sampling locations in the Jiulong River estuary during May 2010.
P. Lin et al. / Marine Chemistry 157 (2013) 93–105
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Table 1 Sampling locations and the concentrations of dissolved P species in surface and bottom waters from the Jiulong River estuary during May 2010. Errors of DIP and DOP are ≤2%. Sample ID
Latitude (°N)
Longitude (°E)
Depth (m)
Salinity
DIP (μM)
DIP/TDP (%)
DOP (μM)
DOP/TDP (%)
Surface waters S1 S2 S3 S4 S5 S6 S7 S8 S9
24°27′44.4″ 24°25′55.5″ 24°24′46.4″ 24°22′54.6″ 24°23′48.5″ 24°24′10.1″ 24°25′12.4″ 24°24′9.8″ 24°27′50.8″
117°48′8.1″ 117°51′31.1″ 117°52′45.7″ 117°54′10.3″ 117°55′0.8″ 117°57′42.8″ 118°01′3.6″ 118°06′19.5″ 118°03′10.2″
9.1 9.4 6.9 6.3 8.5 11.9 12.0 15.3 14.7
0.02 1.0 2.4 7.4 11.7 17.7 19.9 22.8 26.7
2.14 1.26 1.59 1.49 1.26 1.02 0.69 0.52 0.82
80 68 74 73 69 63 53 46 64
0.55 0.60 0.57 0.55 0.56 0.60 0.62 0.61 0.47
20 32 26 27 31 37 47 54 36
Bottom waters S1 S2 S3 S4 S5 S6 S7 S8 S9
24°27′44.4″ 24°25′55.5″ 24°24′46.4″ 24°22′54.6″ 24°23′48.5″ 24°24′10.1″ 24°25′12.4″ 24°24′9.8″ 24°27′50.8″
117°48′8.1″ 117°51′31.1″ 117°52′45.7″ 117°54′10.3″ 117°55′0.8″ 117°57′42.8″ 118°01′3.6″ 118°06′19.5″ 118°03′10.2″
9.1 9.4 6.9 6.3 8.5 11.9 12.0 15.3 14.7
0.1 1.0 4.3 7.6 15.2 19.3 23.3 25.5 25.2
2.12 1.19 1.62 1.49 1.05 0.92 0.40 0.12 0.65
86 71 82 75 70 67 46 17 58
0.34 0.49 0.37 0.51 0.46 0.45 0.46 0.58 0.48
14 29 18 25 30 33 54 83 42
modifications (Lin et al., 2012a,b). In short, 10 mL of the water sample was first mixed well with 1 mL of acidified K2S2O8 solution (50 g/L, pH = 1) in a Teflon vial, and the mixed solution was digested in an oven at 140 °C for 4 h. After digestion, TDP concentrations were measured by the standard phosphomolybdenum blue method using a Cary 300 UV–visible spectrophotometer and 5 cm quartz cuvettes (Parsons et al., 1984). DIP concentrations were directly measured without digestion. Concentrations of DOP were calculated from the difference between TDP and DIP. Standard solutions were treated as samples during sample processing and analysis to ensure data quality. The detection limit was 8–10 nM based on replicate blank sample measurements, with a precision better than 2% for both DIP and TDP (Cai and Guo, 2009; Lin et al., 2012a). Concentrations of total particulate P (TPP = PIP + POP) were determined after high temperature combustion and acid hydrolysis of filter samples (Solórzano and Sharp, 1980). Briefly, the filter samples were first wetted with 0.5 M MgCl2 solution and heated in an oven at 95 °C until dried, followed by ashing in a furnace at 550 °C for 2 h to decompose organic P compounds. The residue was extracted using 1 M HCl solution in the dark at room temperature for 24 h. PIP was directly extracted from filter samples with 1 M HCl solution at room temperature in the dark for 24 h (Aspila et al., 1976). After neutralization and dilution, TPP and PIP were quantified as DIP through the standard phosphomolybdenum blue method, and the concentrations of POP were calculated from the difference between TPP and PIP (Lin et al., 2012a,b).
blue method with a modified chromogenic agent to avoid interference from the extraction solution. The extract solutions from the Labile-P and CFA-P phases were measured for both DIP and TDP to quantify the abundance of organic P in these two particulate P phases. The detection limit was 0.2 nM for particulate P measurements with a high concentration factor (25–55, the ratio of the water sample volume to the volume of extraction solution) in a 5 cm quartz cuvette. 2.6. Measurements of particulate organic matter (POM) and SPM GF/F filter samples were treated with HCl acid fumes overnight and measured for POC and PN concentrations on an elemental analyzer (Chen et al., 2006; Guo and Macdonald, 2006). Concentrations of POC and PN were used along with POP to determine particulate organic C/N and C/P ratios, respectively. The weight difference between filter samples and blank filters was used to calculate suspended particulate matter (SPM) concentrations. In brief, aliquots of whole water samples were filtered through a preweighed 0.45 μm polycarbonate membrane (Whatman) and rinsed with nanopure water 3–4 times to remove salts. Filter samples were weighed after drying at 60 °C until achieving a constant weight (Lin et al., 2012b). 3. Results 3.1. Variations in dissolved inorganic and organic phosphorus
2.5. Sequential extraction of particulate phosphorus The sequential extraction (SEDEX) technique has been frequently used to quantify various particulate P species in sediments, sinking particles, and suspended particulate matter in aquatic environments (e.g. van der Zee et al., 2002; Faul et al., 2005; Lin et al., 2012b). Using the SEDEX technique (Ruttenberg, 1992; Zhang et al., 2010), TPP could be operationally divided into five particulate P phases: (1) loosely adsorbed (exchangeable) P (Labile-P), (2) ferric bound P (Fe-P), (3) authigenic carbonate fluorapatite + biogenic apatite + CaCO3associated P (CFA-P), (4) detrital apatite P (Detr-P), and (5) refractory organic P (Org-P). The extraction of particulate P was carried out in a 25 mL Teflon centrifuge tube with 10 mL of extraction solution based on procedures described in Zhang et al. (2010) and Lin et al. (2012b). After each extraction and centrifugation, the supernatant solution containing the extracted P was determined as DIP using the phosphomolybdenum
Table 1 summarizes the concentrations of dissolved P species in surface and bottom waters from the Jiulong River estuary. Surface water DIP concentrations generally increased from 0.52 μM at the coastal station S8 to 2.14 μM at the river end-member station S1 with an average of 1.20 ± 0.50 μM. Bottom water DIP concentrations were more variable, ranging from 0.12 to 2.12 μM, with an average of 1.06 ± 0.63 μM. These data indicated a strong riverine DIP source although there seemed to be a slight removal during early mixing (Fig. 2). Concentrations of DOP were generally lower and less variable than those of DIP, ranging from 0.47 to 0.62 μM with an average of 0.57 ± 0.05 μM for surface waters, and from 0.34 to 0.58 μM with an average of 0.46 ± 0.07 μM for bottom waters, respectively. Differences in DIP and DOP concentrations between surface and bottom waters at each station are generally small (Table 1), especially in the lower salinity area, showing a fairly well mixed water column in the Jiulong River estuary.
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Sur
Bot DIP DOP
2.5
0.7
2
0.4
0.5
0.3
0.3 30
0
0.5
15
20
25
0
5
10
Salinity
25
100
DIP DOP
80
30
0.2
DIP DOP
80
Percentage
Percentage
20
Salinity
100
60 40 20 0
15
DOP (µM)
0.4
0.5
10
0.6
1
1
5
DOP
0.5
0.6
0
0.7
DIP
1.5
1.5
0
DOP (µM)
DIP (µM)
2
0.8
DIP (µM)
2.5
60 40 20
0
5
10
15
20
25
30
Salinity
0
0
5
10
15
20
25
30
Salinity
Fig. 2. Variations in concentration and percentage of DIP and DOP with salinity in the Jiulong River estuary from surface water (Sur, left panels) and bottom water samples (Bot, right panels) during May 2010.
Within the total dissolved phosphorus (TDP) pool, DIP contributed up to 66 ± 10%, ranging from 46% to 80% in surface waters, while DOP comprised 34 ± 10% of the TDP, varying from 20% to 54%. In bottom waters, 63 ± 21% of the TDP was DIP and 37 ± 21% was DOP. As expected, DIP was the major dissolved P species in the Jiulong River estuary for both surface and bottom waters except two higher salinity (S N 20) stations (Fig. 2). The river end-member DIP concentrations were up to 2.14 μM, which are within the concentration ranges (1.7–7.7 μM) for anthropogenically influenced and eutrophic rivers, such as the Mississippi River (Turner et al., 2003; Cai and Guo, 2009; Shim et al., 2012), Lorie River (Meybeck et al., 1988) and Tamar River (Monbet et al., 2009), and they are considerably higher than those observed for more pristine rivers, such as the Yukon River (Guo et al., 2004, 2012), the Chena River in interior Alaska (Cai et al., 2008), and the Jourdan River in Mississippi (Lin et al., 2012a). Furthermore, these updated DIP concentrations (2.12–2.14 μM) are over 50–100% higher than those measured for the Jiulong River before recent economic development (0.5–1.0 μM between 1982 and 1997, as compiled by Yan et al., 2012), demonstrating an increasing nutrient input from the Jiulong River. 3.2. Variations in particulate inorganic and organic phosphorus Concentrations of suspended particulate matter (SPM) in surface waters ranged from 8.0 to 112 mg/L, with the highest concentration at station S2 and a slight increase at station S7 (Fig. 3). For bottom waters, SPM decreased in general with salinity with a maximum concentration of 123 mg/L at station S7 (Fig. 3), showing a significant sediment resuspension or lateral particulate source at this station. Concentrations of PIP ranged from 0.12 to 4.38 μM (average of 1.35 ± 1.66 μM) and
concentrations of POP varied from 0.09 to 1.81 μM (average of 0.59 ± 0.58 μM), with elevated concentrations at stations S2 and S7 (Table 2). Both particulate P species showed a monotonic decrease with increasing salinity, with elevated concentrations in bottom waters at stations S6 and S7 (Fig. 3). The highest TPP, PIP and POP concentrations observed at station S2 coincided with the highest SPM at the same station. In addition, the evident deviation in SPM and TPP concentrations from the conservative mixing line (Fig. 3) demonstrated strong particle settling or a sink within the estuarine mixing zone, especially in lower salinity areas. While surface water TPP concentrations (in μM) decreased with increasing salinity and showed a predominant river influence, concentrations of PIP and POP expressed as an intensive property (in mg-P/gparticles) became less variable in surface waters (Fig. 3). This indicated similar particulate P sources, although bottom waters had more variable TPP concentrations. For example, both PIP and POP in surface waters changed little before salinity reached 20 (Fig. 3). Interestingly, contents of particulate P (in mg-P/g-particles) were also generally higher at upper estuarine stations except at those influenced by sediment resuspension. However, samples with lower particulate P contents seemed to have lower particulate C/N ratios (Table 2). Higher concentrations of particulate P species (in both μM and mg-P/g) at lower salinity (b2) stations and stations influenced by sediment suspension point to a profound influence of anthropogenic and terrestrial P sources. Variations in PIP and POP concentrations (in mg-P/g-particles) were consistent with high terrestrial particle inputs from the river and sediment resuspension occurring in the lower estuary region as supported by the elevated SPM concentration (Fig. 3). The average TPP concentration (1.59 ± 0.39 mg-P/g) for surface waters in the Jiulong River estuary was evidently higher than those reported for
P. Lin et al. / Marine Chemistry 157 (2013) 93–105
97
Bot 150
100
120
SPM (mg/L)
SPM (mg/L)
Sur 120
80 60 40
60 30
20 0
90
0
5
10
15
20
25
0
30
0
5
10
Particulate P (µM)
Particulate P (µM)
PIP POP TPP
6
4
2
0
5
10
15
20
25
4
2
0
5
10
20
25
30
6 PIP POP TPP
2
Particulate P (mg-P/g)
Particulate P (mg-P/g)
15
Salinity
1.5 1 0.5
0
5
10
15
20
25
4 3 2 1 0
30
PIP POP TPP
5
0
5
10
15
20
25
30
20
25
30
Salinity
Salinity 100
100
%TPP %TDP
%TPP %TDP
80
Percentage
80
Percentage
30
6
0
30
2.5
60 40 20 0
25
PIP POP TPP
Salinity
0
20
8
8
0
15
Salinity
Salinity
60 40 20
0
5
10
15
Salinity
20
25
30
0
0
5
10
15
Salinity
Fig. 3. Variations in suspended particulate matter (SPM), particulate inorganic and organic P concentrations (in both μM and mg-P/g-particles), and the partitioning of P between dissolved and particulate phases with salinity for surface water (Sur, left panels) and bottom waters (Bot, right panels) in the Jiulong River estuary during May 2010.
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Table 2 Concentrations of suspended particulate matter (SPM), particulate organic carbon (POC), particulate nitrogen (PN), particulate inorganic P (PIP), and particulate organic P (POP), as well as the particulate organic C:P and C:N ratios for surface and bottom waters in the Jiulong River estuary during May 2010. Both PIP and POP concentrations are given in extensive (μM) and intensive units (mg-P/g-particles and μmol/g-particles), with errors ≤2%. Sample ID
SPM (mg/L)
POC (μM)
PN (μM)
POP (μM)
PIP (μM)
Par-C:N
Par-C:P
POP (mg-P/g)
PIP (mg-P/g)
POP (μmol/g)
PIP (μmol/g)
Surface waters S1 S2 S3 S4 S5 S6 S7 S8 S9
70.6 112.1 44.9 19.2 14.6 11.4 18.5 4.9 8.0
120 129 81 40 36 22 18 10 10
15.5 14.9 12.0 6.2 6.2 3.8 3.2 1.5 1.6
1.16 1.81 0.83 0.41 0.32 0.25 0.29 0.11 0.09
3.95 4.38 1.59 0.66 0.54 0.37 0.37 0.12 0.14
7.8 8.7 6.7 6.5 5.8 5.8 5.6 6.8 6.0
104 72 97 98 111 90 63 93 104
0.51 0.50 0.58 0.66 0.68 0.66 0.48 0.70 0.36
1.73 1.21 1.10 1.07 1.15 1.01 0.61 0.78 0.56
16.4 16.1 18.6 21.2 22.0 21.4 15.5 22.5 11.7
223.7 134.7 107.5 101.4 92.7 75.7 41.9 45.6 36.7
Bottom waters S1 S2 S3 S4 S5 S6 S7 S8 S9
58.9 46.1 35.2 32.0 17.2 23.7 123.4 32.3 17.0
– – – – – – – – –
– – – – – – – – –
1.28 1.82 1.50 0.47 0.32 0.95 0.95 0.25 0.17
4.35 5.76 3.48 1.26 0.67 1.94 2.01 0.66 0.44
– – – – – – – – –
– – – – – – – – –
0.67 1.22 1.32 0.45 0.58 1.24 0.24 0.24 0.30
2.29 3.87 3.07 1.22 1.20 2.54 0.50 0.63 0.80
21.7 39.5 42.7 14.7 18.6 40.0 7.7 7.7 9.8
308.1 499.6 319.0 119.1 110.8 227.4 54.2 49.6 54.7
“–” denotes “data not available”.
1.54 μM in surface waters. Overall, concentrations of all particulate P species in surface waters decreased with increasing salinity, but an evident increase in CFA-P, Detr-P and Org-P occurred in lower and higher salinity areas (Fig. 4). For bottom waters, all five particulate P species also showed a general decrease with increasing salinity, with the highest concentration in river waters and slightly elevated concentrations in the lower estuary (Fig. 4). Compared with surface waters, bottom waters generally had higher concentrations except for Labile-P (Table 3) and had a similar variation trend for all particulate P species (Fig. 4). Among the five particulate P species, Fe-P had the highest abundance in surface waters, comprising on average 30 ± 6% of the TPP, ranging from 22% at coastal stations to 42% at the river end-member station, followed by Org-P (28 ± 5%), Labile-P (18 ± 4%), CFA-P (15 ± 4%) and Detr-P (8 ± 2%). For bottom waters, Fe-P and Org-P were the two most abundant particulate P species, comprising 30 ± 8% and 28 ± 4% of the TPP pool, respectively, followed by CFA-P
other estuarine systems, e.g., 1.24–1.55 mg-P/g in Delaware estuary (Lebo, 1991); 0.60–0.81 mg-P/g in Changjiang estuary (Yan and Zhang, 2003), and the global average of 1.20 mg-P/g (Froelich, 1988). 3.3. Variations in chemical speciation of particulate P In addition to measurements of PIP and POP, total particulate P was operationally divided into five particulate P species, including Labile-P, Fe-P, CFA-P, Detr-P and Org-P (Table 3). In surface waters, the concentration of Labile-P varied from 0.04 to 0.93 μM with an average of 0.32 ± 0.34 μM, and Fe-P had an average concentration of 0.66 ± 0.81 μM, ranging from 0.05 μM to 2.09 μM in the Jiulong River estuary. In general, concentrations of particulate apatite P were lower compared with the other three particulate P phases, ranging from 0.03 to 1.04 μM with an average of 0.28 ± 0.34 μM for CFA-P, and varying from 0.02 to 0.66 μM with an average of 0.18 ± 0.23 μM for Detr-P (Table 3). Org-P had an average concentration of 0.47 ± 0.50 μM, ranging from 0.06 to
Table 3 Results of sequential extraction analyses with concentrations of different particulate P phases in surface and bottom waters from the Jiulong River estuary during May 2010. The concentrations of organic P in CFA-P fraction were below the detection limit during the sampling period. Errors of concentrations of particulate P species are ≤2%. Labile-IP (μM)
Labile-OP (μM)
Labile-P (μM)
IP/TP-Labile (%)
OP/TP-Labile (%)
Fe-P (μM)
CFA-P (μM)
Detr-P (μM)
Org-P (μM)
Surface waters S1 17.7 S2 32.5 S3 14.8 S4 6.5 S5 5.8 S6 4.9 S7 8.7 S8 2.7 S9 3.9
Sample ID
Particle mass (mg)
0.84 0.76 0.39 0.15 0.14 0.09 0.07 0.03 0.03
0.09 0.06 0.06 0.04 0.05 0.04 BD 0.02 0.01
0.93 0.82 0.45 0.19 0.19 0.13 0.07 0.05 0.04
91 92 87 76 74 68 100 56 73
9 8 13 24 26 32 BD 44 27
2.09 2.00 0.79 0.36 0.27 0.19 0.16 0.05 0.06
0.59 1.04 0.40 0.13 0.13 0.07 0.13 0.03 0.06
0.44 0.66 0.22 0.07 0.06 0.03 0.06 0.02 0.02
0.98 1.54 0.66 0.32 0.21 0.17 0.25 0.08 0.06
Bottom waters S1 14.1 S2 11.5 S3 10.9 S4 10.6 S5 6.0 S6 8.5 S7 37.0 S8 13.2 S9 8.0
0.50 0.72 0.53 0.24 0.11 0.18 0.14 0.05 0.04
0.03 0.04 0.05 0.02 0.01 0.02 0.01 0.01 0.02
0.53 0.76 0.58 0.26 0.12 0.20 0.15 0.06 0.06
95 95 91 93 92 91 92 92 61
5 5 9 7 8 9 8 8 39
2.53 2.92 1.33 0.51 0.30 0.57 0.56 0.22 0.16
0.77 1.24 1.02 0.30 0.15 0.68 0.76 0.23 0.12
0.44 0.77 0.63 0.14 0.07 0.40 0.45 0.10 0.05
1.26 1.80 1.39 0.41 0.24 0.92 0.93 0.24 0.14
“BD” denotes below the detection limit.
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0
S=0.1 S=1 S=4.3 S=7.6 S=15 S=19 S=23 S=25 S=26
Fig. 4. Abundance and partitioning of different particulate P species (in μM and mg-P/g-particles) and their variations with salinity in surface (Sur, left panels) and bottom waters (Bot, right panels) in the Jiulong River estuary.
(21 ± 5%) and Detr-P (11 ± 3%). Labile-P was the least abundant particulate P species (10 ± 3%) in bottom waters, compared to 18 ± 4% in surface waters (Fig. 4), suggesting the influence of sediment resuspension on the particulate P composition in bottom waters, as also supported by the relatively higher Detr-P abundance, especially at stations S6 and S7 in the Jiulong River estuary.
3.4. Results from mixing experiments Results of the laboratory mixing experiment showed a significant addition of DIP, especially when the salinity was b15, suggesting the release of particulate P (Figs. 5 and 6, and see detailed discussion below). The DOP concentration demonstrated an initial dramatic decrease showing considerable removal of DOP before S = 15 (Fig. 5). Within the dissolved P pool, the DIP/TDP ratio was generally higher than the DOP/TDP ratio during the entire mixing experiment even though the
DIP/TDP ratio began to decrease when the salinity was higher than ~20 (Fig. 5). Accompanying the increase in DIP, concentrations of PIP decreased with increasing salinity during the laboratory mixing experiment (Fig. 6), showing an instantaneous transformation between PIP and DIP. However, the increase in DIP was not balanced out by the decrease in PIP during mixing, suggesting that the transformation between organic and inorganic P species could also play a role in controlling the partitioning and speciation of P in the estuary. As shown in Fig. 6, both POP and DOP had much lower abundance than DIP and PIP, and they showed significant removal during mixing, especially DOP when salinity was b5. This indicated that the loss of organic P species could contribute to the increase in DIP during mixing. Interestingly, while specific P species had non-conservative mixing behavior with a dynamic transformation between dissolved and particulate, inorganic and organic phases, both TDP and TPP showed seemingly conservative behavior during laboratory mixing experiments, except
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Salinity Fig. 5. Results of end-member river–sea water mixing experiments, showing the concentrations and partitioning of DIP and DOP, as well as the concentrations and percentage of different particulate P species based on sequential extraction analyses.
in the lower salinity region (Fig. 6). The overall decrease in TPP and increase in TDP in the lower salinity region during mixing experiments pointed to the fact that the upper estuary (S b 5) was the most dynamic region where physicochemical processes were prevalent. Similar to TPP or PIP, almost all particulate P species decreased consistently with increasing salinity, except Org-P (Fig. 5). However, the abundance of each particulate P species in the mixing experiment was also variable, suggesting a dynamic transformation between different particulate P phases during mixing experiments.
3.5. Partitioning of P between dissolved and particulate phases Considering both particulate and dissolved P phases, particulate P was the predominant phase in the total phosphorus pool (TP = TDP + TPP) at lower salinity stations (S b 5), ranging from 54% to 77% for surface waters and from 69% to 71% for bottom waters (Fig. 3), indicating that riverine export of P was mainly in the particulate form. As salinity increased to N5, the TPP/TP ratio sharply decreased and TDP became the predominant P species in surface waters (Fig. 3). For bottom waters, in addition to higher TPP/TP ratios at the river station and lower salinity stations, elevated TPP/TP ratios were also observed in the lower estuary due to the influence of sediment resuspension (Fig. 3). The partitioning of P between dissolved and particulate phases and its particle reactivity can be quantitatively evaluated using its distribution coefficient (Kd). This approach has been used in previous studies to examine the partitioning and adsorption behavior of P in different aquatic environments (e.g., Morris, 1986; Santschi, 1995; Prastka
et al., 1998; Lin et al., 2012a). Similarly, Kd is defined as the partitioning of P between filtrate solution and filter retained particles in a given P pool: Kd ¼
Cp Cd ½SPM
where Cp is the concentration of particulate P (μM); Cd is the concentration of dissolved P in the organic, inorganic, or total P pool (μM); and SPM is the concentration of suspended particulate matter (kg/L or g/mL). Thus, the dimension of Kd becomes mL/g or L/kg. Hereafter, Kd values are reported in logKd. logKd values of inorganic, organic, and total P changed little with salinity in the Jiulong River estuary, ranging from 4.34 to 4.68 in surface waters (Fig. 7). For bottom waters, the resultant logKd values had a relatively higher variability, varying from 4.13 to 5.24, possibly due to the effect of sediment resuspension, especially in the lower estuary (see discussion in previous sections). Although concentrations of TPP or TDP varied considerably (Figs. 2 and 3), nearly constant logKd values across the whole salinity gradient in the Jiulong River estuary were somewhat unexpected (Fig. 7) and different from other estuaries where the partitioning of P between dissolved and particulate phases could be evidently affected by salinity (e.g., Caraco et al., 1990; Santschi, 1995; Turner and Tyler, 1997; Lin et al., 2012a), but agreed well with previous reports for some estuarine systems (e.g., Fang, 2000). Additionally, the average logKd value from both surface and bottom waters (4.57) was similar to those reported for the Humber estuary (4.51–4.66), Amazon estuary (4.62) and Tay estuary (4.89), as compiled by Prastka et al. (1998).
P. Lin et al. / Marine Chemistry 157 (2013) 93–105
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Fig. 6. Partitioning of inorganic, organic and total phosphorus between dissolved and particulate phases from field investigation in the Jiulong River estuary (Field, left panels) and laboratory mixing experiments (ME, right panels). Dash lines represent the hypothetic conservative mixing lines of dissolved and particulate P species.
6
6 PIP POP TPP
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3 0.6 0.8
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LogSPM
Fig. 7. Variation in the distribution coefficient (in logKd) with salinity in the Jiulong River estuary during May 2010, including the data from surface and bottom water samples as well as the relationship between logKd and suspended particulate matter concentration (in logSPM).
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4. Discussion 4.1. Estuarine mixing behavior of dissolved and particulate P species The variation in P concentrations of dissolved and particulate phases clearly show non-conservative behavior for most P species in the Jiulong River estuary (Figs. 2, 3 and 6). The varying extents of removal, addition, and transformation observed for different P species in the Jiulong River estuary are distinctly different from those of N and Si (e.g., Zhang et al., 1999; Yan et al., 2012). Such distribution patterns could be attributed to various physicochemical and biological processes, such as adsorption/desorption, coagulation/flocculation, biological uptake, sediment resuspension, and other end-member inputs. The most dynamic variation in the concentrations of dissolved and particulate P occurred in the low-salinity region (S b 3). In addition, the middle salinity area of the estuary near Jiyu Island (stations S6 and S7 with salinity of 17–23, Fig. 1), the most intensive mixing zone in the estuary, was the other dynamic region with evident changes in the concentration of particulate P species, especially in bottom waters (Figs. 3 and 4). A considerable decline in TDP concentration, especially DIP, was accompanied by an increase in TPP concentration between stations S1 and S2. Most interestingly, the elevated concentration of TPP from stations S1 to S2 (1.08 μM) was comparable to the decrease in TDP concentration (0.83 μM), indicating a transformation of P between particulate and dissolved phases in the upper estuary (Fig. 2 vs. Fig. 3). However, the increase in TDP concentration (0.30 μM) between stations S2 and S3 could not offset the sharp decrease in TPP concentration (3.77 μM), suggesting a strong particulate P removal during early mixing in the estuary. Possible processes for the removal and addition of P species included biological uptake of DIP, adsorption/desorption and release of P from the particulate phase, and precipitation/flocculation of particulate P (see detailed discussion below), as implied by the sharp decrease in SPM concentration with increasing salinity in these regions. After early estuarine mixing, the decrease in TDP concentrations was no longer accompanied by the increase in TPP concentrations in middle and higher salinity areas, showing a general decrease in TPP and TDP concentrations with increasing salinity (Figs. 2 and 3). In addition, DIP concentrations in the middle and higher salinity areas also decreased with increasing salinity, showing a conservative mixing and physical dilution in this region (Fig. 6). An evident increase in DIP concentration but decrease in DOP concentration was found at a higher salinity station (S9), close to Gulangyu and Xiamen Islands, possibly resulting from city sewage water inputs and other sources. As salinity increased, the relative abundance of both DIP and TPP decreased, but the importance of DOP and TDP increased (Figs. 2 and 3), reflecting the decrease in terrestrial inputs of DIP and TPP and the increase in autochthonous sources of DOP in coastal waters (e.g. Jensen et al., 2006; Yao et al., 2009; Lin et al., 2012a). 4.2. Sources and transformation of dissolved and particulate phosphorus Compared with traditionally defined PIP and POP, the speciation of particulate P as determined by sequential extraction could provide more detailed insights into the sources of particulate P and its correlation with dissolved P in aquatic environments. For example, Org-P could be used to demonstrate the influence of in situ biological production or activities in natural waters (Zhang et al., 2004), while Detr-P was considered to be the most inert fraction of particulate P and could be a proxy of terrigenous origin (van Cappellen and Berner, 1988; Ruttenberg, 1992; Hou et al., 2009). As shown in Table 3, the additional input of TPP at station S2 was mostly composed of CFA-P, Detr-P and Org-P. Since the decrease in Labile-P and Fe-P concentrations showed that the removal of DIP could not result from binding to clay or amorphous Fe oxyhydroxide (Lebo, 1991; Fox, 1993; Ruttenberg and Sulak, 2011), two factors
might be responsible for the inverse relationship between TPP and DIP. First, an increase in the concentration of CFA-P (Fig. 4) indicated the addition or formation of CaCO3 or authigenic carbonate fluorapapite and biogenic apatite at station S2. In addition, the significant positive correlation between Org-P and POC or PN (Fig. 8), together with the increase in biogenic apatite and Org-P concentration at station S2, would suggest a strong biological production process drives the addition of TPP at this station. Higher transparent exopolymer (TEP) concentrations in the study area (data not shown) further supported a higher biological contribution. Second, the slight increase in Labile-P, Fe-P and Detr-P abundance in bottom waters (Table 3) pointed to a possible terrigenous TPP input. Lower SPM in bottom waters compared with surface waters at station S2 (Table 2) further negated the importance of sediment resuspension for surface water at this location, but the higher TPP concentration in bottom waters at station S2 (Table 2) suggested different particle sources between surface and bottom waters at S2. Hence, biological production and additional terrestrial apatite might be the dominant factor governing the removal of DIP and providing the additional source of TPP in the upper estuary. As shown in Fig. 2, the addition of DIP between stations S2 and S3 (0.33 μM, Table 1) could be almost balanced out by the decline in the particulate Labile-P concentration (0.37 μM, Table 3), indicating that additional DIP was mainly derived from the release of labile particulate P, especially during early estuarine mixing. Zhang et al. (2004) suggested that the ambient phosphate concentration as well as other environmental conditions, such as the salinity effect (Zhang and Huang, 2011) could determine the extent of P release from TPP. The higher TPP concentration compared with the TDP concentration at station S3 (Tables 1 and 2) supported our hypothesis that Labile-P could be a source of DIP during early estuarine mixing in the Jiulong River estuary. Thus, the labile or exchangeable particulate P could play an important role in regulating the distribution and partitioning of P species in estuarine environments (e.g. Froelich, 1988; De Jonge and Villerius, 1989; Prastka et al., 1998; Zhang and Huang, 2007; Lin et al., 2012a). After early estuarine mixing (S N 3), the decreasing TPP/TP and increasing TDP/TP ratios (Fig. 3) could result from physicochemical processes, biological activities, and the transformation between particulate and dissolved phases in the water column. For the sudden increase in TPP concentration at station S7, mostly from CFA-P, Detr-P and Org-P (Table 3, Figs. 3 and 4), sediment resuspension should be the main cause, judging from the elevated SPM and Detr-P, as well as lower Labile-P and TPP concentrations (in mg-P/g-particles, Figs. 3 and 4) for both surface and bottom waters at this station. In addition, after early estuarine mixing, the abundance of organic P in the Labile-P phase increased from 8% at the river end-member station to 44% at coastal stations (Table 3) suggesting an increasing importance of biological processes when the influence of riverine TPP decreased. This is consistent with the increasing DOP fraction in the TDP pool (Fig. 2). Furthermore, the average particulate C/N (6.6 ± 1.0) or C/P ratio (92 ± 16) for surface waters (Table 2) was somewhat close to the Redfield ratio (C/N = 6.6, C/P = 106), and evidently lower than other estuarine systems, such as the Mackenzie River Shelf (Ruttenberg and Goñi, 1997), Bay of St. Louis (Cai et al., 2012) and Leyre estuary (Canton et al., 2012). Since P-containing organic matter could be preferentially decomposed during transport (Sannigrahi et al., 2006; Cai and Guo, 2009), the C:N:P ratio that was close to the Redfield ratio suggested that the particulate organic matter (POM) in the study area was mostly derived from autochthonous sources rather than terrestrial origins. The occurrence of mangrove vegetation in the lower salinity areas may have an influence on POM quality since mangrove ecosystems generally have high primary productivity, and thus may play a role in the biogeochemical cycling of P through the assimilation of dissolved P, especially in the lower salinity region. This is similar to the cases in the creek waters of a mangrove forest (e.g., Kristensen and Suraswadi, 2002) and the high contents of organic N and P in a mangrove lagoon (Fuentes, 2000).
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200
20
2
y = 9.12 + 90.0x R = 0.92
2
y = 2.39 + 10.2x R = 0.84
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POC (µM)
160 120 80
12 8 4
40 0
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0
0
0.4
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Fig. 8. Relationship between Org-P and particulate organic carbon (POC) or particulate nitrogen (PN) in the Jiulong River estuary during May 2010.
4.3. Comparisons between field studies and laboratory mixing experiments As shown in Fig. 6 and the deviations from the hypothetic conservative mixing lines, the specific addition/removal of P species and their overall mixing behavior in the Jiulong River estuary could not be entirely simulated with the short-term laboratory mixing experiments, especially for the processes such as the removal of DIP and strong particle settling in the upper estuary, as well as the sediment resuspension and biological processes in the lower estuary. The difference in results between the field studies and laboratory mixing experiments suggested that the mixing behavior of P in the Jiulong River estuary was not solely controlled by physicochemical processes. Instead, biological and longer term processes, sediment resuspension, and additional end-member waters from north of Xiamen Island may all play a role in regulating the concentration, speciation, and distribution of P species in the Jiulong River estuary. For example, the distinct variation pattern of POP (Fig. 6) or Org-P (Fig. 4 vs. Fig. 5) between field and laboratory studies also supported the linkage between organic P and biological processes in the Jiulong River estuary (Fig. 8). Short-term laboratory mixing experiments have been used to simulate estuarine mixing and to examine key processes in regulating mixing behavior of different chemical species (e.g. Wang et al., 2010; Lin et al., 2012a). In a previous study, little difference was observed for the mixing behavior of P species between field studies and laboratory experiments in the Bay of St Louis in the northern Gulf of Mexico (Lin et al., 2012a). As pointed out above, the Jiulong River has experienced increasing anthropogenic influence over the past several decades. In contrast, the Jourdan and Wolf rivers, both discharging into the Bay of St. Louis, remained rather pristine with much lower DIP abundance. Thus, in addition to physical conditions such as mixing/flushing time, and primary production in a specific estuary, the abundance of DIP and particulate P composition in river end-member and coastal waters are likely important factors controlling the mixing behavior of P in the Jiulong River estuary.
4.4. Variations in distribution coefficient (Kd) and its relation to the concentration of suspended particles The partitioning of P between particulate and dissolved phases may be largely determined by the chemical properties and the compositions of particulate P or biological processes in the estuary. The correlation between Kd values and salinity might depend on the flushing time of estuarine waters. For example, Caraco et al. (1990) and Lin et al. (2012a) have shown a decrease in logKd values of P with increasing salinity in estuaries with shorter flushing times compared with the time-scale of equilibrium between dissolved and particulate P (Morris, 1990). Thus, less variable Kd values of P in the Jiulong River estuary
might result from the relatively longer flushing time of estuarine waters, as also observed in other estuaries (e.g., Fox et al., 1986; Fang, 2000) and relatively high concentrations of DIP in the water column. In addition, even though concentrations of different particulate P species varied sharply with salinity, the relative abundance of each particulate P species in the TPP pool changed little (Section 3.3, Fig. 4), suggesting little change in particle composition along the salinity gradient and thus similar Kd values in the Jiulong River estuary (Fig. 7). Furthermore, logKd values had a weak correlation with SPM concentration (in logSPM, Fig. 7), showing a weak “particle concentration effect” on the partitioning of P in the Jiulong River estuary. This is somewhat different from those observed for other estuarine systems (e.g., Balls, 1989; Santschi, 1995; Lin et al., 2012a). Weak particle concentration effect observed here also suggested a minor role of colloidal materials in the partitioning of P between particulate and dissolved phases, consistent with generally low colloidal P abundance in the Jiulong River estuary (Chen et al., 2010; Cai et al., unpublished data).
4.5. Changing mixing behavior of P in the estuary Elevated inputs of P and other nutrients through river to estuarine and coastal environments could result in “cultural eutrophication”, and thus lead to considerable attention to changing biogeochemical behavior of P in estuarine systems with increasing anthropogenic influence (Ruttenberg, 2003; Stathan, 2011). Results of our field study in the Jiulong River estuary clearly showed a strong riverine DIP source, a general removal of DIP during early estuarine mixing, and conservative mixing behavior after early estuarine mixing (Figs. 2 and 6), similar to estuaries with anthropogenic influences, such as the Humber and Tanshui river estuaries and the Mississippi River plume (e.g., Prastka and Malcolm, 1994; Fang, 2000; Shim et al., 2012), but distinctly different from more pristine estuaries, such as the Bay of St. Louis and Leyre estuary (e.g., Canton et al., 2012; Lin et al., 2012a). Even though the slight addition or release of DIP from labile and exchangeable particulate P species occurred during early estuarine mixing (see Section 4.2), the extent of DIP addition or the buffering of DIP concentrations by suspended particulate matter (i.e., “P-buffering” mechanism) was evidently reduced and became insignificant in most areas of the estuary, compared with the observation in more pristine estuaries (e.g., Lin et al., 2012a) and the Jiulong River estuary before the recent economic development over the past 20 years (e.g., Yang et al., 1998; Zhang et al., 1999; Yan et al., 2012). Additionally, since the release of DIP could be seen in the laboratory experiment (see Section 3.4), as in more pristine estuaries (e.g., Lin et al., 2012a), rapid removal of particulate P in the upper Jiulong River estuary might be another factor responsible for the lack of a “P-buffering” mechanism in the study area. Consequently, in general, increasing DIP inputs and human impacts on
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the Jiulong River estuary had evidently reduced the relative role of physicochemical processes and buffering capacity of particulate P during estuarine mixing and thus altered the transformation and mixing behavior of P species in the estuary. This is consistent with results observed in the Chesapeake Bay (Conley et al., 1995) and results from model prediction (Prastka et al., 1998). 5. Conclusions The distribution and partitioning of P species and their mixing behavior in the Jiulong River estuary were investigated during May 2010. Phosphate (DIP) was the dominant species in the total dissolved P pool in both surface (66 ± 10%) and bottom waters (63 ± 21%), while DOP predominated only at a coastal station, reflecting a profound influence of anthropogenic inputs from the Jiulong River. Compared to historical data, the concentration of DIP in Jiulong River waters seemed to increase continuously with time, suggesting an increasing impact of nutrients on coastal waters off the Jiulong River estuary. The DIP concentration generally decreased with increasing salinity, showing removal in lower (S b 3) salinity regions but conservative mixing in middle and higher salinity regions. In contrast, DOP concentration was highly variable with a general increase in the DOP/TDP ratio with increasing salinity. The highest (up to 82%) and dramatically decreasing TPP/TP ratio in lower salinity areas as well as the negative correlation between TPP and DIP or TDP support a strong river input of particulate P and an intensive transformation between P species during estuarine mixing, although less so in middle and higher salinity regions. Phase speciation of particulate P characterized using the sequential extraction technique elucidated a biological source for CFA-P and Org-P, a terrestrial input for Detr-P, and a release of P from the Labile-P phase, especially in the upper estuary. Most P species behaved non-conservatively during estuarine mixing in both field studies and laboratory experiments, showing a dynamic transformation between dissolved and particulate organic and inorganic P species in the Jiulong River estuary. However, the apparent mixing behavior of P species observed during short-term laboratory experiments was not the same as that observed in the field with a longer flushing time. It seemed that, in addition to physicochemical processes, strong riverine inputs, biological processes, and hydrological conditions/sediment resuspension all play a role in controlling the mixing behavior of P species and their biogeochemical cycling in the Jiulong River estuary. Values of distribution coefficient of P (in terms of logKd) changed little with salinity in the estuary. A poor correlation between logKd and logSPM or a weak “particle concentration effect” was found in the Jiulong River estuary, consistent with the relatively constant logKd values, long estuarine flushing time, and low colloidal abundance in the study area. Overall, the elevated input of P and increasing anthropogenic influences may alter the transformation mechanism between P species and the mixing behavior of P in the Jiulong River estuary. Acknowledgments We gratefully acknowledge Lili Ma and Liangshi Lin for their assistance during sample collection, Feng Lin, Wangjiang Hu and Wentao He for their help in sample processing and analyses, Dr. Weidong Guo and Xiuli Yan for discussion, and Dr. Sarah Cooley at Woods Hole Oceanographic Institution for technical editing. We also thank the Associate Editor, Dr. Claudia Benitez-Nelson, and two anonymous reviewers for their constructive comments, which improved the presentation of the manuscript. This work was supported in part by Chinese Natural Science Foundation (#40906040 and #41125020), State Oceanic Administration of China through a special scientific research project for public welfare (#2010050012-3), U.S. NSF (OCE #0850957), Natural Science Foundation of Fujian Province (2011J01277), and the Fundamental Research Funds for the Central Universities of China.
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