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Keywords:

  • Ca2+;
  • Pearl River estuary;
  • salinity;
  • transparent exopolymer particles (TEP)

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Distribution of Transparent Exopolymer Particles (TEP) in the Pearl River estuary, China, was investigated during two cruises in August 2009 and January 2010. TEPcolor concentrations were 521.5–1727.4 μg Xeq.L−1 (μg Gum Xanthan equivalent liter−1) in August 2009 and 88.7–1586.9 μg Xeq.L−1 in January 2010, respectively. The size of TEP generally increased in the seaward along the longitudinal section with the dominant size of 2–40 μm during the cruises. Experimental work suggested that both concentration and size of TEP increased with Ca2+ concentration (from 0.8 mmol L−1 to 10 mmol L−1). In the field study, Ca2+ concentration had a positive correlation with TEPcolor concentration in the surface layer with salinity <16. Decrease of TEP concentration seaward from intermediary salinity was partly due to dilution of seawater as well as enhanced aggregation and sedimentation of TEP via increasing divalent cation concentration. TEP concentration and turbidity maximum coexisted at the tip of salt wedge in the bottom layer during the wet season, and positive correlation between TEP and turbidity was observed in the winter. Relationships between TEP and turbidity suggested the important contribution of TEP aggregation to flocculation and sedimentation of particles in estuaries. Different pattern of TEP during two cruises can be attributed to physical process (i.e., mixing type) in estuaries. These findings indicated that formation and distribution of TEP were largely influenced by interaction between physical and biogeochemical processes in the Pearl River estuary. A conceptual model for TEP formation and distribution in the Pearl River estuary was developed.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Since the development of methodology [Alldredge et al., 1993], Transparent Exopolymer Particles (TEP) have been widely studied to address their roles in the fluxes of sinking particles and the functions of food webs in aquatic ecosystems [Azetsu-Scott and Passow, 2004; Logan et al., 1995; Passow, 2002a]. Large portions of TEP are assembled by dissolved or colloidal acidic polysaccharides produced by phytoplankton and bacterioplankton [Alldredge et al., 1993; Passow, 2002a, 2002b]. TEP can also be generated during the sloughing of cell surface mucus and the disintegration of colonial matrices [Hong et al., 1997; Kiørboe and Hansen, 1993; Thornton, 2004]. As a biologically important constituent in the surface euphotic zone [Passow, 2002a; Verdugo et al., 2004], TEP play a key role in regulating aggregation and sedimentation through enhancing coagulation and altering particles density [Engel, 2000; 2004; Mari and Robert, 2008; Passow et al., 2001]. It was suggested that TEP-induced aggregation and sedimentation might be reduced and the vertical carbon flux may be reversed with the increase of acidification [Mari, 2008]. TEP provide surfaces for the colonization by bacteria and also can be readily grazed by euphausiids. This expands the concept of food webs and links the microbial loop with an aggregation web [Dilling et al., 1998; Grossart et al., 1998; Mari and Kiorboe, 1996].

[3] Recent advances in TEP in estuaries environment have highlighted the importance of TEP and their prosecutor (dissolved acidic polysaccharides) for the organic carbon cycling in estuaries [Mari et al., 2012; Thornton, 2009; Wetz et al., 2009; Wurl and Holmes, 2008]. Furthermore, exported acidic polysaccharides from estuaries may potentially enhance particle aggregation over the continental shelves and affect the fate of carbon produced there [Thornton, 2009]. In estuaries, the increase in TEP sticking properties toward high salinities affect the vertical export pump [Mari et al., 2012]. TEP may also have important implication for food web and trace metal cycling in estuaries because of their high abundance and high surface reactivity [Barrera-Alba et al., 2008; Wu et al., 2001]. Estuaries are dynamic aquatic environments with sources of matter from both terrestrial and marine inputs. Estuarine environments are influenced not only by diurnal variability and seasonal changes, but also by tides, fluctuating river flows and multiple inputs. Information on the formation and distribution characteristics of TEP in different estuaries is necessary to evaluate the potential significance of TEP to biogeochemical processes in estuaries.

[4] In order to accurately describe the role of TEP in estuary, it is also necessary to know the change of TEP properties with varying concentrations of divalent cations in estuary. The availability of divalent cations during formation of algal gels determines the size and rigidity of gels [Kloareg and Quatrano, 1988]. As a part of gel, TEP are flexible and sticky. Their high stickiness was directly linked to the presence of a high concentration of polysaccharides within sulfate ester groups [Zhou et al., 1998], which would form cation bridges [Kloareg and Quatrano, 1988] and hydrogen bonds [Chin et al., 1998; Mopper et al., 1995]. Therefore, it was expected that the concentrations of these divalent cations would determine the sticking coefficient that controlled the efficiency of the coagulation processes [Mari and Burd, 1998]. Wetz et al. (2009) found that addition of a certain concentration of Ca2+ (5 mmol L−1) and Mg2+ (12 mmol L−1) could stimulate the formation of TEP in the North Carolina's Neuse River estuary [Wetz et al., 2009], where the surface cation concentrations are comparable to those found in the Pearl River estuary (Ca2+ concentration varied from 0.8 to ∼10 mmol L−1; see Table 1). There is, however, no report on how TEP property change with varying divalent cations concentrations in estuary.

Table 1. The Concentration of Ca2+ (mmol L−1) in the Pearl River Estuary in August 2009 and January 2010 Cruises
StationsAugust 2009January 2010
SurfaceBottomSurfaceBottom
S10.680.710.801.28
S20.680.701.952.41
S30.700.702.462.85
S40.791.635.525.54
S52.115.667.387.39
S64.7910.28.798.76
S74.839.8410.7510.63
S85.5110.2010.9010.58

[5] Estuarine environments often show strong temporal and spatial variability in their physical properties such as temperature and salinity, as influenced by dynamic hydrography and meteorology. Turbulence strength could change the collision rate between suspended particles and their aggregation [Mari and Robert, 2008]. Residence time of water mass was tightly related to bacterial degradation rate that finally controlled the export of TEP [Mari et al., 2007]. Moreover, the physical mixing could lead to change of biogeochemical cycles in estuarine waters [Harrison et al., 2008; Kasai et al., 2010; Lui and Chen, 2011]. It was suggested that mixing process was the main factor influencing pCO2 in the mid-estuary of the Pearl River [Guo et al., 2009]. Mixing processes in large estuary may differ greatly from those in small one due to the variability in river discharge, topographic constraints, wind condition and tidal cycle. Based on the strength of water column stratification or the vertical structure of salinity, mixing type of estuaries can be classified as salt wedge, partially mixed or vertically mixed [Cameron and Pritchard, 1963; Hansen and Rattray, 1966]. Although previous studies suggested that mixing categories as a function of the seasonal cycles in estuary may affect TEP distribution characteristics [Mari et al., 2012; Wetz et al., 2009], their roles on TEP aggregation and distributional patterns are still not well understood.

[6] The work of Wetz et al. [2009] and Mari et al. [2012] has provided us a good context on the distribution of TEP, sticking characteristics and the underlying controlling mechanisms in the North Carolina's Neuse River estuary and the Bach Dang estuary. However, the physical and biogeochemical process varies from estuary to estuary, there is still a lack of understanding on the distributional characteristics of TEP in the Pearl River estuary, particularly its distribution near the bottom part of the estuary. In this study, we are trying to address the following questions based on data collected from the Pearl River estuary: (1) What are the concentration, size spectrum, and spatial/ temporal patterns of TEP in the water's surface and the bottom of the Pearl River estuary? (2) How do TEP properties vary with divalent cations such as Ca2+ and what are the effects of TEP on flocculation and formation of turbidity maximum in the Pearl River estuary? (3) How do hydrodynamics, especially the mixture type, affect TEP distribution in estuarine waters?

2. Material and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. Study Site and Research Cruises

[7] The Pearl River estuary is a semi-enclosed estuary located in the south China coast (Figure 1). It is the second largest river in China. Runoffs from all the tributaries discharge into the SCS through three sub-estuaries: Lingdingyang, Modaomen and Huangmaohai. This study focuses on Humen Outlet and Lingdingyang, which is traditionally regarded as the Pearl River estuary [PRWRC/PRRCC, 1991; Zhao, 1990] (Figure 1). It transports 85 × 106 tons of sediment annually to the South China Sea (SCS) [Li et al., 2006; Zhang et al., 1999]. The net contribution of organic carbon from the Pearl River Delta to the coastal ocean represented approximately 0.1–0.2% of total organic carbon transported by rivers in the world [Ni et al., 2008].

image

Figure 1. Sampling stations of the Pearl River estuary. Zone 1 is from S1 to S3; Zone 2 is from S4 to S6. Zone 3 is from S7 to S8.

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[8] The Pearl River estuary is like an inverted funnel with the narrow neck in the north and wide mouth opening to the south. Most of the Pearl River estuary is very shallow with a water depth from 2 m to 10 m, except for the deep channels and the areas around the outer islands where the water depth varies from 20 m to 30 m. The width is about 4 km at the northern end near Humen, and about 60 km between Lantau Island (Hong Kong) and Macau at the southern end. The length is about 63 km between the two ends. The geographic and topographic features exert dynamic influences on tidal cycles, water circulation and the water column structure. The average river discharge is 10,524 m3 s−1. 80% of the total discharge occurs in wet season between April and September, with the remainder for 20% is between October and March [Zhao, 1990]. During the wet season, the circulation has a two-layer structure in the entire estuary and the estuary is stratified or partially mixed. As the runoff is greatly reduced in the dry season, the estuary is well-mixed by the strong northeast winds [Dong et al., 2004; Xu, 1985; Zhao, 1990]. This seasonality of flow regimes will presumably influence biogeochemical cycling in estuary, which would have important consequences for TEP properties.

2.2. Sampling

[9] In situ water samples were collected during August 2009 and January 2010 cruise. Samples from different stations in the Pearl River estuary (Figure 1) were collected from surface (0.5 m depth) and bottom layers (about 0.5 m above bottom) using 5.0-L Niskin bottle that had been previously acid-washed. Upon collection, samples were stored in a cooler and returned to the lab for processing within 2 h. Vertical profiles of turbidity, salinity, temperature, and pH were measured at each site using a YSI 6600 sonde (YSI Environmental, USA). Concentration of Ca2+ was measured by ICP-AES (Prodigy SPEC Leeman, USA). Chl a concentrations were determined with a F-4500 Fluorometer (Hitachi, JPN) after extraction in 90% acetone for 24 h [Herbland et al., 1985; Parsons et al., 1984].

[10] Free DOM polymers can spontaneously assemble to form polymer gels, which can be enhanced by divalent cations (particularly, Ca2+ and Mg2+) because divalent bonds can stabilize the matrix of polyanionic gels through cation bonding [Chin et al., 1998]. The concentrations of the divalent cation in the Pearl River estuary showed a large gradient [Han, 1998]. Investigating the response of TEP to variations in divalent cation concentration is necessary. In the experiment, only Ca2+ was chosen to be added, as Ca2+ and Mg2+ produce similar effects on bioflocculation in the divalent cation bridging theory [Bruus et al., 1992; Sobeck and Higgins, 2002]. For determination of the effect of Ca2+ concentration on TEP formation, 30 L of water were sampled at a depth of 50 cm at S1 during the winter cruise, with 0.8 mmol/L Ca2+ and a salinity of 0.53, using a 5-L Niskin bottle. The samples were transported back to the laboratory in acid cleaned, polycarbonate carboys (Nalgene) within 1 h.

2.3. TEP Concentration and Size Spectrum

[11] Concentrations of TEPcolor (μg Xeq. L−1, as μg gum xanthan (GX) equivalents L−1) were measured using the technique of Passow and Alldredge [1995]. Triplicate 15–100 ml samples from each station were vacuum filtered (<0.2 bar) through 25 mm diameter, 0.2 μm polycarbonate filters (Millipore). Filters were stained for <5 s with 0.5 mL of 0.02% Alcian Blue 8GX (Amresco) in 0.06% acetic acid (pH 2.5) and then rinsed with 2.0 mL of deionized water. Alcian Blue-stained material was extracted from the filters with 6 mL of 80% sulfuric acid for 2 h on an oscillator. The extracted material was then centrifuged for 5 min at 1000 rev.min−1. Absorbance of the supernatant fluid was measured spectrophotometrically at 787 nm. Alcian Blue absorption was calibrated using a xanthan gum solution (SIGMA) that was processed by tissue grinder and measured by weight. Submicro-TEP of 0.2–0.6 μm was found an important component in the sample collected from Santa Barbara Channel [Passow and Alldredge, 1995], so the pore size of the filter used for detecting TEP was important because smaller size would collect more TEP [Wetz et al., 2009]. During these two cruises, we have compared the different results between 0.2 μm filters and 0.4 μm filters. Average concentration of TEP detected by 0.2-μm filters was 42% higher than that by 0.4-μm filters in the Pearl River estuary (the percentage of increase of TEP-0.2 μm over TEP-0.4 μm is presented in auxiliary material Table S1). The detection limit of the method was 2.35 μg Xeq. L−1. Average absorptions of filter blanks range between 0.160 and 0.220 for 0.2 μm polycarbonate filters.

[12] TEP size spectra were measured according to Mari and Robert [2008]. Two ml samples were filtered onto 0.2-μm polycarbonate filters (<0.2 bar), and then stained with Alcian Blue (pH 2.5) [Alldredge et al., 1993]. The filters were prepared in duplicate, placed onto semi-permanent TEP slides (Cyto-Clear) and stored frozen at −20°C until analysis. For each slide, ten images were randomly screened under a compound light microscope, a digital OLYMPUS camera with a 200–400× magnification. Equivalent spherical diameter (ESD; μm) of individual TEP was calculated with measuring its cross-sectional area by using an image-analysis system (ImagePro Plus, MediaCybernetics). TEPvolume concentration (μm3 mL−1, ppm) was calculated according to data of the TEP size spectra assuming a spherical volume for each particle. The detection limit of the method in terms of TEP diameter was 0.5 μm. The sampling volume used for measuring TEP size spectra (2 mL) was lower than that in previous studies in oceanic water (10 mL) [Alldredge et al., 1993]. A low sampling volume may reduce the detection sensitivity of large-sized TEP, as sizes over 150 μm were rare during those two cruises in the Pearl River estuary. This modification might result in underestimation of TEPvolume concentration.

[13] TEP-carbon concentration (CTEP; μg C L−1) was calculated using the slope (0.75) from equation (1) as follows [Engel and Passow, 2001]:

  • display math

where TEPcolor is the concentration of TEP in the unit of μg Xeq. L−1. We should mention that the slope from each experiment [Engel and Passow, 2001] could range from 0.51 to 0.88 due to difference in phytoplankton specifications.

2.4. Total Organic Carbon

[14] TOC samples were collected using precombusted (450°C, overnight) and acid-washed 20-mL vials and were preserved with 12 μL 85% phosphoric acid (H3PO4). Samples were stored in the dark before analyses and measured using a Shimadzu TOC VCPH analyzer. Potassium phthalate standard calibration were conducted over the range 0 to 250 μmol C L−1. The blank of the analytical system was between 5 and 10 μmol C L−1 and the standard deviation was less than 2% of the mean of triplicate measurements.

2.5. Experiment for Determination of the Effect of the Ca2+ Concentration on TEP Formation

[15] Sample water were sieved through 20 μm nylon mesh and then filtered through 0.2 μm polycarbonate filters (Millipore) of 47 mm diameter at low and constant vacuum pressure (<0.2 bar) to get rid of the relative abundance of phytoplankton or the physiological state of cells under changing osmotic pressure conditions. 600-ml filtrate was brought into the Pyrex flasks (1000 ml) for each of the incubations. CaCl2·2H2O was added to the filtrate up to concentrations of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mmol L−1. Such concentrations were selected due to the results of in the field study (Table 1). The control group (CG) was not changed. To inhibit microbial activity, sodium azide was added to the sample solutions to a final concentration of 0.02%. Turbulence was generated with an orbital shaker (at 140 rpm), which oscillated with an orbit of 3 cm. After being incubated for 30 ∼ 60 min, water samples were collected for TEP concentration and size measurements. Each group was prepared with three replicates. The energy dissipation rate was calculated using the following equation [Colomer et al., 2005; Zirbel et al., 2000]:

  • display math

where Po is the orbital shaker power number (9.96 × 10−5) [Colomer et al., 2005; Zirbel et al., 2000], N is the orbital shaker speed, V is the fluid volume and D is the diameter of the orbit. For a volume V of 0.6 L, and N of 140 rpm, ε can be estimated as 0.11 cm2 s−3. Such value was consistent with those found in the upper mixed layer of the estuary [Mari et al., 2012; Orton and Visbeck, 2009].

2.6. Mapping and Statistics

[16] Analysis of variance or Student's t-test was conducted on the data that met the assumptions of normality and equality of variance. Data that did not meet these criteria were log (n + 1)-transformed before analyses, or nonparametric tests were carried out on the ranks. Paired-sample t-test was performed to compare TEP spatial–temporal differences. Tukey HSD test was used to compare the TEP at each station. Pearson product moment correlation analyze was used to examine the relationship among TEP properties, such as concentration and relevant environmental parameters.

[17] Statistical calculations were conducted using SPSS software. Contour plots and station maps were generated by MATLAB R2008b (Mathworks Inc., USA) and Surfer8 (Golden Software Inc., USA). Conceptual diagram of TEP distribution was created in photoshop8.0 (Adobe Systems Inc., USA). All other figures were generated using Origin7.5 (OriginLab Corporation, USA).

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Salinity Distributions

[18] The Pearl River estuary shows distinct seasonal hydrographic characteristics associated with river runoff and monsoon strength. Water mass was not well mixed in the middle and lower parts of the estuary during the summer cruise of 2009 (Figure 2a). The halocline was evidential from S6 to S8. Using 1 ppt isohaline as indicator, a null point (the tip of salt wedge) was found at Station S4, near the river outlet (Humen) (Figure 2a). During the winter cruise, however, the stronger northeast winds (over 7 m/s) and the lower river runoff in Lingdingyang had led to a well-mixed estuary with a vertically homogeneous distribution of salinity (Figure 2b).

image

Figure 2. Vertical distribution of salinity in the Pearl River estuary in (a) August 2009 and (b) January 2010.

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3.2. Concentration of TEP and Turbidity

[19] In the August 2009 cruise, the TEPcolor concentration varied from 521.5 to 1727.4 μg Xeq.L−1, with an average of 988.6 μg Xeq.L−1 (Figures 3a and 3b). Although intense phytoplankton bloom (average content of Chl a, 72.4 μg L−1) occurred in Zone 1(Figure 4a), the TEPcolor concentrations were higher in Zones 2 and 3 than in Zone 1 (t-test; p <0.05). In contrast, the turbidity in the surface water reduced seaward in Lingdingyang Bay (Figure 3a). TEP distribution in the bottom was different with that in the surface layer (Figure 3b). Two peaks of TEP concentration and turbidity were observed for the bottom water (Figure 3b). The largest concentration of TEP and the maximum turbidity were both presented at S4, near the null point where the residual seawater landward flow is balanced by the seaward residual river flow (Figure 3b). The second peak of TEP concentration and turbidity were at S6 and S7, respectively (Figure 3b). The concentrations of TEP at S4 and S6 differed significantly from those in other stations (Tukey HSD post hoc test; p < 0.05).

image

Figure 3. Spatial variations of turbidity, TEPvolume concentration, and TEPcolor concentration in the Pearl River estuary in August 2009 ((a) surface and (b) bottom) and January 2010 ((c) surface and (d) bottom)). TEPvolume concentration was determined by microscopy method and TEPcolor concentration was determined by spectrophotometry method.

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image

Figure 4. Chl a concentration in the Pearl River estuary in August 2009 ((a) surface and (b) bottom) and January 2010 ((c) surface and (d) bottom).

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[20] During the January 2010 cruise, TEPcolor concentration ranged from 88.7 to 1,586.9 μg Xeq.L−1, with an average of 827.6 μg Xeq.L−1 (Figures 3c and 3d). High surface concentrations were found at S4 (1,586.9 μg Xeq.L−1) (Figure 3c). Near-bottom concentrations ranged from 671.7 to 1,352.6 μg Xeq.L−1 (Figure 3d). In the upstream region of station S3 (Zone 1), TEPcolor concentration at the surface was much lower than that in the near-bottom (t-test; p = 0.026). Difference of TEPcolor concentrations between the surface and the bottom was not significant at Zones 2 and 3 (t-test; p = 0.85), but TEPcolor was significantly higher in Lingdingyang estuary (Zone 2) than those in Zone 1 (usually below 1,000 μg Xeq.L−1) and in Zone 3 (Tukey HSD post hoc test; p <0.05). TEPcolor concentrations were significantly correlated with turbidity during the dry season, with a r2 of 0.86 for the surface and 0.61 for the bottom waters, respectively (p <0.001) (Figures 5c and 5d), which however was not found in summer time (Figures 5a and 5b).

image

Figure 5. Relationship between TEP concentrations and turbidity in the Pearl River estuary during August 2009 ((a) surface and (b) bottom) and January 2010 ((c) surface and (b) bottom).

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[21] Significant differences of surface TEP concentrations (t-test; p < 0.05) were found between the two cruises representing two different seasons, summer and winter. However, the maximum of surface TEP concentrations for each cruise was mostly associated with water salinities close to 15 (13.5 in S7 in the summer and 15.6 in S4 in the winter) (Figure 3). We also found that the distribution of TEPvolume was generally consistent with that of TEPcolor concentration. In present study, there was no significant correlation found between the Chl a content (Figure 4) and the TEPcolor concentration (n = 32, r2 = 0.056, p > 0.05).

3.3. Temporal and Spatial Distribution of TEP Size

[22] Figures 6a and 6b suggested that the dominant sizes of TEP shifted to larger particles along the longitudinal section during the August 2009 cruise. Predominant sizes ranged between 2 and 30 μm (Figures 6a and 6b). In the upstream region (Zone 1), small sizes were dominant (<5 μm). The dominant size increased to about 10 μm at S4 and about 15–30 μm at S7 and S8 together with increase in salinity and cation concentrations (Figure 6a). In Zone 3, the size distributions of TEP were bimodal, mainly in the range of 15–30 and 80–100 μm (Figures 6a and 6b). During January 2010 dry season, the dominant sizes were larger than those during the wet season except for the upper water in the upstream region of the river (<5 μm) (Figures 6c and 6d). At Zones 2 and 3, the size distribution was bimodal, with sizes of 20–40 μm and 90–150 μm (Figures 6c and 6d).

image

Figure 6. Seasonal variations in TEPvolume concentration per size class equivalent spherical diameter (ESD in μm) (μm3 mL−1 μm−1) in the Pearl River estuary: (a) surface in August 2009, (b) bottom in August 2009, (c) surface in January 2010, and (d) bottom in January 2010.

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3.4. Experimental Results

[23] Compared with the control group (with Ca2+ concentration of 0.8 mmol L−1), enrichment of Ca2+ enhanced TEPvolume concentration and size after 30 min (Figures 7a and 7b). About 30 min after filtration, the dominant size varied from 1 to 2 μm at 0.8 mmol L−1 and to 10 μm at 10 mmol L−1 (Figure 7a). TEP size continued to grow with time, reaching an average equilibrium size of 15 μm in 1 h at 10 mmol L−1 (Figure 7b). Although the difference in TEPvolume concentration among the groups (Ca2+ >6 mmol L−1) was not significant (t-test, p >0.05) (Figure 8). However, the dominant ESD continued to increase, suggesting that Ca2+ with concentration greater than 6 mmol L−1 also will affect TEP coagulation (Figures 7a and 7b).

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Figure 7. Variation of Ca2+ concentration with TEPvolume concentration per size class (ESD in μm) (μm3 mL−1 μm−1): (a) 30 min and (b) 1 h.

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image

Figure 8. Variation of Ca2+ concentration with TEPvolume concentration (ppm, μm3 mL−1): (a) 30 min and (b) 1 h.

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3.5. Relationship Between TEP and TOC

[24] The results indicated that TEP were an important pool of organic carbon in the Pearl River estuary. In August 2009, the percentage of CTEP in the TOC pool varied from 5.05% to 14.87%, with an average of 9.76% (Table 2). During the January 2010 cruise, the low river flow in winter reduced terrestrial organic carbon into the river; TOC concentration in winter (average 322.8 μmol L−1) was much lower than that in summer (average 560.7 μmol L−1) (t-test; p <0.05). Therefore, we observed a higher percentage of TEP-C in TOC in January than in August, particularly in Zone 2 (>20%). During both cruises, high percentages of TEP-C in TOC were found in places with high TEP, for instance, S4 and S5.

Table 2. Contribution of TEP-C to TOC in the Pearl River Estuary During August 2009 and January 2010 Cruises
StationsAugust 2009January 2010
SurfaceBottomSurfaceBottom
S15.05%6.56%1.26%11.75%
S26.01%9.04%1.65%11.98%
S38.12%6.17%7.05%12.44%
S410.15%14.87%24.13%18.59%
S512.35%11.35%25.47%25.85%
S67.69%10.47%15.55%23.31%
S711.96%12.98%7.85%12.82%
S812.87%10.57%13.52%14.20%

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Concentration of TEP in the Pearl River Estuary

[25] It has been suggested that about 50% of TEP sample from the Santa Barbara Channel passed through 0.6-μm pore size, whereas they could be retained on the 0.2-μm filter [Passow and Alldredge, 1995]. In this study, we found that average concentration of TEP collected by 0.2-μm filters was 42% higher than that by 0.4-μm filters in the Pearl River estuary. We also found that about 30% of TEP in the Pearl River estuary exists in the size of 0.2–0.4 μm. Results of the North Carolina's Neuse River Estuary where 0.2 μm filter was used for TEP collection suggested that more TEP would be retained on the smaller pore size filters due to TEP's quasi-particulate nature [Wetz et al., 2009]. Furthermore, Passow and Alldredge [1995] had clearly verified that TEP collected with a 0.2-μm filter were not an artifact generated by filtration [Passow and Alldredge, 1995]. Therefore, our findings here supported the previous results of Passow and Alldredge [1995] and Wetz et al. [2009] and suggested that 0.2-μm filtration technique may be a better approach for collecting TEP samples, since it can retain a large amount of small TEP that has been missed by the 0.4 μm filtration.

[26] TEP concentrations were generally higher in the Pearl River estuary than those in the open sea due to higher nutrient concentrations and phytoplankton biomass [Passow, 2002a]. There were several studies on TEP in estuaries found in literature, including North Carolina's Neuse River Estuary [Wetz et al., 2009], Bach Dang estuary [Mari et al., 2012] and the estuarine-lagoon system of Cananéia–Iguape [Barrera-Alba et al., 2008]. Compared to Cananéia–Iguape (TEPcolor maximum of 681.2 μg Xeq.L−1, 0.4-μm filter) [Barrera-Alba et al., 2008], TEP concentrations in our Pearl River estuary were much higher (maximum of 1150 μg Xeq.L−1 using a 0.4-μm filter). The TEPvolume concentrations reported in this study was comparable with those of Bach Dang estuary in the wet season [Mari et al., 2012]. However, our TEP concentrations (TEPcolor maximum of 1727.4 μg Xeq.L−1, 0.2 μm filtration) in the Pearl River Estuary are much lower than those in North Carolina's Neuse River Estuary (TEPcolor >3500 μg Xeq.L−1, 0.2 μm filtration), although the total Chl a was very high in the Pearl River estuary during the summer cruises. The large variability of TEP concentrations among different estuaries could be attributed to the difference in the resident time of the estuary waters [Mari et al., 2007]. The longer renewal rate of the water masses in the Neuse River Estuary (average 51 days) would favor the accumulation of TEP [Christian et al., 1991]. However, TEP was rapidly exported in the Pearl River estuary, where water mass had a low residence time (about 30 day in the dry season and 4–9 day in the wet season) [Han, 1998].

[27] TEP maximum during the two cruises was found at S4 (near the Humen outlet), which is consistent with the trend of decreasing monocarbohydrates and increasing polycarbohydrates from upper estuary to Humen [B. Y. He et al., 2010]. This result should indicate that aggregation and coagulation induced by substantial TEP precursor would lead to the transformation of dissolved organic carbon to particle organic carbon. In addition, we observed a clear decline of TEP concentrations in the surface from intermediate salinity water to seawater during both cruises. Decrease of TEP concentrations from S7 to S8 at the surface (535.8 μg Xeq.L−1) resulted in removal of 40.5% of TEP in August 2009. In contrast, the magnitude of reduction from Zones 2 to 3 (727.4 μg Xeq.L−1) for surface water was higher in January 2010 than in August 2009, reaching 63.4%. This decline indicated intense trapping of TEP in the estuary. TEP aggregation may therefore be one of the major removal mechanisms for high molecular weight organic matter in estuary.

[28] No correlation was found between TEPcolor concentration and the total Chl a, which may be due to delay between the peaks of phytoplankton bloom and TEP. It is possible that there may be a lag between bloom peak and TEP peak, because it usually will take some time for the TEP precursors that are released by phytoplankton to coagulate into TEP particles. On the other hand, the lack of correlation between TEP and Chl a may also be attributed to bacteria activity, which is generally high in the Pearl River estuary given the high phytoplankton biomass and organic carbon concentrations. It was reported that bacterioplankton could modify exopolymer into biologically refractory dissolved organic matter [Barrera-Alba et al., 2008; Rochelle-Newall et al., 2010]. Future studies therefore may need to focus on the combined effects of bacteria and phytoplankton on TEP formations and removals in tropical and sub-tropical estuaries.

4.2. Effect of Ca2+ Concentrations on TEP Formation and Distribution

[29] Divalent cations play an important role on the formation and sustention of TEP, as evidenced by the fact that EDTA chelation of cations in seawater samples could induce TEP disaggregation [Chin et al., 1998]. Wetz et al. [2009] found that addition of 5 mmol L−1 Ca2+ (as CaCl2·2H2O) and 12 mmol L−1 Mg2+ (as MgCl2·6H2O) to the water samples could increase the concentration of TEP in the North Carolina's Neuse River estuary [Wetz et al., 2009]. The effect of divalent cations on TEP formation would be influenced by phytoplankton bloom. To eliminate the interference of phytoplankton bloom, TEP in our Pearl River estuary water samples were filtered before the addition of divalent cations. Our experimental results indicated that addition of Ca2+ could induce TEP assembly and size of TEP increased with Ca2+concentration.

[30] In estuaries, divalent cations were directly related to salinity and determined by the volume ratio of fresh water to saline water [Han, 1998; Surge and Lohmann, 2002]. Salinity and mixing intensity of the water column were found positively correlated with TEP in the surface water of the North Carolina's Neuse river estuary [Wetz et al., 2009]. This finding should indicate that divalent cations could be one of the key factors for TEP formation in estuarine environments. The positive correlation between TEP concentrations and Ca2+ concentration (r = 0.78, p < 0.05) was found in area with surface salinity < 16 except S8 in summer where dilute marine water may decrease the TEP concentration. In higher salinity waters, the stickiness of TEP that is directly linked to salinity parameters (i.e., divalent Ca2+ concentrations) will be higher, leading to increase of TEP sizes and subsequent removal of TEP from water column [Mari et al., 2012].

4.3. Relationship Between TEP and Turbidity

[31] In the surface of the sediment of the Pearl River estuary, polysaccharide (including TEP precursors) was an important component of organic carbon [B. He et al., 2010] and are energy and carbon sources for many heterotrophic organisms in the sediment [Haynes et al., 2007; Middelburg et al., 2000]. It was reported that increase of bio-alteration or biodegradation of polysaccharide by bacterial heterotrophs would generate stickier TEP with older DOM [Rochelle-Newall et al., 2010]. These TEP or their precursors are more physical reactive (i.e., higher stickiness) and would be transported upward from sediment into water columns through resuspension at the middle and lower parts of the estuary [Thornton, 2009; Wai et al., 2004]. These processes enhanced the aggregation and flocculation of the suspended particles. Positive correlation between TEP and turbidity was observed during the well mixed dry season, which indicated that sediment might act as a source of TEP to water column [Thornton, 2009]. Since the physical influences would be reduced or stopped when the estuary was partially mixed or stratified, surface TEP found under these conditions (wet season with reduce vertical mixing) should therefore come from other sources, such as surface phytoplankton production. It had been reported that diatom blooms enhanced flocculation speed and efficiency in the Seine estuary [Verney et al., 2009]. It is thus reasonable to expect that TEP, a product of diatom production, may also be responsible for flocculation in estuaries.

4.4. TEP Distribution and the Mixing Processes

[32] Study of the aggregation and size distribution of TEP in estuaries is often complicated by complex physical flows and fluctuations associated with the change of cation. A conceptual diagram for processes regulating surface and bottom TEP in the Pearl River estuary is shown in Figure 9. The flow regime in August 2009 was partially mixed with the low salinity surface water in the estuary bay, showing a decrease of Ca2+ concentration at the surface from the inlet to the head of the estuary and an increase of TEP concentration and size from upper to lower estuary. However, the concentration of TEP was decreased dramatically in the nearshore waters, which may be due to either the dilution of seawater or the removal of TEP by the sinking of the more dense and compacted large-size particles [Mari et al., 2012]. This pattern of TEP distribution in the surface layer agreed well with those found in North Carolina's Neuse River estuary and Bach Dang estuary [Mari et al., 2012; Wetz et al., 2009]. At the bottom layer, where saltier water prevailed, the hydrographic and biochemical conditions differed substantially from those above. Physical processes were found more important than divalent cations at the bottom of partially mixed areas, such as S4, near the head of the estuary (Humen). S4 was located in the null point that was between the fresh water and salt water, where the aggregation of upper stream TEP precursors would take place. On the other hand, the other source of TEP at S4 might come from the bottom water of the lower estuary, based on the measurement of sediment transport flux in the Pearl River estuary by Wai et al. [2004]. Their results suggested that the sediment is generally transported seaward at Humen (upstream of S4) in the wet season. At the region of 12KM downstream of Humen outlet (downstream of S4), sediment in the lower layer however is transported back landward [Wai et al., 2004]. In the middle and lower estuary, reduced mixing and lack of vertical gradient below halocline would lead to low turbulent intensity [Wolanski, 2007], which could increase the TEP residence time and allow the aggregation to grow to larger sizes. Consequently, TEP at the bottom of the lower estuary were dominated by large particles.

image

Figure 9. Conceptual diagram for TEP distributions in the Pearl River estuary under partially mixed and well mixed estuary.

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[33] During the winter cruise, water mass in the Pearl River estuary was better mixed due to lower runoff and stronger northeast winds. The winter monsoon has a monthly average wind speed of 7–10 m s−1 in the northern SCS [Li et al., 2006], which could generate strong turbulence in the Pearl River estuary [Orton and Visbeck, 2009]. In addition, Ca2+ concentrations were generally much higher in dry season than in wet season. Although phytoplankton biomass was low during dry season, the high coagulation rate induced by strong turbulence and the high cation concentration in the surface layer had resulted in higher surface TEP concentrations in winter than in summer in the Lingdingyang Bay (Zone 2). During dry seasons, resuspension process was mostly driven by tidal currents and wind-forcing in the middle and lower estuary [Wai et al., 2004]. A large amount of dissolved acid polysaccharides (TEP precursor) could be released from sediment during sediment resuspension [Thornton, 2009]. Therefore, our observation of TEP maximum in the water column at station S6 in bottom layer was very likely induced by the resuspension process.

5. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[34] In this study, TEP was found to be an important component of total organic pool in the Pearl River estuary, and the percentage of TEP in total organic pool (carbon content) show higher values (with an average of 14.21%) in dry season than in wet season, which may be due to decrease of anthropogenic and terrestrial inputs in wet season. Our experimental results suggested that addition of Ca2+ could enhance TEP assembly and size of TEP increased with Ca2+ concentration. This finding was consistent with the field observation of increasing TEP concentrations and sizes from fresh waters to seawaters, except for the lower estuary where TEP could be removed by the seawater dilution or sedimentation process. In the summer cruise, TEP maximum in the bottom tended to occur at the tip of salt wedge when the estuary was partially mixed. During the well mixed dry seasons, TEP concentrations showed positive correlations with turbidity. These results suggested that increase of sediment resuspension of TEP (and/or theirs precursors) by stronger wind and tidal current may be responsible for the formation of turbidity maximum with enhanced aggregation and flocculation. These findings suggested that hydrodynamic process might be an important factor for TEP distribution in the bottom layer. Different pattern of TEP during two cruises can be attributed to difference of physical process (i.e., mixing type) in estuaries. The formation and distribution of TEP were largely influenced by interaction between physical and biogeochemical processes in the Pearl River estuary.

[35] To better understand the role of TEP in aggregation and sedimentation and their contributions to carbon cycle in estuaries and adjacent continental shelves, future study may need to address the relationship of TEP with other important factors including bacterial effect, metal concentrations, clay or mud concentrations, and physical forcing, such as one tidal cycle. These are also important steps necessary for developing a fundamental model of TEP aggregation, coagulation, and sedimentation in estuaries eventually.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[36] This research was supported by the National Natural Science Foundation of China (41106106, 41076070 and 41176101), the projects of knowledge innovation program of Chinese Academy of Sciences (KZCX2-YW-Q07-02, KSCX2-YW-Z-1024, KSCX2-EW-G-12C and SQ201005).

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

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