Partition of suspended and riverbed sediments related to the salt-wedge in the lower reaches of a small mountainous river

Marine Geology 264 (2009) 152–164

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Partition of suspended and riverbed sediments related to the salt-wedge in the lower reaches of a small mountainous river James T. Liu ⁎, Jia-Jang Hung, Ya-Wen Huang Institute of Marine Geology and Chemistry, National Sun Yat-sen University Kaohsiung, Taiwan 804-24, Republic of China

a r t i c l e

i n f o

Article history: Received 13 November 2007 Received in revised form 24 April 2009 Accepted 29 May 2009 Communicated by J.T. Wells Keywords: salt-wedge dynamic barrier hydrodynamic sorting particle size flocculation carbon–particle relationship

a b s t r a c t A month-long comprehensive field experiment during the beginning of the flood season of a small mountainous river in southern Taiwan was carried out in 2004. The major goal of the study was to investigate the effect of hydrodynamic sorting related to the salt-water intrusion on the spatial variability of suspended and riverbed sediments and their coupling of different sizes. The experiment included the deployment of an instrumented tetrapod near the river mouth with an upward-looking ADCP and two CTDs mounted at 50 and 100 cm above the bed (cmab), respectively. On three different days along the river, turbidity, salinity and temperature of the water column were profiled; and water samples were taken from the surface and near the bed at different stations. Additionally, one sediment sample was also taken from the riverbed. Suspended sediment concentrations (SSC) were analyzed for five different sizes, i.e. N 500, 250–500, 63–250, 10–63 and 1.2–10 μm by filtration. For each riverbed sample the size-composition was analyzed for the subsample that contained lithogenic and nonlithogenic components; and the subsample having major nonlithogenic components (organic matter, carbonates, and biogenic opal) was removed. The grain-size frequency distributions of the riverbed samples were analyzed using a laser particle analyzer. The results were grouped into the following size classes: N473, 249–473, 62–249, 10–62, and b 10 μm for comparison with those of the suspended sediments near the riverbed. Some suspended sediment samples were analyzed for POC and PON. Some riverbed samples were analyzed for TOC. Statistical methods of linear regression and Empirical Orthogonal (Eigen) Function (EOF) were used in the data analysis. Two-layered estuarine circulation pattern was observed at the tetrapod site. The tidally-driven salt-water intrusion is the major factor influencing the hydrodynamics of the Gaoping River, which in turn, affect the longitudinal and vertical distribution of the suspended sediments and the longitudinal distribution of the riverbed sediments. During the flood, the intrusion front of the salt-wedge creates a dynamic barrier. Upriver from this barrier, the riverbed substrate is coarser, composed of sediments mostly coarser than 249 μm. Within the salt-wedge the riverbed substrate is finer, consisting of mostly mud (b62 μm). The barrier creates a trap on the riverbed immediately seaward from the intrusion front, retaining higher percentages of claysized sediments and TOC. The barrier also creates partition in the terrestrial and marine sources of organic matter in the suspended and riverbed sediments. Within the salt-wedge the major contributor of riverbed TOC is the clay-sized marine sediment transport upriver by the intruding seawater. The terrestrial POC is a minor contributor to the riverbed TOC. The riverbed and suspended sediments are coupled. Most size-classes in corresponding suspended and riverbed sediments have a reciprocal relationship (negative feedback) through resuspension and deposition on the water–bed interface. The only size-class of 62–473 μm on the riverbed and 63–250 μm in the suspension are co-varying (positive feedback). This size class contains largely a transient floc population that is formed and disintegrated in-situ both on the riverbed and in the suspension in the course of a tidal cycle. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: +886 7 525 5144; fax: +886 7 525 5130. E-mail address: [email protected] (J.T. Liu). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.05.005

Suspended particles in estuarine and coastal waters are important carriers of lithogenous materials (silicate minerals and rock fragments), biomass (phytoplanktons, Jennerjahn et al., 2004; Small and Prahl, 2004), heavy metals (Salomons and Forstner, 1984; GESAMP, 1987; Hung and Hsu, 2004), organic substances (Bianchi et al., 2007;

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Goni et al., 2005; Leithold and Blair, 2001; Leithold et al., 2005), and other chemicals that come from either natural or anthropogenic sources. When riverine particles enter the coastal sea, transformations of carbon–particle relationship take place (Keil et al., 1997). Riverine suspended particles have been found to unload a large portion of sorbed terrestrial carbon upon entering the ocean and gradually reload to similar levels with marine carbon (Leithold and Blair, 2001). Particles exported by rivers especially under flood conditions could be identified in continental shelf deposits by many signatures some of which include fine-grained texture having high clay content (Fan et al., 2004; Leithold and Hope, 1999; Leithold and Blair, 2001; Mullenbach and Nittrouer, 2000) and high organic carbon content (Allison et al., 2000). In a detailed review of the role of suspended particles in estuarine biogeochemical cycles, Turner and Millward (2002) classify suspended particles into two major categories: seston and suspended sediments. Seston includes mostly biogenic substances formed in situ. Suspended sediments comprise a wide range of substances including minerals, biogenic and anthropogenic materials (Turner and Millward, 2002). In this paper, we will follow this designation when we mention suspended sediments. Prior to being exported to the coastal sea, terrestrially derived suspended sediments first enter transitional environments such as estuaries. In estuaries, suspended sediments have many properties (Uncles et al., 2006a), of which the most important is the grain size since it determines the fate and transport of suspended sediments and particle adherent contaminants (Fugate and Friedrichs, 2003). Due to its large surface area per unit size, fine-grained particles such as clay have been shown to preferentially adsorb contaminants (Ganju et al., 2004; Turner and Millward, 2002; Uncles et al., 2006b) and organic matter (Goni et al., 2003). Thus, fine-grained particles (silt and clay) play important roles in determining the fluxes and biogeochemical cycles of terrestrial materials through the estuary. However, recently Bianchi et al. (2007) found sandy sediments in the Mississippi River that also provides a pathway equally important to fine-grained sediments for the transport of terrestrially derived organic substances. Many factors influence the size of estuarine suspended sediments in space and time, including the concentration of suspended sediments; clay mineralogy of the primary silicates; physical and physiochemical processes that affect the formation and break up of flocs and aggregates; and biological processes that may also enhance particle aggregation and break down the large particles (Fugate and Friedrichs, 2003; Manning and Bass, 2006; Manning et al., 2006; Turner and Millward, 2002; Wolanski and Spagnol, 2003). The formation and destruction of aggregates may therefore determine the settling fluxes of suspended sediments (Manning and Bass, 2006), and consequently the texture and chemical composition of bottom sediments. Flocculation and grain-size related hydrodynamic sorting in the lower reaches of a river and its estuary could affect the nature of the suspended sediments exported to the coastal sea (Bianchi et al., 2007; Fox et al., 2004; Gordon and Goni, 2004; Thill et al., 2001). The surficial texture in estuaries could be affected by sedimentological, biological, and chemical processes that take place at the sediment–water interface (Garcia et al., 2005). On a larger spatial scale, the watershed, especially of small mountainous rivers that have episodically large floods, controls the properties of estuarine sediments exported to the coastal sea (Leithold and Blair, 2001). Small mountainous rivers located in tectonically active regions contribute nearly half of the global flux of fluvial sediments and carbon to the global ocean (Leithold et al., 2005; Milliman and Syvitski, 1992). It is estimated that over eighty percent of this sediment is provided by rivers on high-standing islands of south Asia and Oceania, including Taiwan, Indonesia, Papua New Guinea, and New Zealand (Leithold and Blair, 2001). Yet, not enough attention has been given to the study of the transport of suspended particles of small mountainous rivers (Maneux et al., 1999). Furthermore, few studies have focused on grain-size related hydrodynamic sorting in

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estuaries of small mountainous rivers and how they affect the carbon– particle relationship. Therefore, the goal of this study is to investigate the coupling between suspended and riverbed sediments through the grain size and organic content in the lower reaches of a small mountainous river to understand the effect of hydrodynamic sorting related to the salt-water intrusion. 2. Study area and background The Gaoping (formally spelled Kaoping) River (KPR) is the largest river in Taiwan (Fig. 1a) in terms of drainage area (3257 km2), the estimated mean annual runoff (8.45 × 109 m3), and the second largest in terms of suspended sediment load (4.9 × 107 T). The sediment yield of the river is about 15,000 T/km2/yr. The headwater of the river originates in the southern part of the Central Mountain Range near Mt. Jade whose elevation is 3997 m above the sea level. Based on the elevation, 47.45% of the drainage basin is above 1000 m; 32.38% is between 100 and 1000 m, and 20.17% is below 100 m. Consequently, the riverbed gradient is 1:15 in the upper reaches, 1:100 in the middle reaches, and 1:1000 in the lower reaches, having the average of 1:150 (7th River Management Office, Water Resource Agency, Ministry of Economic Affairs). Due to the influence of the monsoon climate and typhoons, the annual discharge of the Gaoping River is concentrated in the summer season and early fall (June to October), culminating in August (Liu et al., 2002). About 91% of the annual discharge occurs in the flood season. The physical and chemical weathering rates of the Gaoping River watershed are not only higher than the world average but also higher than the average of small mountainous rivers (Hung et al., 2009). The river is also subject to increasing pollution from the mid- to lower reaches (Hung and Hsu, 2004). There have been no systematic and comprehensive studies of the hydrodynamics and sediment dynamics in the lower reaches of the river. Since the KPR is the major source for terrestrial input to the Gaoping Submarine Canyon (Hung and Hsu, 2004; Liu and Lin, 2004; Liu et al., 2006), it is important to characterize the physical and geochemical nature of suspended particles in the lower reaches of the river before one could quantify its net fluxes to the sea. 3. Field experiments A month-long comprehensive field sampling and monitoring between May 25 and June 25, 2004 during the flood season of the river were carried out in the lower reaches of the KPR (Fig. 1). The major aim of the field experiment was to identify the grain-size composition and organic content of suspended and riverbed sediments and their spatial and temporal variability. 3.1. Tetrapod deployment An instrumented-tetrapod was deployed in the river near the river mouth in the deepest (still-water depth about 2.5 m) river-dominated part of the channel (Fig. 1a). There were two levels of instrumentation at 50 and 100 cmab (cm above bed), respectively. At the lower level there were an upward-looking ADCP (Aquadopp) with bin size of 10 cm and a CTD with an OBS (optical backscatter sensor). At the upper level there was another CTD. The sampling rate was 1 h for all instruments. 3.2. Hydrographic profiling and water and substrate sampling along the river On May 27, June 5, and June 18, salinity, temperature, and turbidity profiles were measured along the river using a CTD with OBS (XR-420 by Richard Brancker). At each station, multiple water samples (10 l each) at surface and near the riverbed (within 5–10 cm from the bed) were taken by personnel from three different laboratories, and one sediment

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Fig. 1. Map showing the deployment site of the instrumented tetrapod and all the sampling stations in the lower reaches of the Gaoping (formally spelled Kaoping) River (KPR) between May 25 and June 25, 2004. Each station label indicates the general location for the three different hydrographic surveys and water and substrate samplings. The insert is a larger-scaled map showing the island of Taiwan.

sample from the riverbed was also taken. There were 10 stations on May 27 and 11 stations each on June 5 and 18 (Fig. 1). 4. Laboratory analyses 4.1. Analyses of water samples Each water sample was filtered sequentially using steel sieves of four different mesh sizes of 500, 250, 63, and 10 μm. Contents retained by the sieves were then washed with distilled deionized water (DDW) to remove sea salt and dried in an oven at 60 °C and weighed to determine the suspended sediment concentration (SSC) in mg/l. The residue after sieving, was again filtered with pre-weighed membrane filter (Nucleopore PC, 1.2 μm) driven by a peristaltic pump. The residue on the filter was washed with DDW to remove sea salt. The washed filter was dried in an oven at 60 °C and then re-weighed. The total suspended material (TSM) in mg/l of different water samples taken on May 25 and June 18 along the river at the same stations and depths were also analyzed by a different laboratory. The same procedure described above was followed, except that only one

filter size of 0.4 μm was used in the filtration. Therefore, in principle, the sum of the 5 SSC values would be closely equivalent to the TSM at the same location. For the sake of clarity in this paper, SSC size-classes will be used to describe partitioned suspended sediment concentration; and TSM for total concentrations of suspended sediments at the same station. The particulate organic carbon (POC) and nitrogen (PON) in the water samples were analyzed by following the same procedure described by Liu et al. (2009). The precisions of POC and PON analyses were ±0.2 μM C and ±0.3 μM N (±1s), respectively, as evaluated from eight replica samples.

4.2. Analyses of riverbed sediment samples 4.2.1. Grain-size analysis of subsamples having all components A subsample of each original riverbed samples was analyzed for the grain-size frequency distribution following the same procedure described in Liu et al. (2009). For the convenience for later comparison with suspended particles, grain-size frequency distribution output of

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Fig. 2. Progressive vectors of the measured instantaneous hourly flow in all the bins between 60 and 410 cmab (cm above the bed) by the upward-looking ADCP on the tetrapod.

LS100 was grouped into the following size classes: b10, 10–62, 62– 249, 249–473, and N473 μm.

superimposed on a two-layered estuarine circulation pattern at the site of the tetrapod.

4.2.2. Grain-size analysis of subsamples having only lithogenic components To obtain the size-composition of only lithogenic sediments, the following components were removed sequentially from the subsample of a riverbed sample: organic substances, CaCO3, and biogenic opal, using the same methods described by Liu et al. (2009). Then, the subsample was analyzed for the grain-size frequency distribution following the same procedures as the original subsamples described before. The output of the LS100 was also grouped into the same 5 sizeclasses.

5.1.2. Sea surface fluctuation and salinity variations with respect to the river discharge and tide The M2 tide was the dominant constituent in both the sea surface (about 22.2 cm) fluctuations and the flow (about 15 cm/s) in all the ADCP bins. The phase of the flow preceded that of the sea surface by about 70°, suggesting that the flooding seawater caused the water level to rise in the river mouth. At 100 cmab, in the course of a tidal cycle the salinity fluctuated widely, having the maximum value corresponding to the flood tide and the minimum value in the ebb (Fig. 3a,b). The upper limit fluctuated between 28 and 33, probably representing the salinity of the coastal water. The lower limit, however, had a much wider range between 0 and 28. The timing of the increase of salinity difference and zero value of the lower salinity often coincided with the ebb, spring tide, and increased river runoff (Fig. 3b,c). The salinity difference between 50 and 100 cmab at the tetrapod site also shows the same pattern of river-modulated tidal variations (Fig. 3c). During the flood, the value was usually around 2, which formed the lower limit of the envelope of the salinity difference. During the ebb the stratification intensified, forming periodic spikes with values that exceeded 8, showing the effect of tidal straining (Uncles, 2002). The stratification was the strongest in the spring tide or at strong river runoff events (indicated by the thick over-bars). It was the weakest during events of reduced river runoff (indicated by the thick dashed over-bar). The decreasing trend of the maximum values of the salinity at 100 cmab and the corresponding increasing trend of the lower values of the salinity difference toward the end of the observation period suggest the increased river runoff as the flood season progressed.

4.2.3. Total organic carbon (TOC) analysis Only the riverbed sediment samples taken on July 18 were analyzed for TOC. Each sample was first screened through a nylon sieve to remove particles bigger than 1 mm. A portion of the sediment sample was washed with Milli-Q water to remove any remaining sea salt and then dried in an oven at 60 °C for at least 24 h. The dried sample was ground to powder using an agate mortar and pestle. Total organic carbon (TOC) was determined with a C/N/S analyzer (Fisons NCS 1500) after removing the inorganic carbon with hydrochloric acid (Hung et al., 2000). 5. Results 5.1. Time series measurements on the tetrapod 5.1.1. ADCP measurements The upward-looking ADCP had 35 bins. The observed vertical structure of the river flow is expressed in terms of progressive vector of the instantaneous hourly flow in each bin (Fig. 2). Basically, there were three layers. The flow in the top 110 cm of the water column was affected by the southerly monsoon wind and deflected to the east. Below this layer is the core of the river runoff about 90 cm thick, flowing straight downriver. About 180 cmab, the flow began to be affected by the combined effect of bottom friction and the tide, showing reduced magnitude and periodic reversals (upriver). The flow in the bottom-most bin (60 cmab) was entirely upriver. However, the bottom layer has the strongest velocity gradient. The characteristics of this structure indicate a wind-influenced surface layer

5.2. Spatial observations along the river The along-river profiling and sampling roughly took about 3–5 h between 10:00 and 15:00 each day during different stages of tide. It coincided with the end of flood tide on May 27, and with the ebbing tide on June 5, and flooding on June 18. On May 27, the fieldwork encompassed the end of the flood and the beginning of the ensuing ebb (Fig. 4a) as indicated by the ‘flat curve’ of measured salinity and temperature at 50 and 100 cmab on the tetrapod.

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Fig. 3. Temporal fluctuations of hourly measurements on the tetrapod including (a) water depth, (b) salinity at 50 and 100 cmab, and (c) the salinity difference between measurements at 50 and 100 cmab (the shaded area shows the range of variability), respectively. The inverse triangles indicate times of the three hydrographic surveys. The solid over-bars indicate episodes of strong river runoff and the dashed over-bar indicates the episode of significantly reduced river runoff.

The fieldwork on June 5 occurred during the ebb and also encountered strong river runoff such that near the end of the sampling, the salinity at 100 cmab was almost zero at the tetrapod site (Fig. 4b). Salinity stratification intensified as the ebb progressed. The decreasing water temperature indicates that the river water was colder than the seawater in early June. On June 18 the fieldwork coincided with ebb tide again and salinity stratification also occurred (Fig. 4c). However, the increasing temperature at the tetrapod site suggests that the river water was warmer than the seawater in the middle of June with the onset of summer. 5.2.1. Longitudinal variability of water-born constituents The full complements of the river water constituents at each station on June 18, May 27, and June 5 are presented by the date in columns (Fig. 5a–c;d–f;g–i), respectively. Since the data set of June 18 (during the ebb) is the most complete (Table 1), for the sake of future discussion this data set is presented first. The data of each day begins with the surface and near bottom salinity and turbidity (Fig. 5a,d,g). On June 16 the salinity and turbidity measurements indicate that the turbidity is closely related to the seawater intrusion whose landward limit was located at St. 8 (Fig. 5a). The surface and near-bed SSC of the five size-classes and TSM are plotted cumulatively in an upwardfining sequence (Fig. 5b,c). The top line of the size class of 1.2–10 μm indicates the sum of the 5 SSC size-classes (equivalent to total TSM). The majority of suspended sediments in the study area are finegrained particles and the 1.2–10 μm size-fraction has the highest concentration at all stations both at the surface and near the bed

(Fig. 5b,c). In general, across the size-classes the suspended sediments have higher concentration near the bed than at the surface except at Sts. 9 and 10 (Fig. 5b,c). The POC, PON values follow closely the SSC of the 1.2–10 μm sizeclass (Fig. 5b,c), showing high association of organic matter with the finest size-fraction (Bianchi et al., 2007; Goni et al., 2003). This suggests that suspended sediment particles finer than medium silt are carriers of organic substances. The values of TSM and the sum of the 5 SSC do not match exactly because these were analyzed from different water samples. However, the turbidity values do follow the same patterns of the total SSC more closely (Fig. 5a,b,c). The SSC plots of May 27 (Fig. 5d–f) and June 5 (Fig. 5g–i) have similar formats without the 1.2–10 μm size-class. On May 27 the gap between the TSM and the cumulative SSC up to 10 μm is roughly equivalent to the SSC of the 0.4–10 μm size-class (not measured). This gap also indicates suspended particle finer than 10 μm comprised the largest size-class both at the surface and near the bottom (Fig. 5e,f). There was a higher amount of the size-class finer than 10 μm seaward of the seawater intrusion (Sts. 5–6) at both the surface and near the bottom. During the ebb and strong river runoff on June 5 (the near bottom sample at St. 1 was excluded for contamination by the riverbed substrate), the SSC value of each size-fraction was generally one order of magnitude higher than their counterparts on the other two days (Fig. 5h,i). The high SSC values corroborate with the higher FTU values in the turbidity measurements (Fig. 5g). The three data sets suggest the salt-wedge and the river runoff have combined influence on the spatial distribution patterns of the river water constituents.

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components show noticeable change of size-compositions to become finer at Sts. 4 and 7 and coarser at Sts. 8 and 10 (Fig. 6e). One striking difference between the June 5 data set and the two previous ones is the total absence of sizes coarser than 249 μm (Fig. 6f) in the original samples. Upriver from St. 2, the river runoff almost flushed out the saltwater except at St. 6 where a small pocket of seawater (Fig. 6f) was trapped. After the chemical removal the increase of coarse fractions at all stations making the mean particle much coarser is equally striking (Fig. 6g). Detailed cross-comparison among the three data sets will be made later. 6. Statistical analyses 6.1. Empirical Orthogonal (eigen) Analysis (EOF) All the data sets contain spatially correlated variables of water column and riverbed properties taken at various stages of tide. Therefore, a multivariate analysis technique EOF was used first to identify major modes of correlated variance for a sharper focus in the data interpretation. Again, only the June 18 data set is presented. In the analysis, the data set was divided into two subsets. The first subset was focused on the suspended sediment dynamics. This included the salinity (terrestrial-versus-marine physical indicator); turbidity (parameter reflecting the amount of SSC); all the SSC size-classes (fundamental physical property of the particles); and POC and PON (geochemical properties associated with the particles and carbon is also a substance of global interest) at the surface and near bottom of the water column. The second subset was focused on the interface dynamics between near-bed suspended sediments and riverbed sediments. The correlation (normalized covariance) matrix of each subset was analyzed separately using EOF.

Fig. 4. Hourly salinity and temperature measured at 50 and 100 cmab at the tetrapod site from 11:00 to 15:00 on (a) May 27, (b) June 5, and (c) June 18.

5.2.2. Analysis results of riverbed samples Since the June 18 data set has the most variables it is presented first. On this day the grain-size composition of both riverbed sediment samples with and without nonlithogenic components show a marked difference between Sts. 7 and 8 (Fig. 6a,b). Upriver from St. 8, the substrate was composed of material mostly coarser than 249 μm. Downriver from St. 7, the riverbed substrate was composed mostly of mud (finer than 62 μm). The chemical removal of nonlithogenic components altered the grain-size compositions of the original samples in a spatial way. At the seaward-most three stations the loss of nonlithogenic components makes the mean grain-size slightly coarser and the loss was mostly in the silt and clay fractions (Fig. 6b). Upriver from St. 8, the chemical removal makes the sediments noticeably coarser (Fig. 6b) as indicated by the mean grain size. Corresponding to the high clay concentration on the riverbed between Sts. 4 and 7, the percentage of TOC is also elevated (Fig. 6c). On May 27, there were only 10 stations and there was no sample from St. 1. Except at St. 10, the original riverbed substrate was mainly composed of mud (Fig. 6d). The samples without nonlithogenic

6.1.1. The water column subset Since POC and PON are not available at St. 11, data at this station were excluded in the analysis. At each station, 18 variables were used in the analysis (Fig. 7a). This is a highly correlated data set. The first two modes alone explain over 80% of the correlations (Fig. 7a,b). Since the third mode only explains 7% of the data, only the first two EOF modes are presented. The eigenvectors show the groupings of the 18 variables in the data set (Fig. 7a). In the first mode, which describes 61.1% of the data, the negative group is dominated by the surface and near-bottom salinity and all near-bottom SSC size-classes except for the finest size and turbidity. The rest of the variables are grouped by the positive sign. The eigenweightings of the first mode indicate a spatial partition of the stations having the demarcation region between Sts. 7 and 8 (Fig. 7b). This mode describes the longitudinal (landward) decrease of the salinity in the water column and the decrease of the near-bottom concentration of N500, 250–500, 63–250 and 10–63 μm size-classes and the turbidity (in the negative group); and the landward increase of the all the SSC size-classes in the surface water and POC and PON both in the surface and near-bottom, and the 1.2–10 μm in the near-bottom water (in the positive group). Within these two opposing longitudinal trends the salinity stratification drastically decreased and the vertical homogeneity of the SSC increased at the demarcation region except for the finest size-fraction. The second mode explains 21.1% of the data. This mode describes the vertical gradient and the lack thereof in the SSC of individual sizeclasses, turbidity, POC and PON as indicated by the opposite signs in the eigenvector plot (Fig. 7a). This stratification pattern is weighted heavily at St. 2 (stratification), and 10 (vertical homogeneity) (Fig. 7b). Therefore, the second mode is interpreted as the vertical gradient mode. 6.1.2. The interface subset There were 21 variables in the substrate subset having to do with the water–bed interaction. For lack of TOC data, St. 11 was excluded in

158 J.T. Liu et al. / Marine Geology 264 (2009) 152–164 Fig. 5. River water constituents measured on June 18 (a–c), May 27 (d–f), and June 5 (g–i). On each day, the salinity and turbidity measured at surface and near bottom by using XR-420 are presented first. On June 18, the salinity is followed by the cumulative suspended sediment concentration (SSC) of five particle size-classes (N 500, 250–500, 63–250, 10–63, and 1.2–10 μm) in mg/l; TSM (N 0.4 μm); POC and PON (missing St. 11) at the surface (b) and near bottom (c). On May 27, the salinity is followed by the cumulative SSC of four particle size-classes (N500, 250–500, 63–250, and 10–63 μm) and TSM (N0.4 μm) in mg/l at the surface (e) and near bottom (f). On June 5, the salinity is followed by the cumulative SSC of the same four particle size-classes as in May 27 in mg/l at the surface (h) and near bottom (i).

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Table 1 Availability of suspended particle related data set. Sampling day

May 27 June 5 June 18

SSC (mg/l)

POC, PON

N 500 (μm)

250–500 (μm)

63–250 (μm)

10–63 (μm)

√ √ √

√ √ √

√ √ √

√ √ √

TSM (mg/l) N 0.4 (μm)

Turbidity (FTU)

Number of stations

√

√

√

√

10 11 11

1.2–10 (μm)

√

√

Remark: The check mark indicates availability.

the analysis. This set is not as correlated as the water column subset for having greater complexities. It takes 3 modes to explain over 80% of the data (Fig. 7c). The first mode, which explains 47.2% of the data, describes the longitudinal gradient of the decreasing near-bed salinity and the percentage of finer riverbed sediments of 10–62 and b10 μm size-classes (for both original and lithogenic subsamples) (Fig. 7c). The spatial structure of this mode is similar to that of the first mode in the water column subset in that there is a demarcation region between Sts. 7 and 8 (Fig. 7b,d). The second mode explains 24.2% of the data. This mode describes the opposite trends of suspended and the riverbed sediments on the interface, which are especially elucidated by the corresponding suspended and riverbed size-classes that have opposite eigenvector signs including N500 μm (bed N473 μm), 250–500 μm (bed 249– 473 μm), and 10–63 μm (bed 10–62 μm) (Fig. 7c). Along the river, the increase/decrease of SSC values of these size-classes in the water column correspond to the decrease/increase of equivalent size-classes on the riverbed. This suggests an exchange at the interface through entrainment (decrease of percentage on the interface but increase in the concentration in the water column) and deposition (vice versa). The third mode explains 13.4% of the data. The negative group in this mode has only one size size-class of 63–250 μm in the water column and 62–249 μm on the riverbed and the near bottom salinity (Fig. 7c) indicating coupling between the suspended and riverbed sediments of this size through salinity. The positive group on the other hand includes all other suspended and riverbed sediment sizeclasses. The eigenweightings of this mode do not particularly reveal any consistent spatial characteristics, whose interpretation is not straightforward. 6.2. Linear regression analysis Each data set collected on three different days was divided into the water column subset and water–bed interface subset. Only the analysis results on the June 18 data are presented. Within this data set, the water–column related grain-size variables (percentages of 5 size-classes) were regressed against POC and PON percentages and the turbidity measurements for the surface (Table 2a) and near bottom samples (Table 2b). The results for the surface variables show that all size-classes are carriers of POC and PON (statistically significant positive correlations) except for particles greater than 500 μm (Table 2a). Also, suspended particles of all sizes contributed to the measured turbidity (Table 2a). The results are quite different for the near-bottom variables. Only the finest size-class (1.2–10 μm) has significant positive correlation with POC and PON (Table 2b), suggesting only this size-class is the carrier near the bottom of the water column. Furthermore, only the two sizeclasses greater than 250 μm have significant correlation with turbidity, suggesting that the turbidity measurements near the riverbed were influenced by factors other than the suspended sediments (Table 2b). Since salinity comes from the seawater, negative correlations with the salinity imply terrestrial (fluvial) origin, and the positive correla-

tions imply marine origin. The regressions against the salinity show that at the surface the three finer size-classes, total SSC, TSM, POC, and PON have significant negative correlations (Table 2c) implying that they are of terrestrial origins. Near the bottom, only the finest size-class (1.2–10 μm) in the suspended sediments, POC and PON are clearly terrestrial, the other size-classes do not have significant correlations with the salinity (Table 2c). The coupling between comparable size-class of the near-bed SSC and riverbed sediments was explored using the linear regression analysis. The results show that positive water–bed coupling only exists in the 62–473 (62–250 μm) original size-class (Table 3a). The same result is also indicated in the third EOF mode of the interface subset (Fig. 7c). Results of regression against the salinity show that the two coarsest size-classes on the riverbed (N473 and 249–473 μm, original and lithogenic subsamples) have significant negative correlations, suggesting terrestrial origin (Table 3b). On the other hand, the two finest size-classes (10–62 and b10 μm) suggest marine origin (Table 3b). Only the 62–473 μm size-class is neutral with the salinity (no correlation, no trend, Table 3b). This singularity is also depicted in the third EOF mode of the interface subset (Fig. 7c). 7. Discussion In the lower reaches of this small mountainous river, the hydrodynamics are largely controlled by the salt-wedge. Because of its shallowness, friction dominates over the acceleration so that mixing is suppressed (Parker, 1991; van Maren and Hoekstra, 2004). Consequently, within the realm of salt-water intrusion, the water column remains stratified (Fig. 3). During the flood tide the tidal straining and increased shear associated with the flood current increased the mixing and reduced the stratification (Fig. 3c) (Uncles, 2002). During the ebb the river runoff ‘slides’ over the salt-wedge (Figs. 3 and 4). The buoyancy suppressed the mixing, creating greater salinity stratification (Fig. 3c). Consequently, in the course of a tidal cycle, the vertical stratification would decrease during the flood tide and increase during the ebb. Strengthened river runoff also has the same effect as the ebbing tide. Periodic stratification and destratification due to the tide and river runoff has been reported by van Maren and Hoekstra (2004). 7.1. Grain-size partition related to the salt-wedge On May 27 and June 18 the riverbed landward of the salt-wedge was coarse-textured whereas inside the salt-wedge the riverbed was composed of much finer texture (Fig. 6a,d). There are two possible causes. First, when the river runoff encounters the salt-wedge front, its carrying capacity for the suspended sediment load decreases causing the coarsest sizes to settle to the riverbed. The second possibility involves flocculation. Flocculation could take place in the fresh water (Fox et al., 2004) or where the salinity is low (1–5, Eisma et al., 1994; 0–2, Gibbs et al., 1989) near the seawater–saltwater interface. However, the concentrations of coarse suspended sediments

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Fig. 7. EOF analysis results of the water column subset on June 18 showing the first two modes of (a) eigenvectors, and (b) eigenweightings; and the results of the interface subset showing the first three modes of (c) eigenvectors, and (d) eigenweightings.

are not noticeably elevated around the salt-wedge (Fig. 5). As suspended particles increase in size due to flocculation, their settling velocities increase (Manning and Bass, 2006). This suggests that flocculated coarser sediments are at least partially removed from the water column due to rapid deposition (Fox et al., 2004; Thill et al., 2001) on the riverbed near the vicinity of the salt-wedge interface. This is probably why the two coarsest size-classes on the riverbed indicate terrestrial origin (Table 3b).

During the flood, after passing the dynamic barrier fine-grained terrestrial suspended sediments are mostly carried seaward by the river runoff over the salt-wedge. Inside the salt-wedge, the flood current carries fine-grained marine suspended sediments upriver (Table 3b). However, the blockage by the saltwater–river water interface causes the percentage of the mud (especially the clay to finesilt fraction, 1.2–10 μm) on the riverbed to increase, forming a trap on the riverbed for TOC (Fig. 6c). On the other hand, the water column is

Fig. 6. Sample analysis results of riverbed sediment samples taken on June 18 (a–c), May 27 (d–e) and June 5 (f–g). The format for the grain-size data is the same for all the three days including the cumulative percentage of the 5 size-classes and the mean particle size (μm) of the subsamples containing lithogenic and nonlithogenic components and the surface and near-bed salinity at each station (a,d, and f), and subsamples without nonlithogenic components (b,e, and g). On June 18, an additional information of the concentration of TOC is plotted against the percentage of the finest size-class (b 10 μm) (c).

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Table 2a Linear regression analysis results (r, p) among percentages of different size-classes versus POC, PON, and turbidity measurements in the surface water on June 18. Size-class

POC

PON

r N 500 μm 250–500 μm 63–250 μm 10–63 μm 1.2–10 μm

p

+ 0.4240 + 0.8674 + 0.8573 + 0.8946 + 0.9339

0.2220 ⁎0.0012 ⁎0.0015 ⁎0.0003 ⁎b 0.0001

r + 0.2295 + 0.7795 + 0.7798 + 0.7875 + 0.8773

Turbidity p

r

0.5236 ⁎0.0078 ⁎0.0078 ⁎0.0068 ⁎0.0009

+ 0.6410 + 0.8559 + 0.9004 + 0.8616 + 0.9393

p ⁎0.0336 ⁎0.0008 ⁎0.0002 ⁎0.0007 ⁎b0.0001

‘⁎’ Signifies statistical significance (p b 0.05).

Table 3a Linear regression analysis results (r, p) between the corresponding size-fraction of the percentages of riverbed and near-bottom suspended sediments (in parenthesis) concentration on June 18. Size-class N473 (N500) μm 249–473 (250–500) μm 62–249 (63–250) μm 10–62 (10–63) μm b10 (0.4–10) μm

Original samples

Lithogenic samples

r

p

r

p

+ 0.2782 + 0.3299 + 0.8600 − 0.1520 − 0.5025

0.4074 0.3218 ⁎0.0008 0.6550 0.1152

+ 0.1885 + 0.3027 + 0.4862 + 0.049 − 0.5531

0.5788 0.3655 0.1294 0.8859 0.0776

‘⁎’ Signifies statistical significance (p b 0.05).

Table 2b Linear regression analysis results (r, p) among percentages of different size-classes versus POC, PON, and turbidity measurements in the near bottom water on June 18. Size-class N 500 μm 250–500 μm 63–250 μm 10–63 μm 1.2–10 μm

POC

PON

Turbidity

r

p

r

p

r

p

− 0.4428 − 0.4973 − 0.3934 − 0.4192 0.8523

0.2001 0.1436 0.2608 0.2279 ⁎0.0017

− 0.4945 − 0.5289 − 0.4292 − 0.4592 0.8828

0.1462 0.1160 0.2159 0.1873 ⁎0.0030

+ 0.6217 + 0.9808 + 0.4028 + 0.4398 + 0.3031

⁎0.0412 ⁎b0.0001 0.2193 0.1759 0.3649

‘⁎’ Signifies statistical significance (p b 0.05).

better mixed during the flood as indicated by the low salinity difference (Fig. 3b), which allows greater production of turbulence shear leading to the breaking down (size reduction) of loosely flocculated particles (Fugate and Friedrichs, 2003; Manning et al., 2006), leading to higher amount of finer suspended sediments in the water column. 7.2. Terrestrial versus marine source partition related to the salt-wedge The removal of the nonlithogenic components in the riverbed subsamples generally results in the increase (coarsening) or decrease (fining) of coarse fractions in a subsample (Fig. 6). The coarsening or fining depends on the sampling location relative to the salt-wedge. If the sample was located in the region under the dominant influence of the river runoff, including the entire (except for St. 1) sampling region taken on June 5 (Fig. 6f) and the region landward from the salt-wedge front taken on May 27 (Fig. 6d) and June 18 (Fig. 6a), the sample would become coarser. On the other hand, if the sample was located within the salt-wedge taken on June 18 (Fig. 6a) and May 27 (Fig. 6d) it would become finer. The difference in the above textural changes could shed some light on the sediment source. Two of the major components that were

Table 2c Linear regression analysis results (r, p) among percentages of different size-classes, total SSC, TSM, POC, and PON versus salinity measurements in the surface and near bottom of the water column on June 18. Size-Class N 500 μm 250–500 μm 63–250 μm 10–63 μm 1.2–10 μm Total SSC TSM POC PON

Surface Samples

Bottom Samples

r

p

r

p

− 0.3462 − 0.5964 − 0.6509 − 0.6734 − 0.8094 − 0.7912 − 0.8391 − 0.7522 − 0.7945

0.2969 0.0528 ⁎0.0300 ⁎0.0231 ⁎0.0025 ⁎0.0037 ⁎0.0012 ⁎0.0121 ⁎0.0060

− 0.1886 − 0.1533 + 0.3653 + 0.4302 − 0.8219 − 0.2582 + 0.4425 − 0.9378 − 0.9472

0.5787 0.6526 0.2692 0.1866 ⁎0.0019 0.4434 0.1729 ⁎b0.0001 ⁎b0.0001

Remarks: ‘+’ Means positive trend, ‘−’ Means negative trend, ‘⁎’ Signifies statistical significance (p b 0.05).

removed from the original samples have different particle-size affinity characteristics. The organic matter has high affinity to fine-grained particles. The carbonate is generally associated with coarse-grained particles (Vilas et al., 2005) in estuarine environments. Consequently, chemically removing these substances would also eliminate the particles onto which they adhere. In other words, the coarsening change is the result of removing finer organics from the total; and the fining change is the result from removing coarser carbonates. Near the surface particulate organic matter (POM, including POC and PON) is carried by all size-classes except for the coarsest size (N500 μm). Among them, 63–250, 10–63, and 1.2–10 μm are also associated with fresh water (Tables 2a,c). Near the bottom, POM is only associated with the finest size-class (1.2–10 μm) who is also of terrestrial origin (Tables 2b,c). These facts suggest that the 1.2–10 μm size-class with the associated POM is advected by the river runoff in the entire water column. Yet, the equivalent size-class (b10 μm) on the riverbed shows marine influence (Table 3b). Linear regression shows no relation between the suspended sediments and riverbed sediments of this size-class (Table 3a). In fact, the longitudinal gradient (effect of opposing advections by river runoff and seawater intrusion) EOF mode of the interface subset shows that the near-bed POC and TOC are de-coupled (Fig. 7c). Yet, TOC is grouped with the near-bed salinity in this mode. In the vertical gradient mode, POC and TOC are coupled through the finest size-class (1.2–10 vs. b10 μm). Based on these facts, one can deduce that the particulate organic matter in the river water is a minor contributor to the organics on the riverbed. The major source for riverbed TOC comes from the sea. It is very likely that after unloading terrestrial organic carbon upon exiting the mouth of Goaping River, suspended sediments picked up marine carbon, a process described by Leithold and Blair (2001). Subsequently, this ‘marine organic carbon’ was carried upriver as bed load during the flood within the salt-wedge. Since carbonates are associated with coarser size-classes (Vilas et al., 2005), after its removal samples showing the fining trend are located within the salt-wedge on May 27 and June 18. This implies that the carbonate was transported into the lower part of the river from the sea. In contrast, on June 5 under strong river runoff, most sediment on the riverbed was fine-grained particles. The significant coarsening in all the samples after chemical removal was caused by the removal of terrestrial organics. Table 3b Linear regression analysis results (r, p) among riverbed sediment size-percentage and near-bottom salinity measurements on June 18. Size-class N473 μm 249–473 μm 62–249 μm 10–62 μm b10 μm

Original samples

Lithogenic samples

r

p

r

p

− 0.932 − 0.935 + 0.196 + 0.752 + 0.687

⁎b 0.0001 ⁎b 0.0001 0.5627 ⁎0.0075 ⁎0.0195

− 0.920 − 0.938 + 0.150 + 0.746 + 0.642

⁎b0.0001 ⁎b0.0001 0.6598 ⁎0.0084 ⁎0.0305

Remarks: ‘+’ Means positive trend, ‘−’ Means negative trend, ‘⁎’ Signifies statistical significance (p b 0.05).

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7.3. A transient floc population related to the salt-wedge Except for the 62–473 (62–250) μm size-class most size-classes in corresponding suspended and riverbed sediments have a reciprocal relationship (negative feedback) through resuspension and deposition on the water–bed interface as pointed out by the 2nd EOF mode of the interface subset. Only sediments in this size-class have a synoptic co-varying relationship (positive feedback). It is because this sizeclass is composed mostly of flocs that formed during the later part of the ebb and destroyed in the ensuing flood. As the flocs formed in the water during ebb, they settle quickly to the riverbed. As the velocity gradient in the lower water column increased during the ensuing flood, the previously deposited large flocs are entrained from the water–bed interface and subsequently broken-down as the result of increased turbulence. The in-situ formation and rapid deposition, entrainment and subsequent disintegration of flocs take place on the time scale of a tidal cycle. These processes caused the suspended and riverbed sediments of this size-class to co-vary as revealed by the positive linear regression (Table 3a). Since salinity is involved in the flocculation process, this size-class is also grouped with the salinity in the 3rd EOF mode of the interface subset (Fig. 7c). Because of the transient and in-situ nature of its formation and destruction, this sizeclass is considered neither terrestrial nor marine (Table 3b).

7.4. Hydrodynamic sorting and source-to-sink implication related to the salt-wedge Patterns of grain-size partition as the result of hydrodynamic sorting have emerged from this study. The dynamic front created by the salt-wedge is a major ‘sorter’ for both the suspended and riverbed sediments. The hydrodynamic sorting is most effective during the flood when the salt-wedge is formed. On the landward side of the saltwedge front, terrestrially-derived and flocculated coarse sediments are deposited to the riverbed. Inside the salt-wedge, clay-sized sediments and its associated organic carbon are trapped on the riverbed immediately seaward of the barrier, clearly indicating a carbon–particle relationship. On the other hand, strong river runoff coinciding with the ebb could suppress the salt-wedge that resulted in little sorting on the riverbed on June 5. The sediments exported during this time would be predominantly fine-grained (mostly clay-size) having high terrestrial organic carbon content. The flocculation process, which is spatially associated with the salt-wedge, is another important element in the sorting process. The size-class of 62–473 (63–250) μm contains a transient population of flocs that are formed and destroyed in-situ.

8. Conclusions Since suspended sediments are vehicles by which geochemical signals are delivered their size is an important parameter that controls the distribution and transport of particulate substances in an estuarine environment. In our study, we find that the grain-size partition in both the suspended and riverbed sediments is a result of hydrodynamic sorting mainly through the presence of the salt-wedge. Major findings of this study are as follows: 1. The lower reaches of the Gaoping/Kaoping River display a windinfluenced two-layered circulation pattern resembling that of a large coastal plain estuary. 2. The hydrography of the shallow Gaoping River Estuary is controlled by the interplay between the tide and river runoff and is noticeably stratified due to insufficient vertical mixing resulting in a distinct pattern of tidally-modulated stratification and de-stratification that is also affected by the river runoff.

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3. In the lower reaches of the Gaoping River, the clay-to-fine-silt (1.2– 10 μm) size-class in the suspended sediments has the highest mass concentration (mg/l). 4. On the landward side of the salt-wedge on the riverbed, the sediments are significantly coarser than that on the seaward side of the salt-wedge. 5. Immediately seaward of the salt-wedge front on the riverbed, there is an area of high percentage of fine-grained (b10 μm) sediments, which cause the trapping of TOC. 6. POM (POC and PON) in the river water is of terrestrial origin. At the surface of the river, POM is associated with sediments finer than medium sand (b250 μm). Near the riverbed, POM is only associated with particles finer than fine silt (b10 μm). 7. TOC on the riverbed is associated with seawater, and has weak link with terrestrial POC in the river water. 8. A transient floc population is identified that contains the suspended sediment size-class of 63–250 μm and the corresponding riverbed sediment size-class of 62–249 μm. This floc population is formed in-situ in the course of a tidal cycle; it is neither terrestrial nor marine. Acknowledgements The funding for this study was provided by the Taiwan National Science Council under Grant Numbers NSC 93-2611-M-110-013 and NSC 94-2611-M-110-002 to J.T. Liu; and NSC 94-2611-M-110-016 to J.-J. Hung. J.-J. Hung also thanks the partial support from ‘Aim for the Top University Plan’ of the National Sun Yat-sen University and Ministry of Education (97C030302) for manuscript preparation. We are grateful to Jeff C. Huang, Fanta Hsu, and Gina Lee for their help in the fieldwork and data and sample analysis. The Seventh River Management Office of the Water Resources Agency provided runoff records and the historical crosssectional survey data. Marilyn Ritzer-Liu proofread a later version of the manuscript. We thank Dr. John T. Wells and two other anonymous reviewers for their constructive comments and helpful suggestions to improve the manuscript. References Allison, M.A., Kineke, G.C., Gordon, E.S., Goni, M.A., 2000. Development and reworking of seasonal flood deposit on the inner shelf off the Atchafalaya River. Cont. Shelf Res. 20, 2267–2294. Bianchi, T.S., Galler, J.J., Alisson, M.A., 2007. Hydrodynamic sorting and transport of terrestrially derived organic carbon in sediments of the Mississippi and Atchafalaya Rivers. Estuarine, Coast. Shelf Sc. 73, 211–222. Eisma, D., Chen, S., Li, A., 1994. Tidal variations in suspended matter floc size in the Elbe River and Dollards Estuaries. Neth. J. Aquat. Ecol. 28, 267–274. Fan, S., Swift, D.J.P., Traykovski, P., Bentley, S., Borgeld, J.C., Reed, C.W., Niedoroda, A.W., 2004. River flooding, storm resuspension, and event stratigraphy on the northern California shelf: observations compared with simulations. Mar. Geol. 210, 17–41. Fox, J.M., Hill, P.S., Milligan, T.G., Boldrin, A., 2004. Flocculation and sedimentation on the Po River Delta. Mar. Geol. 203, 95–107. Fugate, D.C., Friedrichs, C.T., 2003. Controls on suspended aggregate size in partially mixed estuaries. Estuarine, Coast. Shelf Sci. 58, 389–404. Ganju, N.K., Schoellhamer, D.H., Warner, J.C., Barad, M.F., Schladow, S.G., 2004. Tidal oscillation of sediment between a river and a bay: a conceptual model. Estuarine, Coast. Shelf Sci. 60, 81–90. Garcia, T., Velo, A., Fernandez-Bastero, S., Gago-Duport, L., Santos, A., Alejo, I., Vilas, F., 2005. Coupled transport-reaction pathways and distribution patterns between siliciclastic–carbonate sediments at the Ria de Vigo. J. Mar. Syst. 54, 227–244. GESAMP, 1987. Land–sea boundary flux of contaminants, contributions from rivers. Report and Studies No. 32, Unesco, Paries, 49 pp. Gibbs, R.J., Tshudy, D.M., Konwar, L., Martin, J.M., 1989. Coagulation and transport of sediments in Gironde Estuary. Sedimentology 36, 987–999. Goni, M.A., Teixeira, M.J., Perkey, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coast. Shelf Sci. 57, 1023–1048. Goni, M.A., Cathey, M.W., Kim, Y.H., Voulgaris, G., 2005. Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions. Estuarine, Coast. Shelf Sci. 63, 683–700. Gordon, E.S., Goni, M.S., 2004. Controls on the distribution and accumulation of terrigenous organic matter in sediments from the Mississippi and Atchafalaya river margin. Mar. Chem. 92, 331–352.

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