Variability of bulk organic δ13C and C/N in the Pearl River delta and estuary,
southern China and its indication for sources of the estuarine sediment
Fengling Yua*, Yongqiang Zongb, Jeremy M. Lloyda, Guangqing Huangc, Melanie J.
Lengd,e, Christopher Kendrickd, Angela L. Lambd, Wyss W.-S Yimb
a
Department of Geography, University of Durham, South Road, Durham DH1 3LE, UK
Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR,
China
c
Guangzhou Institute of Geography, 100 Xian Lie Road, Guangzhou 510070, China
d
NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham,
NG12 5GG, UK
e
School of Geography, University of Nottingham, Nottingham NG7 2RD, UK
____________________________________________
*Corresponding author. Tel: +44 (0)191 3341817; Fax: +44 (0)191 3341801; Email address:
fengling.yu@durham.ac.uk
b
Abstract
This paper presents carbon isotope composition (δ13C) and C/N of organic matter from
source to sink in the Pearl River catchment, delta and estuary, and discusses the
applicability of δ13C and C/N as indicators for sources of estuarine sediment. In addition
to the 91 estuarine surface sediment samples, other materials collected in this study cover
the main sources of organic material to estuarine sediment. These are: terrestrial organic
matter (TOM), including plants and soil samples from the catchment as well as mangrove
leaves and sediment from the salt marshes; estuarine and marine particulate organic
carbon (POC) from both summer and winter. Results show that average δ13C of estuarine
surface sediment increases from –25.0 ±1.3‰ from the freshwater environment to –21.0
±0.2‰ from the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ±0.2. In
the source areas, C3 plants have lower δ13C than C4 plants of –29.0 ±1.8‰ and –13.1
±0.5‰ respectively. δ13C increases from –28.3 ±0.8‰ in the forest soil to around –
24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C4
grasses. The δ13CPOC increases from –27.6 ±0.8‰ in the freshwater areas to –22.4 ±0.5‰
in the marine-brackish-water areas in winter, and ranges between –24.0‰ and –25.4‰
from freshwater areas to brackish-water areas in summer. Comparison of the δ13C and
C/N between sink and source indicates a weakening TOM and freshwater POC input in
the surface sedimentary organic matter seawards, and a strengthening contribution from
the marine organic matter. Thus we suggest that the bulk organic δ13C and C/N can be
used as indicators for sources of the sedimentary organic matter. Organic carbon in
surface sediments derived from anthropogenic sources such as human waste, organic
pollutants from industrial and agricultural activities accounts for less than 10% of the
total organic carbon (TOC). Although results also indicate elevated δ13C of sedimentary
organic matter due to some agricultural products such as sugarcane, C3 plants are still the
dominant vegetation type in this area, and the bulk organic δ13C and C/N is still an
effective indicator for sources of estuarine sediments.
Keywords: δ13C, C/N, sediment source, Pearl River, estuary, southern China
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1.
Introduction
Preservation of organic matter in estuarine and coastal sediments is an important
process in the global carbon cycle as more than 90% of the carbon buried in the oceans
occurs in continental margin sediments (Emerson and Hedges, 1988; de Haas et al., 2002).
In estuarine areas, organic matter can be supplied both from autochthonous sources (such
as plants growing on the sediment surface) and allochthonous sources (organic material
transported predominantly by the tide or a river). Better constraints on the sources of
organic matter in marine sediments are needed to understand the processes responsible
for its preservation (Mayer, 1994). Such understanding will help estimate the possible
contribution of different sources of organic matter to marine organic matter, relating to
the global cycle of carbon (Schlunz et al., 1999; Gaye et al., 2007). Bulk organic δ13C and
C/N have been widely used to elucidate the source and fate of organic matter in the
terrestrial, estuarine and coastal regions (e.g. Hedges and Parker, 1976; Fontugne and
Jouanneau, 1987; Goñi et al., 1997; Bianchi et al., 2007; Middelburg and Herman, 2007;
Harmelin-Vivien M. et al., 2008; Ramaswamy et al., 2008) and also as proxies in
palaeoclimatic studies (e.g. Chivas et al., 2001; Lamb et al., 2006; Zong et al., 2006;
Wilson et al., 2005; Mackie et al., 2007).
The Southeast Asian area is one of the most densely-populated areas in the world, as
well as one of the most industrialized regions. The contribution of organic carbon from
land to the ocean via major fluvial systems in this area has not been well estimated. Some
studies have been carried out to assess the contribution of terrigenous organic matter to
the South China Sea (e.g. Hu et al., 2006), as well as a number of studies investigating
the modern-day organic carbon isotopic signature from the Pearl River delta and estuary
(Dai et al., 2000; Chen et al., 2003, 2004; Jia and Peng, 2003; Callahan et al., 2004; Zong
et al., 2006). However, a detailed understanding of the range of sources of organic matter
and their relative contribution to the bulk sediment organic carbon within an estuary is
still rather poorly constrained. Detailed investigation of δ13C and C/N of the organic
matter through the full estuarine complex (from freshwater to marine environments) is
one way to address this problem. Here we aim to use this technique to establish the full
range of δ13C and C/N of the organic matter in the Pearl River estuary from source to sink.
We will then assess the suitability of this proxy as an indicator for dominant vegetation
type, sediment source and environmental conditions as well as the role of anthropogenic
influence on the δ13C and C/N in the Pearl River estuary.
2.
Materials and methods
2.1. The Pearl River delta and estuary
The Pearl River delta is located at 21°20´-23°30´N and 112°40´-114°50´E (Fig. 1a),
and formed during the last 9000 years (Zong et al., 2009). The Pearl River (Zhujiang) is
the general name for the three rivers (the East, the North and the West, Fig. 1a) that flow
into the Pearl River delta-estuary before entering the South China Sea. It is the second
largest river in China in terms of water discharge (about 330 ×109 m3/year; Hu et al.,
2006). The River is 2214 km in length (the West River), and it drains an area of 425,700
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km2 (Li et al., 1990). The suspended sediment concentration in the Pearl River is
relatively low compared with other major Asian rivers (Zong et al., 2009), with a mean
concentration of about 0.172 kg m-3 and an annual flux of about 30.64 106 t (Wai et al.,
2004). About 92-96% of the suspended sediment is discharged during the wet season
(from April to September, monsoon summer), with maximum river discharge occurring
in July. The warm/hot wet season is followed by a cool/cold dry season (monsoon winter
from October to March). Approximately 80% of sediment influx to the estuary is
deposited within the Pearl River estuary, the remaining being transported to the South
China Sea (Xu et al., 1985).
The coastal waters in the vicinity of the Pearl River estuary are influenced by three
water regimes: the Pearl River discharge, oceanic waters from the South China Sea and
coastal waters from the South China Coastal Current (Morton and Wu, 1975; Yin et al.,
2004). These water regimes are subjected to two seasonal monsoons. In winter, the
northeast monsoon prevails, the China Coastal Current dominates the coastal waters of
Hong Kong, and freshwater discharge is at its lowest. In summer when the monsoon
blows from the southwest direction and the Pearl River discharge reaches the maximum,
the interaction of the estuarine plume and oceanic waters from the South China Sea
dominate the coastal water regime (Yin et al., 2004).
The Pearl River delta is within the tropical climate zone, with mean annual
temperature ranging from 14 to 22°C across the basin and precipitation ranging from
1200 to 2200 mm year-1 (Zhang et al., 2008). Subtropical and tropical forests are the
dominant vegetation type in the Pearl River catchment (Winkler and Wang et al, 1993).
The natural plants in this area are dominantly C3 plants, mixed with some C4 grasses,
while sugarcane (a C4 plant) is one of the major agricultural products in the lower deltaic
area today.
2.2. Filed data collection
2.2.1. Terrestrial organic matter
Terrestrial organic matter (TOM) includes land plants and soil organic matter (Fig. 1b;
Table 1, 2). Plant samples collected include: 1) C4 plants, including general C4 grasses,
such as Panicum maximum Jacq. and sugarcane (Saccharum officinarum). As this study
also examines δ13C and C/N of agricultural plants, it is necessary to separate sugarcane
from other non-agricultural grasses. The ‘general C4 grasses’ are the non-agricultural,
naturally-grown C4 grasses in this area; and 2) C3 plants, including general C3 plants (e.g.
Pteris semipinnata, Pinus sp.), mangrove (Avicennia, Kandelia obovata, Bruguiera
gymhorrhiza, Acanthus ilicifolius, Aegiceras corniculatum, Avicennia marina, Ficus
microcarpa) and agricultural plants e.g. banana (Musa acuminata), lotus (Nelumbo
nucifera), reed (Phragmites australis) and rice (Oryza sativa). Here, the ‘general C3
plants’ are the non-agricultural, naturally-grown C3 plants in this area. Representative
plant organs were collected and initially stored in paper bags until dried at 50ºC over
night in an oven. Soil samples were collected from c. 3 cm depth from under the surface
(to avoid fresh plant roots and disturbed soil, Liu et al., 2003), and stored in polyethylene
tubes in a fridge before sample preparation and analysis. As both C3 and C4 plants grow
in the terrestrial area, organic matter in terrestrial soil samples is a mixture of both kinds
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of plants. Soil samples are clustered into the following groups: forest soil, mangrove soil,
riverbank soil and agricultural soil, depending on sample location as well as the dominant
vegetation type. Plants and soil samples were collected in June 2006, except for
agricultural plants (banana, lotus, reed, rice and sugarcane) and agricultural soil samples
which were collected in August 2007.
2.2.2. Estuarine organic material
Estuarine organic material includes seasonal particulate organic carbon (POC; mainly
derived from freshwater or marine phytoplankton and terrigenous organic matter) and
estuarine surface sediments (Fig. 1b; Table 3, 4; Table 5). A total of 49 POC samples
(PE41-92, without PE76-77) were collected in winter (December 2006), and 45 POC
samples (PE41-75) in summer (June 2006) including 35 samples on filter paper and 10
samples using a 70 µm net. POC samples were collected by filtering water samples using
the fibreglass filter paper (Fisher Brand MF200) in the laboratory. POC samples were
also collected using a 70 µm net from some sites in June 2006 (no POC on net was
collected during winter due to small size of the boat). Samples were then washed from
the net into a sampling tube, and refrigerated prior to analysis. A total of 91 surface
sediment samples (PE1-PE92, without PE43; Fig. 1b) were obtained using a grab sampler
from a boat. The top 10 cm sediment was collected, which potentially represent sediment
deposited during the past 6-10 years, according to the sedimentation rate of around 0.8
cm/year on shoals within the estuary (Li et al., 1990). Samples were sealed in
polyethylene tubes and stored in fridge at 2-3ºC before analysis.
A total of three field campaigns were carried out to collect these surface sediment and
POC samples. Surface sediment samples PE01-40 were collected by Zong et al. (2006) in
June 2005. Surface sediment sample PE41-77 were collected by this study in June 2006,
and PE78-92 were collected in December 2006. Summer POC samples PE41-75 were
collected in June 2006 and winter POC samples PE78-92 were collected in December
2006.
2.2.3. Estuarine environmental variables
Environmental variables including water salinity (Sal), water depth (WD), pH, total
dissolved solid (TDS) and water temperature (Temp), were measured from the same
sampling sites as the POC samples (Fig. 1b; Table 3). Summer water salinity and mean
water depth from PE78-PE92 were interpolated based on a bathymetry map of the area.
Mean water salinity for sites of PE41-PE92 is the mean value of seasonal values
measured. Water salinity was measured using the Practical Salinity Scale. Mean water
salinity and mean water depth from PE1-PE40, PE76 and PE77 were interpolated based
on bathymetry maps of this area. These variables were measured at 50 cm below water
surface using a YSI meter.
2.3. Laboratory methods
2.3.1. Sample preparation
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Soil and sediment samples were prepared using 100 ml 5% HCl to remove carbonates.
They were then washed 3 times with deionised water using the fibreglass filter paper,
before being dried down at 50ºC overnight, homogenised in a pestle and mortar, and
weighed (25-50 mg) for δ13C and C/N analysis.
Plant samples are placed in a –80ºC freezer overnight followed by freeze drying for 24
hours. These samples were then crushed and homogenised in a ball mill at a speed of 500
rpm for 5-15 minutes (depending on type of plants) and then weighed (1-2 mg) for δ13C
and C/N.
POC samples on the filter paper were dried in the field, and were difficult to remove
from filter papers. Some were successfully removed by gently scrapping off the filters
while others that could not be removed had to be analysed while attached to the filters.
The POC on the filters could only be measured for C/N ratios (rather than TOC and TN)
as absolute weights could not be measured (Table 3).
2.3.2. Sample analysis
Carbon isotopes and C/N analyses were performed by combustion in a Carlo Erba
NA1500 (Series 1) on-line to a VG Triple Trap and Optima dual-inlet mass spectrometer,
with δ13C calculated to the VPDB scale using a within-run laboratory standards (BROC)
calibrated against NBS-19 and NBS-22. Replicate analysis of well-mixed samples
indicated a precision of ±<0.1‰ (1SD). C/N was determined by reference to an
Acetanilide standard. Replicate analysis of well-mixed samples indicated a precision of
±<0.1%. The isotopic ratios are expressed as δ13C, in units of per mille (‰).
Organic matter is depleted in 13C relative to V-PDB, so δ13C of organic material is
negative. The weight ratio of total organic carbon (%TOC) to total nitrogen (%TN),
giving C/N is usually measured along side δ13C and help to distinguish carbon sources.
Particle size analysis was carried out using a laser granular meter (Coulter LS 13200).
Values of the δ13C, C/N, particle size and environmental parameters in this paper are
presented in the format of ‘average value ±standard deviation (SD)’. Values of δ13C or
C/N of some samples are not available due to low content of organic matter in those
samples.
3.
Results
3.1. Hierarchical Cluster Analysis
All 92 samples were subjected to Hierarchical Cluster Analysis (using SPSS for
Windows 15.0) based on winter salinity and sand concentration. The cluster analysis
defined 9 groups (G1-G9) with a clear geographical control (illustrated in Fig. 2; Table 5).
Surface sediment samples are from the 91 sampling sites (without site PE43) covering
G1-G9. The POC samples are only from the western estuary where suspended sediment
concentration was high enough to allow measurements to be made, with samples ranging
from G2-G6 for winter POC and G2-G5 for summer POC.
The 9 groups defined by selected environment variables clearly highlight a trend from
the full-freshwater environment (G1-G3) to the full-marine environment (G8-G9), via
brackish areas (G4-G7) (Fig. 2; Table 6). The freshwater groups have low annual
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average water salinity of 3.4±3.6, which increases to 15.8±5.3 and 28.4±3.1 in the
brackish-water areas and reaches 32.9±0.8 under the full marine environment (Table 6).
Mean water depth is lower than 5 m in the freshwater area, getting deeper seawards, and
becomes deeper than 20 m in the shallow marine area. Meanwhile, average particle size
gets finer offshore with a reduction in sand component. All variables tend to be more
stable in the marine area than in the freshwater area (Table 6).
In addition to the spatial diversity of the environmental conditions, seasonal variability
is also observed. For G2-G5, mean winter salinity is 14.0±11.6, ranging between 0.1 and
33.9. This is significantly higher than that in summer of 3.1±4.8, ranging between 0.1 and
18.2 (Table 3). A similar situation is found in the concentration of total dissolved solids
(TDS) of 15.5±12.1 g l-1 in winter compared to 2.0±2.7 g l-1 in summer. Areas under the
freshwater- and fresh-brackish conditions are more sensitive to seasonal changes than
those under marine- and marine-brackish conditions, as seasonal variation of these
environmental parameters are larger for G2 than G5.
3.2. Terrestrial organic matter
3.2.1. Plants
Plant samples collected in this study are composed of two main groups: C3 plants
(dominant vegetation in the Pearl River delta), and some C4 plants. Results show that C3
plants in general have lower δ13C with an average of –29.0±1.8‰ compared to C4 plants
with an average of –13.1±0.5‰ (Fig. 3a). C3 plants generally have the lowest δ13C of –
29.9±1.3‰, followed by the agricultural C3 plants (e.g. rice, lotus and banana) and
mangroves, with δ13C values of –28.2±1.4‰ and –27.1±1.7‰ respectively (Fig. 3b). C4
plants (mostly grasses) have δ13C of –13.2±0.5‰, slightly lower than sugarcane which
has δ13C of –12.7±0.2‰ (Fig. 3b).
C/N of C3 and C4 plants are widely variable (7.4-61.8 for C3 plants and 8.2-40.3 for C4
plants) and overlap significantly, with the average value of 22.7±11.6, and 24.6±9.4
respectively (Fig. 3a). It is impossible to distinguish between the two plant types based on
C/N alone. Within the C3 plants, agricultural plants have lower C/N (13.7±2.7) than other
C3 plants (21.7±10.7) and mangroves (31.0±12.5). Within the C4 plants, sugarcane has a
slightly higher C/N of 30.6±8.4 compared to other C4 plants of 21.5±8.9 (Fig. 3b).
3.2.2. Terrestrial soil
Terrestrial soil samples have δ13C between –28.9‰ and –19.3‰, and C/N between 7.7
and 20.4, with a negative correlation between δ13C and C/N (Fig. 4a). As expected soil
samples from different areas of vegetation cover, have different δ13C and C/N values.
Soil samples from pine forest have the lowest δ13C of –28.3±0.8‰, but the highest C/N
of 17.9±3.6. Soil samples from riverbanks and mangrove areas have similar δ13C and C/N,
around –24.1‰ and 12.5 respectively, although both δ13C and C/N are relatively more
variable in the mangrove soil than in the riverbank soil (Fig. 4b). Soil samples from
farmlands have the highest δ13C of –21.7±0.7‰ and the lowest C/N of 8.9±1.1 (Fig. 4b).
3.3. Estuarine particulate organic carbon
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3.3.1. POC collected on filter paper
Overall, δ13C of POC shows very little if any seasonal variations, the average δ13C of
POC in summer (–24.5‰) is within error of winter (–24.2‰) (there is slightly more
variation in winter (SD=1.9) than in summer (SD=1.0)). C/N of the winter POC ranges
between 6.0 and 9.0, with a mean value of 7.1±0.9. It is slightly lower than the summer
POC, ranging between 8.0 and 10.2, averaging 9.2±1.1 (Fig. 5).
δ13C and C/N of POC varies between groups from different environments. In winter,
freshwater POC has the lower δ13C (–27.6±0.8‰, G2) than the marine-brackish POC (–
22.4±0.5‰, G6), while samples from the fresh-brackish water area have δ13C between
the two (–24.5±1.3‰ for G4; –23.6±1.2‰, G5) (Fig. 6a). Variation in C/N of winter
POC between groups is not as significant as δ13C (Fig. 6a). In summer, the average
δ13CPOC of each group is similar and does not show any trend between groups.
Differences in both δ13C and C/N of summer POC is not significant (Fig. 6b).
δ13C and C/N of POC show good correlation between salinity and other environmental
conditions (Fig. 6c, d), especially in the winter. In winter, δ13C increases from –27.6‰ in
the freshwater area further inland to around –21.6‰ in the marine-brackish area near the
estuary mouth, with increasing salinity (Fig. 6c). Correlation between C/N and
environmental variables is not as significant as that of δ13C. C/N is found to be high in the
middle estuary (7.6±1.0, G5) and decreases landwards and seawards (<7.0). In summer,
both δ13C and C/N are more uniform than winter and not clearly correlated to the
environmental variables (Fig. 6d).
3.3.2. POC collected by a 70 µm net
Estuarine POC is composed of organic carbon from fine-size plankton and relatively
coarse TOM. As the size of plankton (e.g. diatoms) is usually smaller than 80 μm,
organic matter collected on filter paper has a higher proportion of plankton/algae, while
those collected in a 70 µm net presents higher proportion of TOM over plankton. Results
show that the POC samples from the 70 µm net have a lower average δ13C of –
25.6±1.0‰, compared to the filter paper sample from the same location (i.e. –24.3±1.0‰
in summer and –24.2±1.5‰ in winter, Fig. 7). The average C/N of the POC from the net
samples is 10.7±4.5, which is higher and more variable than that of the POC collected on
filter paper of 9.4±1.1 in summer and 7.8±1.0 in winter respectively (Fig. 7). POC values
from the samples collected using the net fall into two groups. POC from locations further
seaward, such as PE65, PE67, PE68, and PE71 (Fig. 1b), tend to have lower δ13C and
C/N than POC on filter paper from both seasons, while those from the inner estuarine
area have similar δ13C with the POC from the filter papers but higher C/N (Fig. 7).
3.4. Estuarine surface sediment
The estuarine surface sediment samples have δ13C ranging from –27.0‰ to –20.8‰
and C/N ranging between 20.1 and 6.5, with δ13C and C/N negatively correlated (Fig. 8a).
With the environmental change from the full-freshwater environment to the full-marine
environment from G1 to G9, a clear trend is observed in both δ13C and C/N values. δ13C
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becomes progressively higher from G1 to G9, increasing from –25.0±1.3‰ in G1 to –
21.0±0.2‰ in G9 (Fig. 8), with increasing water salinity and decreasing sand
concentration (Table 5). Meanwhile, C/N also shows a clear trend decreasing from
15.2±3.3 in G1 to 6.8±0.2 in G9. G3 does not fit in this trend with δ13C of –23.0±1.6‰
and C/N of 11.1±1.3, which is lower than G4 (Fig. 8b). Both δ13C and C/N are more
stable in the marine environment compared to the freshwater (Table 5).
4.
Discussion
4.1. Terrestrial organic matter and its sources
Plants from the study area can be placed in two main groups, C3 and C4 plants, based
on the way they fix carbon from atmospheric CO2 during the photosynthetic process. This
process produces significant difference in the δ13C signature of the organic carbon from
these two groups of plants. C3 plants use the Calvic-Benson cycle (C3 pathway) (Craig,
1953; Park and Epstein, 1960), through which they absorb and conserve 12C from
atmospheric CO2 more effectively compared to the Hatch-Slack pathway used by C4
plants (Hatch and Slack, 1970). This results in greater discrimination against 13C in C3
plants than C4 plants, with C3 plants producing lower δ13C around –28.1‰ (Graig, 1953;
Fry and Sherr, 1984; O’Leary, 1985) than C4 plants with values around –13.0‰ (Bender,
1971; Fry and Sherr, 1984; Emerson and Hedges, 1988). The C3 plants analyzed in this
study have an average δ13C of –29.9‰ (Fig. 3), about 1.8‰ lower than values reported
by other studies. This might be due to the high precipitation in the Pearl River delta area,
with precipitation over 1500 mm/yr (Li et al., 1990). A similar case has been reported by
Austin and Vitousek (1998) where δ13C of C3 plants shifts from –29.9‰ to –25.6‰,
when the sampling site changed from an area of 5000 mm/yr precipitation to an area of
only 500 mm/yr. The average δ13C of C4 plants from this study is –13.1‰ (Fig. 3),
similar to other studies (e.g. Bender, 1971; Fry and Sherr, 1984; Emerson and Hedges,
1988).
Terrestrial soil is one of the direct sinks of the organic carbon from the surface
vegetation, and its organic carbon signature depends greatly on the dominant vegetation
type. This explains the variation in organic carbon isotope signatures found in soil
samples from different areas of the Pearl River delta, e.g. forest, riverbank and tidal flats
(Fig. 4). Soils from the forest mainly receive organic carbon from C3 plants (e.g. pine),
and tend to have low δ13C of –28.3±0.8‰, but high C/N of 17.9±3.6 (Fig. 4b). Riverbank
soil samples are from the flood plain along the North River, dominated by shrubs and
grass. The increasing proportion of C4 grass on the flood plain produces relatively higher
δ13C in the riverbank soil (–24.1±1.0‰) compared to the forest soil (Fig. 4b). Similar
δ13C and C/N are found in the riverbank and mangrove soil (–24.0±1.9‰) and are due to
the similar proportion of C4 plants growing on the flood plain and the tidal flat. Other
studies also report the δ13C of soils from C4-dominated areas as being around –21.0‰,
and around −24.0‰ from areas mixed by C3 and C4 plants (Driese et al; 2005; Saia et al.
2008). Many studies have used δ13C of terrestrial soil to reflect different vegetation types
(Mackie, et al., 2005; Fan et al., 2007; Cao et al 2008; Driese et al., 2008; Saia et al.,
2008). Usually, C/N of soil organic matter is lower than 20. Although the range of C/N is
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not as wide as the C/N of either C3 or C4 plants, it is still impossible to use C/N alone to
distinguish different sources of the TOM.
4.2. Estuarine particulate organic carbon and its sources
In the Pearl River delta, terrestrial organic matter enters the river system in the form of
plant fragments and soil organic matter, and mixes with in situ (freshwater, brackishwater, marine) plankton and algae within the estuary. Accordingly, within the estuarine
area, the POC receives carbon from the TOM and in situ aquatic productivity (freshwater,
brackish-water or marine plankton). The terrigenous freshwater POC, having δ13C around
−28.7‰ (Fig. 5 and 6), is delivered into the estuarine system by the river runoff. It dilutes
the signal produced by the brackish-water and marine POC which usually has δ13C
around −21.2‰ and C/N ≤ 11.0 (Fig. 5 and 6). Higher δ13C in marine POC compared
with riverine POC has also been reported from other coastal areas (Middelburg and
Nieuwenhuize, 1998; Countway et al., 2006; Bianchi et al., 2007; Middleburg and
Herman, 2007; Zhang et al., 2007; Bird et al., 2008). Wu et al. (2007) report δ13C of the
POC varying from –24.4‰ in the Yangtze River to –21.0‰ on the East China Sea Shelf.
Middleburg and Herman (2007) suggested that the correlation between δ13CPOC and
salinity is more significant in river-dominated estuaries, such as the Rhine estuary and the
Douro estuary, than in tidal-dominate estuaries such as the Gironde estuary and the Loire
estuary, Western Europe.
In the Pearl River estuary, the balance between TOM and in situ plankton in the
estuarine POC is strongly influenced by the strength of the freshwater runoff. It is,
therefore, the strength of runoff that appears to control the spatial and temporal variation
of the carbon isotopic signature of the estuarine POC. In winter, significant reduction in
monsoonal precipitation results in a remarkable reduction in the TOM input into the
estuary, and therefore the POC within the estuary is dominated by in situ phytoplankton
(e.g. diatoms, dinoflagellates, green algae, euglenoides). δ13C of the winter POC shows a
clear trend from freshwater areas (around –28.0‰) to the marine-brackish areas (around
–22.0‰), reflecting the variation in δ13C and C/N of plankton under different
environments (Fig. 6a). In the Pearl River estuary, δ13C of the freshwater POC typically
ranges from –30‰ to –25‰, and the brackish POC typically ranges from –22‰ to –26‰
(Fig. 6c, d) which is also reported by other studies (Lamb et al., 2006; Middelburg and
Herman, 2007; Tesi et al., 2007). The low C/N (7.1±0.9) of winter POC supports the
assumption that winter POC is dominated by plankton, as the phytoplankton tends to
have low C/N of 5-7 (Lamb et al., 2006). In summer the high freshwater flux brings
larger amounts of TOM into the estuary (clearly seen in the amount of sediment collected
by the filter in the summer compared to the winter in this study) resulting in a higher
proportion of TOM in summer POC, suggested by the relatively higher C/N. At the same
time the influence of the strong freshwater flux extends further seawards of the estuary,
and produces a relatively uniform mix of POC within the estuary. This mixing and
homogenisation of the POC throughout the estuary reduces the spatial variability in δ13C
of the summer POC. This is indicated by the considerable overlap in δ13C and C/N of the
summer POC from groups G2 to G5 across the estuary (Fig. 6b).
The POC collected with the 70 µm net is assumed to have a higher proportion of plant
fragments than the POC collected on the filter paper as a higher proportion of the
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relatively smaller phytoplankton can pass through the net, but are trapped on the filter.
The POC from the net would, therefore, be expected to record (on average) lower δ13C
and have higher C/N than the POC from the filter paper. This is indeed the case (Fig. 7)
with POC samples from the net having an average δ13C of –25.6‰ and C/N of 10.7
compared to values of –24.3‰ and 9.5 for samples from the filter paper. Previous studies
suggest that the range of δ13CPOC is small in estuaries of high turbidity, between –24‰
and –26‰ (Fontugne and Jouanneau, 1987; Tan et al., 2004), which reflects the
dominance of terrigenous POC input into the estuary over other sources. This further
supports the suggestion that the proportion of TOM in the POC is one of the main factors
influencing δ13C and C/N of POC, while the strength of the freshwater flux controls
seasonal and spatial distribution of the TOM in the POC. However, during periods of
reduced freshwater flux (during winter in the Pearl River estuary area for example) in situ
productivity will tend to be the dominant source of POC and, hence, be the important
control on spatial variability of δ13C of POC (illustrated by winter distribution of δ13C,
Fig. 6a).
4.3. Estuarine surface sediment and its sources
Comparison of δ13C and C/N from a wide range of potential sources of sediment to the
Pearl River estuary with measurements of bulk sediment samples from the estuary makes
it possible to identify the dominant sources of organic sediment to the estuary floor.
Organic matter within the surface sediment is mainly composed of the POC, especially
from the marine surface sediment (Liu et al., 2007), while the organic matter of the
surface sediments from freshwater environments is a combination of freshwater in situ
plankton productivity and the TOM. Spatially, δ13C increases from –25.0‰ to –21.0‰
from river tributaries to marine areas with C/N decreasing from 15.2 to 6.8. This trend
indicates a decreasing TOM contribution into the surface sediment organic matter, with
increasing POC contribution seawards. Inner estuarine areas near the river mouth
predominantly receive TOM delivered by the freshwater runoff. The proportion of TOM
in the surface sedimentary organic matter decreases seawards as the influence of the
freshwater runoff becomes weaker. In the marine area, surface sedimentary organic
matter is dominated by the POC as TOM flux is weakened by the tide influence.
Applicability of δ13C and C/N of the surface sediment for indicating sources of the
sediment has been explored in previous studies in the estuarine and continental areas
based on the sedimentation rates (Hedges and Parker, 1976; Goñi et al.,. 1997; Hu et al.,
2006; Ramaswamy et al., 2008). Hu et al. (2006) examined organic carbon, total nitrogen,
δ13C and δ15N of surface sediments from the Pearl River estuary and adjacent shelf,
suggesting proportionally higher terrestrial-derived organic carbon at the river mouth and
the western coast compared to marine-origin organic carbon. Using a mixing model
(Schultz and Calder, 1976), Hu et al. (2006) further suggest the algal-derived organic
carbon content is estimated to be low (≤0.06%) at the river mouth and higher (up to
0.57%) on the adjacent inner shelf. A decreasing contribution of TOM in the surface
sediment organic matter is also suggested by Zong et al. (2006) by examining the bulk
organic δ13C and C/N of the surface sediment, as well as diatom records.
Detailed bulk organic δ13C and C/N of the organic matter from potential source areas
of the surface sediment in the Pearl River estuary investigated in this study support the
- 11 -
spatial distribution of the TOM and POC in the estuarine area. Different dominant
organic matter sources between the freshwater and marine environments generate the
lower δ13C but higher C/N in the freshwater sediment compared to the marine sediment
(Fig. 8b). Surface sediments from the freshwater area of this study have low δ13C (e.g. –
23.9‰ for surface sediment and –26.3‰ for the POC on average in G2). Surface
sediments from the brackish/marine environment, with little freshwater influence, have
high δ13C (e.g. −21.0‰ in G9, Fig. 8b) due in part to higher δ13C of the marine POC (–
24.0‰ on average in G5 and –22.4‰ for G6, Fig. 6). C/N is useful for separating algae
and TOM, with algae having C/N between 5 and 8, while TOM has C/N typically higher
than 15 (Meyers, 1997). Low C/N of the surface sediments at the marine end (e.g. 6.9 for
G9, Fig. 8b) suggests higher organic matter contribution from marine algae (Hu et al.,
2006; Wu et al., 2007).
Factors that influence the organic carbon isotopic signature of POC, therefore, also
become significant for the signature of the surface sediment, such as strength of the
freshwater flux and hydrological conditions. The contribution of marine algae to the
sedimentary organic carbon is limited by the turbidity and nutrient content of the water
column (Yin et al., 2004; Huang et al., 2003; 2004). Turbidity is higher at the river mouth
(nearer the source of sediment flux), the contribution of in situ phytoplankton
productivity to the sediment signal is less here than further down the estuary towards the
inner-shelf area. A similar phenomenon has been reported from other estuaries, e.g. the
Gironde estuary, western Europe (Fontugne and Jouanneau, 1987) and the Yangtze River
estuary, China (Wu et al., 2007).
4.4. Influences on the bulk organic δ13C and C/N
4.4.1 Anthropogenic influences
It has been suggested that anthropogenic influence from the Pearl River delta, such as
the agricultural products, human wastes and industrial inputs, has become an important
factor that changes the organic carbon isotopic signature of the surface sediments in the
estuary (Jia and Peng, 2003; Owen and Lee, 2004). Our results show that agricultural C3
plants have marginally higher δ13C (–28.2±1.4‰) than general C3 plants (–29.9±1.3‰),
as well as the agricultural C4 plants and general C4 grass (Fig. 3). Correspondingly, δ13C
decreases from agricultural soil samples to riverbank and mangrove soil, and the forest
soil has the lowest δ13C of all (Fig. 4). C/N increases through these soil types, with the
forest soil having the highest C/N. This increase in δ13C and decrease in C/N found in
agricultural soil might be due to the fertilization process (O’Leary, 1985). Sediments
deposited during recent decades tend to have elevated δ13C, which appear to coincide
with rapid urbanisation, industrialisation and reclamation in this area (Owen and Lee,
2004; Hu et al., 2008). For example, well-nourished plants show higher δ13C than plants
deficient in nitrogen and/or potassium (less fertilized) (O’Leary, 1985).
As a relatively densely populated area, sewage could be a potentially important
contributor to the sediment organic matter in the Pearl River estuary. Sediments from
Victoria Bay, Hong Kong, were significantly contaminated by sewage (Hsieh, 2006). The
δ13C of these sediments varies from −28.59 to −22.60‰ and C/N from 10.71 to 14.73
(Hsieh, 2006), which must include the influence of sewage. Here we have undertaken
- 12 -
biomarker analysis of the Pearl River surface samples (David Strong, pers. comm.) and
show the lack of coprostanol and other 5-beta stanols indicating that sewage is an
insignificant contributor to the organic carbon in these sediments. The main organic
pollutants found in the estuarine area are the polycyclic aromatic hydrocarbons (PAHs)
and polychlorinated biphenyles (PCBs) and organochlorinate pesticides (Kang et al.,
2000). Concentrations of the total PAHs, PCBs and organochlorinate pesticides in surface
sediments from the river to the estuarine area are 1 167 – 21 329 µg g-1, 10.2-485.5 µg g-1
and 5.8-1 658 µg g-1 respectively (Kang et al., 2000). When compared to the TOC values
from surface samples analysed here of 0.5-2% TOC (i.e. 5 000-20 000 mg kg-1) the
concentration of these pollutants is actually very low. Sediments with the highest
concentration of these organic pollutants are from areas near Guangzhou and/or Maucau
according to Kang et al. (2000). For example, concentration of organochlorinate
pesticides in sediments from areas investigated by this study is usually between 5 and 70
µg g-1 (Kang et al., 2000). Overall we consider that the contribution of organic carbon
from these pollutants is insignificant.
A possibly more important contributor that might influence δ13C value of the sediment
is crude oil. The light oil from the Pearl River estuary is characterized by low density
(less than 0.83 g cm-3), high contents of saturated hydrocarbons (71%-93%) and no
asphaltenes (Guo and He, 2006). δ13C of the crude oil from Pearl River estuary is
between −27.09‰ and –26.48‰ (Guo and He, 2006), slight heavier than that of C3 plants.
However, according to biomarker results (David Strong, pers. comm.), the total amount
of lipids from crude oil in these samples ranges from 100 to 500 µg/g, which compared to
the average TOC values of 0.5-2% (i.e. 5-20 mg/g, Table 5), indicates a maximum
concentration of 10% (0.5% - 10%). Furthermore, biomarker results also suggested that
the apolar freactions found in the crude oil would most likely already include the PCBs.
These data indicate that sedimentary organic carbon contributors other than plants,
planktons and algae, such as human waste, organic pollutants and crude oil, are less
important. The maximum concentration of organic carbon from these anthropogenic
sources is less than 10%. Thus, in spite of the influences of organic carbon from other
sources, bulk organic δ13C and C/N is still a good indicator for the source of estuarine
sediment at the broadest scale from terrestrial, lowest deltaic or marine areas.
4.4.2 Degradation
Change in bulk sediment organic δ13C and C/N over time due to decomposition is an
important consideration in coastal palaeoenvironmental research (e.g. Wilson et al., 2005;
Lamb et al., 2006). For example, in the coastal wetlands of Louisiana, USA, sediment
δ13C shifts between −0.5‰ and −3.3‰ from the surface vegetation (Chmura et al., 1987).
Changes in C/N can also occur during decomposition, particularly in the early stages
(Valiela et al., 1985). Lamb et al. (2006) reviewed different studies and concluded that in
inter-tidal and supra-tidal sediments, this may result in much lower surface sediment C/N
compared with the overlying vegetation, whereas in sub-tidal sediments, C/N is
commonly slightly higher than in the suspended sediment phase.
The influence of degradation varies across the Pearl River delta area and estuary. In
the Pearl River delta, results show that soil organic matter generally has higher average
δ13C but lower C/N (Table 2) than that of overlying vegetation (Table 1). For example,
- 13 -
average δ13C of mangrove soil is 3.1‰ higher than that of overlying mangrove plants,
and average C/N of mangrove soil is 12.4 compared to 31.0 for that of mangrove plants.
Degradation effect might be an important reason for differences in δ13C and C/N between
SOM and overlying plants. However, SOM receives organic carbon from all overlying
vegetation which is a mixture of both C3 and C4 types. Thus differences of δ13C between
the mixed type of organic matter (SOM) and individual plants should also be significant.
In the Pearl River estuary, sedimentary organic matter is mainly derived from riverine
and marine areas (Zong et al., 2006). Much of the terrigenous component of riverine
organic matter may have already been extensively degraded on land or further upstream
and should be relatively resistant to further degradation (Hedge and Keil, 1995).
Degradation of in situ POC might become significant for surface sediment δ13C. Results
show that POC from both seasons exhibit much lower C/N ratios than surface sediments
(Table 3, 5). The contrast is particularly obvious in fluvial areas (e.g. G2, G4), and
becomes more similar in marine area (e.g. G6). Similar phenomenon in C/N ratio has
been reported by Cifuentes (1991) from the Delaware Estuary, southeast USA. Cifuentes
(1991) has also reported differences in δ13C values between the two phases. In the upper
estuary, δ13C of POC (−28.9‰) was lower than that of surface sediment (−26.3‰), whilst
in the lower estuary, suspended sediment δ13C (−20.1‰) was higher than surface
sediment δ13C (−23.5‰) (Cifuentes, 1991). However, changes in δ13C between POC and
surface sediment in the Pearl River estuary are not so significant when POC from both
summer and winter are taken into consideration. This suggests that differential
decomposition of organic detritus may partially explain these observations especially in
C/N, seasonality may be more important in the Pearl River estuary, and possibly in the
Delaware Estuary, USA (Cifuentes, 1991).
Degradation can have a potentially significant influence on both sediment δ13C and
C/N. Early loss of labile material in vascular vegetation can lead to significant shifts in
soil δ13C and C/N but is insufficient to prevent the distinction between sediments from C3
and C4 vegetated soil. The degradation of POC in estuarine areas suggests using either
proxy in isolation might produce misleading results. It is important to combine δ13C and
C/N when indicating sediment sources and environmental changes from coastal areas
(Wilson et al., 2005; Zong et al., 2006).
5.
Conclusions
1. In the Pearl River delta, while C3 plants have lower δ13C than C4 plants, –29.0±1.8‰
and –13.1±0.5‰ respectively, C/N of both C3 and C4 are widely variable and the
overlap between groups is considerable.
2. δ13C and C/N for terrestrial soil organic matter are highly related to the source of
organic matter. Soil samples from C3-plant-dominant areas, e.g. the forest, tend to have
lower δ13C than those from C4-grass-dominant areas, e.g. the riverside area. Their C/N,
however, are widely variable and significantly overlap. Thus, while the δ13C of soil
can be used to indicate the predominant vegetation type, C/N cannot.
3. δ13C and C/N of POC show significant variability due to seasonal variation of
freshwater flux resulting in changes the balance between the TOM and in situ
phytoplankton in the POC.
- 14 -
4. δ13C of surface sediment in the estuary increases from –25.0‰ at the freshwater end to
–21.0‰ at the marine end, with C/N decreasing from 15.2 to 6.8, suggesting a
weakening terrestrial/freshwater input and a strengthening marine contribution
seaward.
5. Organic carbon from other sources might influence δ13C of surface sediments.
Agricultural input might elevate δ13C of sedimentary organic matter due to enhanced
fertilization process and, more significantly, by an increase in the proportion of C4
plants such as sugarcane grown in the catchment area. Organic carbon contributed
from crude oil and other anthropogenic inputs such as PAHs, PCBs and
organochrolinate pesticides, counts up to 10% of the TOC in the sediment according to
biomarker results, which might also elevate organic δ13C of the sediment.
6. Degradation accounts for some of the shift in δ13C and C/N between soil and overlying
vegetation, as well as between surface sediments and POC. However, the change is not
significant enough to prevent application of δ13C and C/N as indicator for dominant
overlying vegetation type and sediment source.
7. Based on the analysis of modern samples of organic matter from source to sink in the
Pearl River catchment, delta and estuary, we conclude that it is possible to use δ13C
and C/N of bulk organic matter to indicate vegetation type of the source area,
sedimentary environment such as range of salinity, and dominant sources of estuarine
sediment at the broadest scale from terrestrial, estuarine and marine environment.
Acknowledgements
This research is part of the PhD project sponsored by NERC/EPSRC (UK) through the
Dorothy Hodgkin Postgraduate Award (to FY). This research is also supported by the
University of Durham through a special research grant (to YZ), the NERC (UK) Radiocarbon
Laboratory Steering Committee (1150.1005) (to YZ) and the NERC Isotope Geosciences
Facilities Steering Committee through the organic isotope analyses awarded (IP/883/1105)
(to YZ). We also acknowledge support from the QRA (Quaternary Research Association),
the DGGA (Durham Geography Graduates Association), University College of Durham
University and the BSRG (British Sediment Research Group) for grants awarded to FY to
complete fieldwork and all laboratory visits. This research was also supported by the
Research Grants Council of the Hong Kong SAR through research grants HKU7058/06P
and HKU7052/08P (to W.W.-S. Yim). The authors also thank the director of the
Environmental Protection Department, Hong Kong SRA for the collection of surface
sediment samples and water salinity from the Hong Kong area.
- 15 -
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- 20 -
Figure captions
Fig. 1 Study area, showing the Pearl River delta and estuary in the East Asian area (a) and
sampling sites (b). There is no surface sediment from site PE43 and no winter POC
from site PE76 and 77. ‘PE’ stands for the ‘Pearl River estuary’.
Fig. 2 Clustering groups for the estuarine surface sediment samples and POC samples
identified by winter salinity and sand concentration. Groups G1-G3 are from
freshwater areas; groups G4-G7 are from brackish areas and G8-G9 are from
marine areas.
Fig. 3 Bi-plot of the δ13C and C/N of C3 and C4 plants. a: individual values for each plant
with the average value of C3 and C4 plants; b: average values of different types of
plants within the C3 and C4 groups (error bars are the standard deviation).
Fig. 4 Bi-plot of the δ13C and C/N of terrestrial soil samples. a: individual values for each
soil sample; b: average values of different types of soil with stand deviation.
Fig. 5 Bi-plot of seasonal δ13C and C/N of the POC.
Fig. 6 A summarized bi-plot of seasonal δ13C and C/N (a: winter and b: summer) and biplot of seasonal δ13C and water salinity (c: winter and d: summer) of the POC for
each group indentified by cluster analysis.
Fig. 7 Comparison δ13C and C/N values between the POC on 70 µm net and summer
POC on filter paper.
Fig. 8 Bi-plot of the δ13C and C/N of surface sediment groups. a: scattering value of the
individual sample; b:average δ13C and C/N values of the seven groups with
standard deviation shown.
- 21 -
Tables
General terrestrial C3 plants
Table 1 Results of the plants samples and their sampling sites.
Agricultural C3
plants
Ave.
Mangroves
C3 plants
Ave.
Ave.
General C4
plants
Ave.
Agricult
ural C4
plants
C4 plants
Ave.
Ave.
Ave.
δ13C (‰)
-28.5
-29.9
-29.3
-28.1
-30.3
-28.1
-31.0
-29.9
-28.3
-29.6
-29.6
-31.4
-30.2
-30.7
-28.9
-29.5
-28.7
-28.7
-28.3
-30.6
-30.9
-30.2
-29.9
-32.6
-31.7
-30.4
-31.6
-30.0
-31.8
-28.0
-28.3
-28.4
-32.1
-29.9 ±1.3
-28.2
-30.2
-28.7
-28.6
-28.9
-27.3
-25.8
-28.2 ±1.4
-28.9
-27.9
-28.1
-28.5
-25.0
-25.0
-24.0
-28.4
-25.6
-28.3
-28.1
-27.4
-27.1 ±1.7
-29.0 ±1.8
TOC (%)
41.6
46.7
42.0
43.3
45.3
40.1
45.3
40.9
41.1
42.3
38.6
45.1
44.3
36.6
37.0
39.2
39.6
48.1
44.9
42.2
44.4
47.2
38.0
43.2
39.2
40.8
42.3
41.1
40.3
45.6
48.1
35.2
48.0
42.3 ±3.5
42.0
35.7
38.0
40.0
39.4
43.2
44.1
40.3 ±3.0
45.3
46.6
49.1
45.8
43.6
42.2
40.7
46.7
41.1
45.4
44.2
45.1
44.6 ±2.4
42.6 ±3.4
TN (%)
3.1
3.7
2.3
5.9
3.9
1.9
1.8
2.9
4.2
2.0
2.0
1.4
2.1
1.9
3.5
2.4
3.8
0.8
1.4
1.9
2.1
1.6
1.9
2.1
1.7
1.9
3.1
1.2
0.9
2.1
1.6
2.4
2.0
2.4 ±1.1
2.8
2.3
3.4
3.3
3.5
2.4
3.5
3.0 ±0.5
2.0
1.4
0.9
1.3
0.9
2.2
3.0
1.3
1.8
1.7
1.0
2.3
1.7 ±0.6
2.3 ±1.0
C/N
13.4
12.5
18.1
7.4
11.5
21.4
25.4
14.0
9.8
21.0
19.1
31.2
20.8
19.2
10.7
16.3
10.4
61.8
31.1
22.2
20.9
30.3
20.1
20.5
23.5
21.7
13.5
33.3
43.1
21.6
30.2
14.6
24.2
21.7 ±10.7
15.1
15.3
11.1
12.2
11.1
18.3
12.6
13.7 ±2.7
22.5
32.8
53.2
34.5
46.3
19.1
13.4
36.6
22.4
27.2
45.2
19.4
31.0 ±12.5
22.7 ±11.6
Type
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
Dizygoyheca elegantissima
Pteris semipinnata
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
C3 Plant
Label
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
B12
B13
B14
B15
B16
B17
B18
B19
B20
B21
B22
B23
B24
B25
B26
B27
B28
B29
B31
B32
B35
B47
Site
N1
N2
N2
N2
N3
N3
N3
N3
N4
N4
N4
N4
N4
N5
N5
N5
N5
N6
N6
N6
N6
N6
N6
N6
N6
N6
N6
N6
N6
E1
E1
E1
E1
Reeds
Reeds
Rice
Rice
Rice
Banana
Lotus
F3
F5
F10
F11
F13
F12
F14
E5
E5
E5
E5
E5
E5
E5
Avicennia
Kandelia obovata, Rhizophoraceae
Kandelia obovata, Rhizophoraceae
Kandelia obovato, Rhizophoraceae
Bruguiera gymhorrhiza, Rhizophoraceae
Acanthus ilicifolius, flowers
Aegiceras corniculatum, Myrsinaceae
Acanthus ilicifolius, Acanthaceae
Acanthus ilicifolius, Acanthaceae
Avicennia marina, Avicenniaceae
Ficus microcarpa, Moraceae
Ficus microcarpa, Moraceae
B30
B36
B45
B42
B44
B37
B38
B40
B41
B43
B39
B46
E2
E3
E4
E4
E4
E3
E3
E3
E3
E4
E3
E4
-13.1
-13.2
-13.7
-13.7
-13.3
-12.3
-13.2 ±0.5
-12.9
-12.8
-12.5
-12.7 ±0.2
-13.1 ±0.5
35.5
39.8
40.0
41.2
38.8
39.7
39.2 ±1.9
41.6
42.4
41.4
41.8 ±0.5
40 ±2.0
4.3
1.2
1.7
1.7
1.5
2.8
2.2 ±1.2
1.0
1.6
1.6
1.4 ±0.3
1.9 ±1.0
8.2
33.1
23.9
23.8
26.1
14.2
21.5 ±8.9
40.3
25.9
25.6
30.6 ±8.4
24.6 ±9.4
C4 grass
C4 grass
C4 grass
Panicum maximum Jacq
C4 grass
C4 grass
F6
F1
F2
B33
B34
F4
E5
E5
E5
E1
E1
E5
sugarcane
sugarcane
sugarcane
F7
F8
F9
E5
E5
E5
- 22 -
Forest
soil
Table 2 Results of the soil samples and their sampling sites.
Riverbank soil
Ave.
Mangrove soil
Ave.
Agricult
ural soil
Ave.
Ave.
δ13C (‰)
-28.9
TOC (%)
2.7
TN (%)
0.2
C/N
15.4
Label
A5
Site
N4
-27.7
3.9
0.2
20.4
A9
N6
-28.3 ±0.8
3.3 ±0.9
0.2 ±0.0
17.9 ±3.6
-23.2
-22.9
-24.9
-23.6
-25.0
-23.6
-24.0
-25.6
-24.1 ±1.0
1.9
1.2
2.0
1.4
2.6
0.9
3.1
3.4
2.1 ±1.0
0.2
0.1
0.2
0.1
0.1
0.1
0.2
0.3
0.2 ±0.1
12.1
9.8
12.7
11.1
17.3
10.7
13.4
12.7
12.5 ±2.3
A1
A2
A3
A4
A6
A7
A8
G1
N1
N2
N3
N4
N4
N5
N5
E5
-25.6
-23.8
-24.7
-24.2
-26.1
-25.4
-19.3
-22.5
-25.8
-23.0
-23.8
-24.0
-24.0 ±1.9
2.7
1.1
1.8
2.4
1.8
1.1
4.4
0.9
2.4
2.5
0.7
1.5
1.9 ±1.0
0.3
0.1
0.2
0.2
0.1
0.1
0.3
0.0
0.1
0.2
0.1
0.1
0.2 ±0.1
9.7
8.6
9.3
11.4
12.9
11.0
15.7
19.3
18.8
10.4
10.7
10.8
12.4 ±3.6
A10
A11
A12
A13
A14
A15
A16
A17
A18
A19
A20
PE76
E3
E3
E3
E3
E3
E3
E3
E3
E4
E4
E4
E1
-21.2
-22.5
-21.5
-21.7 ±0.7
1.6
1.7
2.3
1.9 ±0.4
0.2
0.2
0.3
0.2 ±0.1
9.9
9.1
7.7
8.9 ±1.1
G2
G3
G4
E5
E5
E5
- 23 -
Table 3 Results of the POC samples and water conditions of the sampling sites (to be continued).
Gr.
PE 41
δ13C-W (‰)
C/N-W
δ13C-S (‰)
C/N-S
Sal-W (‰)
Sal-S (‰)
PH-W
PH-S
TDS-W (g/l) TDS-S (g/l) DO-W (%)
DO-S (%)
Temp-W (ºC) Temp-S (ºC)
-27.3
6.4
-25.0
8.3
0.9
0.1
7.7
7.5
1.2
0.1
98.5
79.3
17.4
28.2
PE 42
G2
6.6
-25.4
9.0
0.2
0.1
7.5
7.4
0.2
0.1
94.0
79.6
16.9
29.2
PE43
-27.8
6.9
-25.2
8.6
0.1
0.1
7.5
7.4
0.2
0.1
93.0
94.0
17.2
28.9
PE 44
-28.5
6.9
-25.2
8.5
0.1
0.1
7.6
7.4
0.2
0.1
93.3
88.6
17.1
28.8
PE 45
-28.3
6.5
-25.4
8.1
0.1
0.1
7.5
7.4
0.2
0.1
94.0
88.7
16.7
28.9
PE 46
-27.5
8.1
-24.8
9.0
0.1
0.1
7.5
7.4
0.2
0.1
96.5
87.8
16.6
28.8
PE 47
-26.2
6.5
-23.3
9.6
0.1
0.1
7.6
7.5
0.2
0.2
85.5
81.0
17.7
27.9
-27.6±0.8
6.8±0.6
-24.9±0.7
8.7±0.5
0.2±0.3
0.1±0.0
7.5±0.1
7.4±0.0
0.3±0.4
0.1±0.0
93.5±4.1
85.6±5.6
17.1±0.4
28.7±0.4
-23.0
10.0
0.8
0.1
7.6
7.6
Ave.
PE 48
G4
27.8
-24.7
7.5
-22.9
9.9
0.5
0.1
7.6
7.5
0.6
0.2
87.5
84.8
17.5
28.0
PE 50
-26.0
7.6
-23.4
7.6
1.8
0.1
7.6
7.5
2.2
0.2
89.5
75.1
17.6
27.8
PE 51
-25.2
7.5
-24.3
11.3
1.5
0.1
7.6
7.6
1.8
0.2
89.0
76.3
17.7
27.8
PE 52
-26.0
7.5
-23.6
10.2
1.5
0.1
7.6
7.7
1.9
0.2
90.2
76.3
17.6
27.9
PE 53
-26.0
7.3
-24.1
10.1
4.3
0.1
7.7
7.6
5.0
0.2
101.9
72.5
17.2
28.1
PE 56
-24.7
7.2
-24.8
9.2
12.5
0.1
7.8
7.8
13.5
0.2
115.0
83.8
16.6
27.8
PE 54
-27.7
6.4
-25.6
6.9
2.3
0.1
7.7
7.6
2.8
0.2
96.6
73.8
17.6
27.7
-25.8±1.0
7.3±0.4
-24.0±0.9
9.4±1.5
3.1±4.0
0.1±0.0
7.7±0.1
7.6±0.1
4.0±4.4
0.2±0.0
95.7±9.9
79.3±6.8
17.4±0.4
27.8±0.1
4.8
0.1
PE 76
Ave.
92.1
PE 49
Ave.
G3
0.2
PE 55
-26.4
6.8
-24.9
8.8
6.3
0.1
7.8
7.7
7.2
0.2
106.6
84.5
17.1
27.6
PE 59
-23.7
6.5
-25.9
8.1
23.1
2.9
8.0
7.9
23.6
3.8
112.0
86.5
18.3
28.5
PE 60
-24.0
7.7
-26.1
8.8
26.9
5.9
8.0
7.8
27.1
6.9
109.9
79.7
18.5
28.3
PE 61
-23.9
7.7
-24.7
9.8
22.1
4.0
7.9
7.7
22.8
4.6
108.7
77.2
18.6
28.4
-24.5±1.3
7.2±0.6
-25.4±0.7
8.9±0.7
16.6±10.3
2.6±2.5
7.9±0.1
7.7±0.1
20.2±8.9
3.9±2.8
109.3±2.3
82.0±4.3
18.1±0.7
28.2±0.4
S: summer; W: winter; ‘Gr.’ shows different clustering groups of the POC samples.
24
Table 3 Results of the POC samples and water conditions of the sampling sites (continued).
Gr.
G5
PE 69
PE 70
PE 71
PE 72
PE 73
PE 64
PE 57
PE 58
PE 62
PE 63
PE 75
δ13C-W (‰)
C/N-W
δ13C-S (‰)
C/N-S
Sal-W (‰)
Sal-S (‰)
PH-W
PH-S
TDS-W (g/l) TDS-S (g/l) DO-W (%)
DO-S (%)
Temp-W (ºC) Temp-S (ºC)
-22.3
-21.6
-22.5
-21.8
-21.7
-23.9
-24.0
-24.8
-25.2
-23.8
-25.0
7.5
8.3
6.8
8.6
8.7
8.4
9.9
6.1
7.3
8.9
8.9
-24.4
-25.7
-26.1
-25.6
-25.7
-23.3
-23.0
-23.1
-23.0
-23.3
-22.9
8.8
10.6
31.5
33.6
33.9
32.8
32.3
18.6
11.8
13.1
11.6
13.4
14.1
3.4
6.6
5.8
3.6
5.7
0.2
0.1
1.0
0.2
0.1
0.1
8.1
8.15
8.1
8.1
8.2
8.0
7.9
7.9
7.9
7.9
7.9
8.2
8.1
8.1
8.3
8.2
8.0
7.9
7.9
8.1
8.0
8.0
31.3
33.2
33.5
32.5
32.0
19.5
12.8
14.1
12.6
14.0
14.8
4.7
7.6
6.7
4.3
7.7
0.2
0.2
1.4
0.2
0.2
0.2
113.2
111.6
110.7
116.8
113.1
113.6
116.0
115.7
104.0
114.3
113.0
80.4
79.6
81.2
86.4
93.7
97.0
92.3
89.2
86.1
100.1
102.8
18.1
18.6
18.5
18.1
18.1
17.0
16.5
17.1
17.4
16.6
17.5
28.1
28.2
28.2
28.2
28.2
28.8
27.9
28.1
27.9
28.6
28.2
14.8
7.0
10.1
9.8
11.4
9.9
8.0
11.3
PE 77
PE 65
PE 66
PE 67
PE 68
PE 74
PE 78
PE 79
PE 80
PE 81
Ave.
G6
Ave.
PE 82
PE 83
PE 84
PE 85
PE 86
PE 87
PE 88
PE 89
PE 90
PE 91
PE 92
-23.6
-24.5
-24.9
-25.1
-23.9
-23.4
-23.2
-22.9
-24.3
7.3
8.2
6.5
6.8
6.8
7.1
6.4
6.9
7.1
-24.1
-24.6
-24.7
-24.6
-25.1
8.7
8.1
9.6
9.4
8.2
22.2
23.9
22.3
25.7
24.3
25.4
25.5
16.9
18.1
0.8
5.5
2.7
4.9
2.0
13.5
15.2
18.2
15.8
8.1
8.0
8.0
8.1
8.1
7.9
8.0
7.9
7.8
8.0
7.9
8.3
8.2
8.0
22.8
24.4
22.9
26.7
24.8
25.8
25.9
27.2
28.7
1.1
6.5
3.3
5.7
2.3
113.0
115.0
117.0
117.2
115.4
91.6
95.8
93.6
94.7
92.7
90.2
88.1
85.1
88.7
17.1
16.9
16.9
17.6
17.6
17.6
17.9
18.2
18.0
29.1
29.5
29.1
28.2
28.3
-23.6±1.2
7.6±1.0
-24.3±1.1
9.5±1.2
22.2±7.6
5.4±5.7
8.0±0.1
8.1±0.1
24.0±7.2
3.3±2.9
109.8±8.6
89.6±6.7
17.6±0.6
28.4±0.5
-23.0
-22.6
-23.3
-21.9
-22.6
-21.7
-22.5
-22.6
-22.2
-21.8
-22.3
7.2
6.9
5.9
5.7
6.4
6.2
6.1
6.9
5.9
6.1
6.3
32.9
31.9
30.9
35.0
31.3
34.7
30.8
32.5
34.3
35.1
34.8
18.6
22.0
20.3
24.0
25.4
27.5
20.0
25.0
26.2
28.2
28.4
7.9
8.1
8.1
7.9
8.1
8.2
8.1
8.0
8.2
8.2
8.0
32.5
31.7
30.6
34.4
31.1
34.1
30.7
32.2
33.8
34.5
34.2
96.4
96.0
99.0
101.0
98.5
97.0
94.1
94.8
96.3
97.4
93.5
18.8
19.0
19.2
19.8
18.8
19.4
19.0
18.9
19.3
19.7
19.4
-22.4±0.5
6.3±0.5
33.1±1.7
24.1±3.5
8.1±0.1
32.7±1.6
96.7±2.2
19.2±0.3
S: summer; W: winter; ‘Gr.’ shows different clustering groups of the POC samples.
25
26
Table 4 Results of the POC samples on the70 µm net (summer) and on the filter paper
Summer
POC (70 µm
net)
PN50
PN51
PN52
PN53
PN56
PN57
PN63
PN64
PN72
PN65
PN67
PN68
PN71
PN73
Ave.
SD
δ13C
C/N
-25.9
-25.8
-24.8
-24.4
-25.4
-25.1
-25.1
-25.1
-24.2
-27.2
-26.2
-27.0
-27.2
18.4
14.6
12.1
12.5
9.7
12.7
13.6
13.8
15.3
5.7
4.6
6.5
5.0
5.5
+10.7
+4.5
-25.6
+1.0
Summer
POC (filter
paper)
PS50
PS51
PS52
PS53
PS56
PS57
PS63
PS64
PS72
PS65
PS67
PS68
PS71
PS73
δ13C
C/N
-23.4
-24.3
-23.6
-24.1
-24.8
-23.0
-23.3
-23.3
-25.6
-24.1
-24.7
-24.6
-26.1
-25.7
-24.3
+1.0
7.6
11.3
10.2
10.1
9.2
9.8
8.0
10.1
8.7
9.6
9.4
+9.5
+1.1
Winter POC
(filter paper)
δ13C
C/N
PW50
PW51
PW52
PW53
PW56
PW57
PW63
PW64
PW72
PW65
PW67
PW68
PW71
PW73
-26.0
-25.2
-26.0
-26.0
-24.7
-24.0
-23.8
-23.9
-21.8
-23.6
-24.9
-25.1
-22.5
-21.7
-24.2
+1.5
7.6
7.5
7.5
7.3
7.2
9.9
8.9
8.4
8.6
7.3
6.5
6.8
6.8
8.7
+7.8
+1.0
PS: summer plankton samples on filter paper; PW: winter plankton samples on filter
paper; PN: summer plankton samples on a 70µm net; S.D.: standard deviation.
- 27 -
Table 5 Results of the surface sediment and environmental variables of the sampling sites
(to be continued)
Gr.(n) Sample ID Long (ºE) Lat(ºN)
δ13C (‰) C/N
TOC (%) TN(%)
PE 17
PE 18
G1(4) PE 19
PE 20
Ave.
PE 16
PE 15
PE 14
PE 10
PE 09
PE 11
G2(14) PE 12
PE 13
PE 41
PE 42
PE 44
PE 45
PE 46
PE 47
Ave.
PE 48
PE 49
PE 50
PE 51
G3(8) PE 52
PE 53
PE 56
PE 54
Ave.
PE 76
PE 01
PE 55
PE 08
PE 03
G4(11) PE 05
PE 06
PE 07
PE 59
PE 60
PE 61
Ave.
-26.6
-23.3
-25.1
-25.0
-25.0±1.3
-23.7
-25.4
-22.7
-22.8
-23.6
-23.4
-22.7
-24.5
-24.8
-23.4
3.79
0.91
1.88
2.13
2.2±1.2
1.02
1.28
0.74
1.14
0.99
1.50
1.11
1.30
1.39
0.79
0.28
0.22
1.37
0.45
1.0±0.4
0.98
1.03
1.41
0.75
0.52
1.14
0.77
1.54
1.0±0.3
1.49
1.02
1.40
1.51
1.17
1.32
1.27
1.13
1.34
0.66
0.38
1.2±0.4
113.57
113.55
113.52
113.48
22.95
23.00
23.05
23.10
113.58
113.62
113.65
113.64
113.67
113.58
113.54
113.49
113.53
113.44
113.37
113.33
113.47
113.36
22.88
22.82
22.77
22.68
22.63
22.76
22.78
22.80
22.77
22.80
22.88
22.88
22.86
22.78
113.39
113.46
113.52
113.54
113.57
113.59
113.55
113.62
22.75
22.72
22.68
22.66
22.64
22.61
22.49
22.57
113.65
113.64
113.63
113.72
113.74
113.83
113.80
113.76
113.72
113.75
113.72
22.44
22.58
22.53
22.61
22.45
22.54
22.52
22.56
22.47
22.50
22.55
20.1
12.6
13.8
14.4
15.2±3.3
13.4
14.6
11.4
11.4
13.0
13.4
13.9
16.3
15.9
11.7
10.5
10.2
-23.0
12.8
-26.8
12.9
-23.9±1.2 13.0±1.8
-22.0
10.7
-21.9
9.2
-26.7
11.4
-22.6
10.9
-23.8
13.4
-22.0
10.7
-22.2
10.0
-22.9
12.3
-23.0±1.6 11.1±1.3
-24.0
10.8
-24.2
17.5
-23.9
17.1
-24.1
12.7
-23.1
11.5
-23.3
12.3
-23.5
14.6
-23.5
17.7
-23.4
10.5
-24.2
14.9
-27.0
15.1
-24.0±1.1 14.1±2.7
0.11
0.07
0.14
0.15
0.1±0.0
0.08
0.09
0.07
0.10
0.08
0.11
0.08
0.08
0.09
0.07
0.03
0.02
0.11
0.04
0.1±0.0
0.09
0.11
0.12
0.07
0.04
0.11
0.08
0.13
0.1±0.0
0.14
0.06
0.08
0.12
0.10
0.11
0.09
0.06
0.13
0.04
0.03
0.1±0.0
Mean
Sand (%) Clay (%)
Sal. (‰)
5.1
42.8
15.0
3.2
34.9
20.7
2.6
29.3
22.5
1.4
23.7
24.2
3.1±1.6 32.7±8.2 20.6±4.0
6.4
33.3
19.5
8.2
22.6
24.0
12.2
8.4
32.4
9.8
12.5
30.5
11.6
24.0
22.0
3.3
12.1
21.3
1.6
44.0
13.2
0.8
34.2
11.8
1.6
20.6
12.7
1.6
53.6
8.2
0.4
67.4
10.3
-0.2
65.6
7.3
-0.1
19.4
23.7
-0.1
53.1
11.9
4.1±4.6 33.6±20.0 17.8±8.1
0.4
24.8
20.7
0.4
11.7
23.3
1.0
8.3
30.7
1.0
27.2
19.6
2.0
20.5
12.9
3.0
1.7
25.7
6.5
20.6
27.2
3.9
6.6
22.6
2.3±2.1 15.2±9.3 22.8±5.4
2.5
14.9
23.9
6.0
32.9
12.8
7.0
55.8
7.3
14.5
19.9
25.4
19.9
22.3
26.1
23.4
18.9
29.1
20.3
16.8
28.2
17.2
30.8
17.8
16.0
12.7
23.9
19.3
63.2
8.6
14.0
42.5
17.9
14.5±6.7 30.1±17.1 20.1±7.7
‘Gr.(n)’ shows the number (n) of samples in each clustering groups (Gr.).
- 28 -
Silt (%)
WD (m)
42.2
2.1
44.4
2.4
48.2
2.3
52.1
2.1
46.7±4.4 2.2±0.1
47.2
2.5
53.5
10.5
59.2
3.8
57.0
5.0
53.9
2.1
66.6
2.2
42.8
4.0
54.0
3.2
66.7
5.0
38.2
2.2
22.3
2.2
27.1
1.8
56.9
3.5
35.0
3.8
48.6±13.8 3.7±2.2
54.5
2.3
65.0
2.9
61.0
3.2
53.2
5.2
66.6
4.1
72.7
7.1
52.2
3.5
70.8
6.0
62.0±8.0 4.3±1.7
61.2
7.1
54.3
4.4
37.0
8.0
54.7
2.8
51.6
5.0
52.0
4.0
55.0
5.8
51.4
4.8
63.4
15.0
28.2
18.0
39.5
16.0
49.8±10.7 8.3±5.4
Table 5 Results of the surface sediment (groups) and environmental variables (continued)
Gr.(n) Sample ID Long (ºE) Lat(ºN)
G5(22)
PE 69
PE 70
PE 71
PE 72
PE 73
PE 02
PE 64
PE 57
PE 58
PE 62
PE 63
PE 75
PE 77
PE 65
PE 66
PE 67
PE 68
PE 74
PE 78
PE 79
PE 80
PE 81
113.72
113.77
113.78
113.75
113.75
113.69
113.65
113.65
113.68
113.68
113.66
113.68
113.68
113.62
113.60
113.64
113.68
113.70
113.60
113.65
113.70
113.60
22.34
22.37
22.38
22.39
22.35
22.50
22.38
22.45
22.46
22.58
22.45
22.49
22.49
22.33
22.29
22.31
22.32
22.36
22.25
22.25
22.25
22.19
PE 82
PE 83
PE 84
PE 85
PE 86
PE 87
PE 88
PE 89
PE 90
PE 91
PE 92
113.65
113.70
113.62
113.67
113.60
113.65
113.77
113.76
113.75
113.73
113.80
22.19
22.19
22.13
22.13
22.08
22.08
22.25
22.19
22.13
22.08
22.10
PE 29
PE 30
PE 31
PE 32
PE 33
PE 34
PE 39
PE 40
PE 35
PE 36
PE 37
PE 04
PE 38
114.08
114.12
114.02
113.96
114.10
113.98
113.93
113.96
113.95
113.90
113.90
113.83
113.89
22.23
22.25
22.26
22.22
22.32
22.35
22.46
22.48
22.36
22.32
22.38
22.46
22.43
PE 25
PE 26
PE 27
PE 28
114.27
114.23
114.18
114.08
22.24
22.21
22.22
22.19
PE 21
PE 22
PE 23
PE 24
114.45
114.45
114.45
114.32
22.37
22.29
22.22
22.20
Ave.
G6 (11)
Ave.
G7(13)
Ave.
G8(4)
Ave.
G9(4)
Ave.
δ13C (‰)
C/N
-23.1
-23.6
-23.8
-22.0
-22.9
-23.7
-23.2
-22.7
-23.2
9.9
12.0
11.6
9.9
10.3
11.7
9.9
10.7
11.4
8.5
11.2
11.8
11.7
11.3
8.9
8.9
12.2
11.1
8.4
9.1
9.7
8.8
10.4±1.3
9.4
9.6
9.4
9.7
10.3
10.3
9.4
9.7
9.9
10.6
8.8
9.7±0.5
8.8
9.0
7.8
8.0
10.3
10.1
9.3
9.0
10.6
9.0
10.4
11.1
12.3
10.4±3.0
8.0
7.1
7.7
10.0
8.2±1.2
6.8
6.8
6.5
7.1
6.8±0.2
-23.5
-23.1
-22.3
-23.5
-22.9
-22.6
-23.4
-22.9
-23.1
-23.1
-23.4
-23.1
-23.1±0.4
-22.9
-23.2
-23.3
-22.9
-23.4
-23.0
-23.2
-23.2
-22.9
-23.1
-22.5
-23.1±0.2
-22.3
-23.2
-21.8
-21.9
-24.1
-24.1
-23.8
-23.9
-22.2
-22.8
-23.2
-23.9
-22.8
-23.1±0.9
-21.5
-21.0
-21.8
-22.5
-21.7±0.6
-21.0
-20.8
-21.2
-21.1
-21.0±0.2
TOC (%) TN(%)
1.23
1.38
0.95
0.96
1.13
1.69
1.27
1.35
1.26
0.25
1.21
1.11
1.11
0.71
0.94
1.07
1.13
1.38
1.24
1.25
1.26
1.24
1.1±0.3
1.13
1.16
1.23
0.90
1.50
0.93
1.09
1.10
1.09
0.91
0.98
1.1±0.2
1.02
0.83
1.13
0.62
0.59
0.78
1.05
1.23
1.45
0.39
0.56
0.90
0.75
0.9±0.3
0.57
0.68
0.92
1.60
0.9±0.5
0.68
0.75
0.63
0.92
0.7±0.1
0.12
0.12
0.08
0.10
0.11
0.14
0.13
0.13
0.11
0.03
0.11
0.09
0.10
0.06
0.11
0.12
0.09
0.12
0.15
0.14
0.13
0.14
0.1±0.0
0.12
0.12
0.13
0.09
0.15
0.09
0.12
0.11
0.11
0.09
0.11
0.1±0.0
0.12
0.09
0.14
0.08
0.06
0.08
0.11
0.14
0.07
0.04
0.05
0.08
0.06
0.1±0.0
0.07
0.09
0.12
0.09
0.1±0.0
0.10
0.11
0.10
0.13
0.1±0.0
Mean
Sand (%)
Sal. (‰)
20.8
12.5
24.1
15.7
24.0
15.8
17.9
16.8
20.3
18.4
11.9
24.6
13.5
36.0
10.7
13.3
12.0
12.5
11.0
37.8
13.1
9.3
11.0
26.9
10.9
23.5
14.1
12.8
13.8
22.0
15.5
7.1
18.5
19.9
17.0
16.2
17.6
20.3
19.9
9.0
22.1
13.7
20.3
16.1
16.4±4.4 18.2±7.9
23.0
16.9
25.9
14.7
24.3
15.4
27.4
19.6
28.8
17.1
30.0
20.9
25.4
16.0
28.5
14.8
29.4
18.6
30.7
16.9
30.9
15.9
27.7±2.7 17.0±2.0
31.3
15.8
32.1
2.6
30.4
6.8
31.4
8.3
31.5
29.0
30.3
18.9
24.9
4.0
20.3
13.8
30.5
13.9
30.2
24.9
29.8
47.8
26.0
35.7
29.8
11.9
29.1±3.4 18.0±13.2
33.5
21.3
32.4
16.5
32.4
9.9
31.1
23.5
32.4±1.0 17.8±6.0
33.3
17.3
33.4
14.4
33.5
19.8
33.5
3.4
33.4±0.1 13,7±7.2
Clay (%) Silt (%) WD (m)
33.3
31.3
28.4
30.5
28.9
28.6
11.1
29.8
29.2
11.2
27.0
20.8
17.8
34.5
31.1
38.3
18.4
33.0
33.2
32.5
35.2
31.3
28.0±7.4
28.2
29.7
29.5
25.2
28.7
20.2
30.2
25.6
24.3
19.8
21.5
25.7±3.9
24.8
31.0
32.8
15.8
21.7
24.9
32.3
23.3
29.3
22.4
15.2
21.5
28.2
24.9±5.7
17.6
17.4
26.0
18.1
19.8±4.1
18.8
21.6
24.6
27.8
23.2±3.9
‘Gr.(n)’ shows the number (n) of samples in each clustering groups (Gr.).
- 29 -
54.1
13.5
53.0
20.0
55.8
13.0
52.8
15.0
52.7
15.0
46.8
6.9
53.0
7.0
57.0
7.0
58.3
10.0
51.0
8.0
63.7
5.5
52.3
7.0
58.7
5.4
52.7
5.0
46.9
3.5
54.6
13.0
61.6
12.0
50.8
9.0
46.5
6.2
58.6
5.3
51.1
5.8
52.6
5.3
53.8±4.5 9.0±.4.3
54.9
7.0
55.6
6.0
55.1
6.8
55.2
8.5
54.2
6.0
58.9
8.5
53.8
6.3
59.6
8.5
57.1
8.5
63.3
11.5
62.6
14.0
57.3±3.3 8.3±2.5
59.4
8.0
66.5
35.0
60.4
8.0
75.9
6.0
49.2
20.0
56.2
11.0
63.8
4.0
62.9
3.0
56.8
14.0
52.6
5.0
37.0
20.0
42.8
4.8
59.9
8.0
57.2±10.1 11.3±9.1
61.1
21.0
66.1
14.0
64.1
14.0
58.4
14.0
62.4±3.4 15.8±3.5
63.9
24.0
63.9
25.0
55.6
28.0
68.7
31.0
63.0±5.5 27.0±3.2
Table 6 Spatial variation of the organic carbon signature and environmental conditions
within the Pearl River estuary
Environment δ13C (‰)
C/N
TOC (%) TN (%)
Mean Sal.
Sand (%) Clay (%) Silt (%) WD (m)
(‰)
Freshwater
-23.8±1.5 12.7±2.3 1.2±0.7 0.1±0.0 3.4±3.6 27.8±17.7 19.8±7.0 52.4±12.7 3.7±2.0
(G1-G3)
Fresh-brackish
-23.4±0.8 11.6±2.5 1.1±0.3 0.1±0.0 15.8±5.3 22.1±12.8 25.3±8.3 52.5±7.2 8.8±4.6
(G4-G5)
Marine-brackish
-23.1±0.6 9.7±1.0 1.0±0.3 0.1±0.0 28.4±3.1 17.5±9.7 25.3±4.9 57.2±7.7 9.9±6.9
(G6-G7)
Marine
-21.4±0.5 7.5±1.1 0.8±0.3 0.1±0.0 32.9±0.8 15.8±6.5 21.5±4.2 62.7±4.2 21.4±6.8
(G8-G9)
- 30 -
Figures
Fig. 1
- 31 -
Fig. 2
- 32 -
-10.0
a
δ13 C(‰)
-15.0
Average value
General C4 plants
Sugarcane (C4)
General C3 plants
Agricultural C3 plants
Mangroves (C3)
C4 plants
(24.6, -13.1)
-20.0
-25.0
-30.0
C3 plants
(22.7, -29.0)
-35.0
0.0
-10.0
10.0
20.0
40.0
b
50.0
60.0
70.0
Sugarcane (C 4 )
General C4 plants
-15.0
δ13 C(‰)
30.0
-20.0
-25.0
Agricultural C3 plants
Mangroves (C 3 )
-30.0
General C3 plants
-35.0
0.0
10.0
20.0
30.0
C/N
Fig. 3
- 33 -
40.0
50.0
-18.0
a
Agricultural soil
Riverbank soil
-20.0
Mangrove soil
Forest soil
δ13C(‰)
-22.0
-24.0
-26.0
-28.0
-30.0
6.0
-20.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
b
Agricultural soil
-22.0
Mangrove soil
-24.0
δ 13C(‰)
Riverbank soil
-26.0
Forest soil
-28.0
-30.0
+6.0
+8.0
+10.0
+12.0
+14.0
+16.0
C/N
Fig. 4
- 34 -
+18.0
+20.0
+22.0
-21.0
Winter
Summer
-22.0
-23.0
δ 13C(‰)
-24.0
-25.0
-26.0
-27.0
-28.0
-29.0
5.0
6.0
7.0
8.0
C/N
Fig. 5
- 35 -
9.0
10.0
11.0
- 36 -
-21.0
Winter δ13 C - C/N
-22.0
G6
Winter δ13 C - salinity
G5
-24.0
G4
-25.0
G4
-25.0
G3
G3
-26.0
c
G6
-23.0
G5
-24.0
-26.0
-27.0
-27.0
G2
G2
-28.0
-28.0
-29.0
-29.0
5.0
6.0
7.0
-21.0
8.0
9.0
10.0
Summer δ13 C - C/N
11.0
b
-22.0
0.0
5.0
10.0
-21.0
15.0
20.0
25.0
30.0
35.0
Summer δ13 C - salinity
40.0
d
-22.0
-23.0
-23.0
G3
-24.0
δ13C(‰)
-21.0
-22.0
-23.0
δ13C(‰)
a
G5
G2
-25.0
-27.0
-28.0
-28.0
-29.0
-29.0
5.0
6.0
7.0
8.0
9.0
G4
-26.0
-27.0
10.0
11.0
G5
G2
-25.0
G4
-26.0
G3
-24.0
0.0
5.0
10.0
15.0
20.0
25.0
Salinity (PSU)
C/N
Fig. 6
- 37 -
30.0
35.0
40.0
-22.0
-23.0
δ13 C(‰)
-24.0
9.5, -24.3
-25.0
10.7, -25.6
-26.0
Summer POC on NET
Summer POC on filterpaper
-27.0
Group average
-28.0
4.0
6.0
8.0
10.0
12.0
C/N
Fig. 7
38
14.0
16.0
18.0
-20.0
a
G9
G8
G7
G6
G5
G4
G3
G2
G1
-21.0
-22.0
δ13C(‰)
-23.0
-24.0
-25.0
-26.0
-27.0
-28.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
-20.0
20.0
b
G9
-21.0
G8
-22.0
δ13C(‰)
G7
G5
G3
-23.0
G6
G4
-24.0
G2
-25.0
G1
-26.0
-27.0
6.0
8.0
10.0
12.0
14.0
C/N
Fig. 8
39
16.0
18.0
20.0