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Total organic carbon fluxes of the Red River system
(Vietnam)
Thi Phuong Quynh Le,
1
*Viet Nga Dao,
1
Emma Rochelle-Newall,
2
Josette Garnier,
3
XiXi Lu,
4
Gilles Billen,
3
Thi Thuy Duong,
5
Cuong Tu Ho,
5
Henri Etcheber,
6
Thi Mai Huong Nguyen,
1
Thi Bich Ngoc Nguyen,
1
Bich Thuy Nguyen,
1
Nhu Da Le
1
and
Quoc Long Pham
1
1
Laboratory of Environmental Chemistry, Institute of Natural Product Chemistry, Vietnam Academy of Science and Technology, 18
Hoang Quoc Viet Road, Cau Giay, Hanoi Vietnam
2
Institut de Recherche pour le Développement (IRD), iEES -Paris, UMR 242, 32 avenue Henri Varagnat, 93143 Bondy cedex, France
3
University Paris 6, CNRS, UMR 7619, Metis, 4 Place Jussieu, Paris 75005, France
4
Department of Geography, National University of Singapore, Arts Link 1, Singapore 117570, Singapore
5
Institute of Environmental Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay, Hanoi
Vietnam
6
University of Bordeaux 1, Bordeaux, France
Received 16 July 2015; Revised 3 January 2017; Accepted 3 January 2017
*Correspondence to: Thi Phuong Quynh LE, Laboratory of Environmental Chemistry, Institute of Natural Product Chemistry, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet Road, Cau Giay, Hanoi, Vietnam. E-mail: quynhltp@yahoo.com
ABSTRACT: Riverine transport of organic carbon from terrestrial ecosystems to the oceans plays an important role in the global car-
bon cycle. The Red River is located in Southeast Asia where river discharge, sediment loads and fluxes of elements (carbon, nitrogen
and phosphorus) associated with suspended solids have been dramatically altered over past decades as a result of reservoir impound-
ment and land use, population, and climate change. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concen-
trations were measured monthly at four stations of the Red River system from January 2008 to December 2010. The results reveal that
POC changed synchronically with total suspended solids (TSS) concentration and with the river discharge, whereas no clear trend
was observed for DOC concentration. The mean value of total organic carbon (TOC =DOC +POC) flux in the delta of the Red River
was 31.5 × 10
13
± 4.0 × 10
13
MgC.yr
1
(range 27.9–35.8 × 10
13
MgC.yr
1
which leads to a specific TOC flux of 2012 ± 255 kgC.
km
2
.yr
1
during this 2008–2010 period. About 80% of the TOC flux was transferred to the estuary during the rainy season as a con-
sequence of the higher river water discharge. The high mean value of the POC:Chl-a ratio (1585 ± 870 mgC.mgChl-a
1
) and the
moderate C:N ratio (7.3 ±0.1) in the water column system suggest that organic carbon in the Red River system is mainly derived from
erosion and soil leaching in the basin. The effect of two new dam impoundments in the Red River was also observable with lower
TOC fluxes in 2010 compared with 2008. Copyright © 2017 John Wiley & Sons, Ltd.
KEYWORDS: dissolved organic carbon; particulate organic carbon; soil erosion; human impacts; Red River
Introduction
Rivers play an integral role in the global carbon cycle through
their transport and transformation of terrestrial organic carbon
(Hope et al. 1994; Burdige 2007; Bauer et al. 2013).The quan-
tity and quality of riverine dissolved and particulate organic
carbon (DOC and POC) are affected by both natural (e.g. plate
margin tectonics, volcanic deposits, high altitude, steep slopes,
and high intensity rainfall) and anthropogenic factors (e.g. de-
forestation, reservoir impoundment, agriculture, high popula-
tion density and urbanization) (Ludwig et al. 1996; Robertson
et al., 1996; Meybeck et al., 2003). POC and DOC concentra-
tions can therefore also be considered as being important indi-
cators of river water quality (Ni et al., 2008).
The total organic carbon (TOC = POC + DOC) load exported
from rivers to the global ocean is approximately 0.43 Gt.yr
1
,
of which 55–60% are as DOC and 40–45% POC (Ludwig
et al. 1996; Hedges et al. 1997; Schlunz and Schneider,
2000). Meybeck (1982) who examined riverine transport of
TOC from different climatic regimes proposed that carbon flux
from tropical regions was responsible for nearly 60% of the
global flux. This was further highlighted by Huang et al.
(2012) who noted that although tropical regions occupy only
42.7% of the global land surface such regions contribute ap-
proximately 66.2% of global freshwater outflow, 73.2% of
global sediment load, and 61% of terrestrial net primary
production.
In mainland Asia, rivers play an important role in the global
delivery of total suspended solids (TSS) and associated ele-
ments (e.g. C, N, P, Fe) to the coastal ocean. This is primarily
due to particularly high chemical and mechanical erosion rates
(up to 0.5 mm yr
1
) in the region (Meybeck et al., 1989; Ludwig
et al. 1996; Dupre et al. 2002). Moreover, during recent de-
cades, sediment loads and material fluxes have altered
EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms (2017)
Copyright © 2017 John Wiley & Sons, Ltd.
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/esp.4107
drastically as a result of reservoir impoundment, land use, pop-
ulation, and climate change (Walling and Fang, 2003; Lu 2004;
Vinh et al. 2014). Therefore, knowledge of particulate and dis-
solved carbon fluxes from large Asian rivers is essential for the
accurate quantification of geochemical cycles in the context of
global change studies.
The Red River (Vietnam and China) is characteristic of a
South-east Asian river system that is strongly affected by natu-
ral constraints and human activities. Previous studies have
shown that human activities in the basin have induced
changes in hydrology and in the suspended sediment and
associated element (N, P) loads of the Red River (Dang et al.
2010; Le et al. 2014). However, knowledge of the source,
transport and flux of terrestrial organic carbon, which is
closely related to the hydrology and to TSS in the Red River
system, is still limited.
This paper aims: (i) to present the results of the POC and
DOC concentrations in the Red River system with monthly ob-
servations from January 2008 to December 2010; ( ii) to quan-
tify the variability of TOC fluxes in the Red River system over
the same period; and (iii) to identify some of the factors that
may control organic carbon fluxes in the Red River system.
Study site and method
Study site
Geomorphology of the Red River basin
The Red River covers a watershed of 156 451 km
2
, of which
50.3% is in Vietnam, 48.8% in China and 0.9% in Laos. The
main branch of the Red River (called the Thao river) has two
major tributaries, the Da and the Lo rivers that join at Viet Tri.
The Red River then forms a large delta before flowing into the
Tonkin bay (South-east Asian Sea) through four distributaries
at Ba Lat, Lach Gia, Tra Ly and Day (Figure 1).
The mountain areas that dominate the upstream part of the
Red River are tectonically active and unstable, and this, com-
bined with intense rainfall, causes high erosion (Fullen et al.
1998) with the highest mean erosion rate over the 1993–1996
period of 10.69 t ha
1
(Barton et al. 2004). This may affect the
carbon loss from the land to river. Sandstones or mudstones
of mixed colors including red, purple, bluish gray, yellow and
gray-white in the upstream Chinese section of the river are
widely exposed to erosion giving the red color to the water
(Chinadata 1998) whereas in the Vietnamese part, soils are
Figure 1. The Red River basin and sampling sites. The four reservoirs (Thac Ba, Tuyen Quang, Lai Chau and Hoa Binh) are also noted on the map.
The hydrological sample sites (Yen Bai on the Thao river, Vu Quang on the Lo River, Hoa Binh on the Da River and Hanoi on the downstream main
axe of the Red River) are denoted by filled circles. [Colour figure can be viewed at wileyonlinelibrary.com]
LE T. P. Q. ET AL.
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
mostly (70%) grey and alluvial soils (MOSTE, 1997). The red
color, especially in the upstream part, may reflect the high soil
erosion and red laterite soils going into rivers in the upstream
Red River system (Van Maren, 2004).
Meteorological characteristics
The climate in the Red River basin is monsoonal with two dis-
tinct seasons. The rainy season lasts from May to October and
often accounts for 85–90% of the total annual rainfall. The
dry season covers the period from November to April. The cli-
mate results in a hydrologic regime that is characterized by
large runoff during the wet season and low runoff during the
dry season (Le et al. 2012).
Reservoir impoundment
Most dams and reservoirs in Vietnam have been constructed for
multiple purposes including flood control, irrigation, hydro-
power, water supply and flow management. There are four
large-dammed reservoirs located in the Red River basin. The
Hoa Binh and the Son La reservoirs are on the Da River and
the Tuyen Quang and the Thac Ba reservoirs are on the Lo
River. In addition, two more dams have been constructed in
the upstream Da River: the Huoi Quang dam (expected to start
operating in late 2015; Agence Française du Développement,
2014) and the Lai Chau dam (expected to start operating in
2016; AF-Consult 2013) (Table I, Figure 1).
Land use and population
Land use is very diverse from one upstream sub-basin to
another as well as between the upstream sub-basins and the
delta area. Industrial crops dominate (58%) in the Lo basin,
forests and bare land (74%) in the Da basin, and paddy rice
fields (66%) in the delta area. The Thao basin is characterized
by a larger diversity of land use including forest, paddy rice
fields, and industrial crops (85%) (Le et al. 2014).
The population of the Red River basin is growing at an an-
nual rate of about 2.0%. Population density varies considerably
within the whole Red River basin with less than 100 inhabitants
km
2
in the upstream mountainous region and more than 1000
inhabitants km
2
in the delta region. Le et al. (2014) reported
that even though the urban population is rapidly increasing,
the rural population still accounts for the largest proportion in
the river basin (about 70% in the 2010s).
Data collection
Daily river discharge data at the outlet of the three main tribu-
taries and in the main axe (in the delta area in Hanoi) of the
Red River from 2008 to 2010 were obtained for four hydrolog-
ical stations (Hoa Binh, Yen Bai, Vu Quang and Hanoi) which
were the daily mean values from ADCP measurements. These
data were collected from the Vietnam Ministry of Environment
and Natural Resources (MONRE, 2008–2010). The Hoa Binh
station (in Hoa Binh province) is located in the Da outlet; the
Vu Quang station (in Phu Tho province) is in the Lo outlet;
the Yen Bai station (in Yen Bai City) is in the Thao outlet, and
the Hanoi station (in Hanoi city) is representative of the down-
stream course of the Red (Hong) river (Figure 1). During the
3-year study periods (200–2010), the mean annual river flows
downstream of the Thao, Da, Lo Rivers and in the main axe
of the Red River at the Hanoi station were 632 ± 691,
1531 ± 759; 855 ± 1408, and 2109 ± 1722 m
3
s
1
, respectively.
Sampling and laboratory measurements
Monthly sampling campaigns were conducted from January
2008 to December 2010 at the outlet of the Da, Lo and Thao
rivers whereas weekly water samplings were done at the
Hanoi site, main downstream section of the Red river. Water
samples were collected at the four gauging stations (Figure 1)
and returned to the laboratory in a cooler. In the laboratory, a
known volume of well-mixed sample was filtered immediately
by vacuum filtration through precombusted (at 450°C for 6 h)
glass fibre filters (Whatman GF/F, 47mm diameter). The filters
were then kept in a freezer (20°C) until analysis of TSS, C/N
ratio and POC. A 30 mL sub-sample of filtrate was acidified
with 35 μL 85% H
3
PO
4
acid and then stored at 4°C in amber
glass bottles until measurement of DOC concentration.
For the measurement of TSS, each filter was dried for 1 h at
105°C and then weighed. Taking into account the filtered
volume, the increase in weight of the filter represented the total
TSS per unit volume (mg L
1
).
POC concentrations and C/N ratio were determined on the
same filters of each sample. After weighing, the filters were
acidified with HCl 2 N and then dried at 60°C for 24 h. POC
concentrations were then measured using a LECO CS 125
analyser (Etcheber et al. 1999; Veyssy et al., 1999). C/N ratios
were obtained using a Perkin-Elmer 2400 series II CHNS/O
Elemental Analyzer (Perkin Elmer, Inc. 2005). DOC concentra-
tions were analysed with an ANATOC II (SGE, Australia) total
organic carbon analyser (SGE, 2002).
Chlorophyll awas determined by the acetone extraction
method (Lorenzen 1967). The sample (250 mL) was filtered
through a glass fiber filter Whatman GF/C (47 mm diameter)
followed by overnight extraction in 10 mL of 90% acetone.
The absorbance of the sample was measured at 750 nm and
650 nm in a 1 cm path length glass cuvette using a Jasco
V-630 (Japan), before and after acidification by 0.3N HCl,
and the concentration of Chlorophyll awas determined
according to the equations of Lorenzen (1967).
Table I. major characteristics of the reservoir dams in the upstream Red River basin and water discharge output from dams to downstream river
(IMRR Project)
Name
Date of
impoundment
River basin
area, km
2
Storage
capacity,
Mm
3
Reservoir
surface
area, km
2
Water level,
(normal) m
Mean annual
water
discharge
Qo (m
3
s
1
)
Maximum
water
discharge
(m
3
s
1
)
Dam
length
(m)
Mean
depth
Thac Ba (Lo river) 1972 6170 2.9 235 58 190.5 420 657 42
Hoa Binh (Da river) 1989 57 285 9.5 208 115 1781 2400 660 50
Son La (Da river) 2010 43 760 9.3 224 215 1530 3438 961.6 60
Tuyen Quang (Lo river) 2010 14 972 2.3 81.5 120 317.8 750 1105.4 70
Huoi Quang (Da river) 2014 2824 16.3 8.7 370 1581 383.1 267 144–182
Lai Chau (Da river) 2015 26 000 0.7 39.6 295 851 1617.2 612 80.5
TOC FLUXES OF THE RED RIVER
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
All samples, standards and blank measurements were mea-
sured in triplicate. The final result of each variable was the
mean of the triplicate measurements. Analytical error was
within 5%.
Calculation of organic carbon flux
Daily TOC flux at the outlet of each tributary (Thao, Da and Lo)
and at Hanoi station (main downstream axe of the Red River)
was calculated based on the daily total organic carbon
(TOC = DOC + POC) concentrations (Daily TOC concentration
was considered as equal to the same value as monthly TOC)
with the daily river discharge. Annual TOC flux export (Mg
yr
1
) at the outlet of the Thao, Da and Lo and at Hanoi was cal-
culated as the sum of daily export at each tributary and at
Hanoi over the course of a year (2008, 2009 and 2010).
The complexity of the hydrological network in the Delta area
and the lack of regular monitoring at the outlets of each of the
numerous branches of the Red River discharging into Tonkin
Bay (Luu et al., 2010), prevent accurate estimates of the total
riverine TOC delivery. To overcome this shortage, the TOC
fluxes at the outlet of the whole Delta area in this study were
calculated by extrapolating the flux measured at the Hanoi
gauging station taking into account the respective Delta area
in the watershed, according to the following formula:
Flux at delta outlet = (Flux at Hanoi –Σflux upstr.
Tribut.) × tot.delta area/delta area at Hanoi + Σflux upstr. Tribut.
Statistical analysis
All statistical analytical results were performed with Student t-
tests to test the difference of variables (DOC, POC) values be-
tween two seasons (wet and dry) and at each station on the
measured mean variables. Probabilities (p) were determined
and a Pvalue of <0.05 was considered to be significant.
Results
Total suspended solids (TSS)
TSS varied by a factor of about 2000 over the study period
(from 1 to 1796 mg L
1
). The highest value was recorded at
the Hanoi station during a peak of exceptional flood in
November 2008 coincident with the highest recorded water
discharge (10 600 m
3
s
1
). Mean TSS concentrations at Yen
Bai (195 ± 194 mg L
1
) were higher than at Hoa Binh, Vu Quang
and Hanoi (26 ± 34 and 47 ± 48 mg L
1
, 136 ± 91 mg L
1
, re-
spectively) (P<0.05) (Table II). For the whole Red River system,
the mean TSS concentration over the 3-year study was
115 ± 168 mg L
1
. Mean concentrations of TSS during the rainy
and dry seasons for the whole Red River system were 294 ± 569
and 113 ± 428 mg.L
1
, respectively (P<0.05).
Particulate and dissolved organic carbon
POC concentration varied by almost a factor of 100, from
0.1 mgC L
1
to 9.0 mgC L
1
, with a mean value of
1.5 ± 1.5 mgC L
1
(Table II). Of the four monitoring stations,
POC concentrations at Hoa Binh and Vu Quang were generally
lower than at Yen Bai and Hanoi in the main branch of the river.
POC concentrations varied seasonally and the highest POC
concentrations were observed during the rainy season (from
May to October) and the lowest values during the dry season
at all four stations (Figure 2). At almost all the sites, except
for the Hoa Binh site, the mean POC values in the rainy
seasons were higher than those in the dry season (P<0.05)
(e.g. 3.7 ± 2.0 mgC L
1
in rainy season vs 1.1 ± 1.1 mgC L
1
at
Yen Bai station) during the 3-year observation period.
DOC concentrations were also highly variable over the study
period and varied from 0.1 mgC L
1
to 8.5 mgC L
1
, with a
mean value of 2.0 ± 1.2 mgC L
1
for the whole Red River sys-
tem. In contrast to POC concentration, no clear seasonal trend
was found for DOC concentration (Figure 2). The mean DOC
concentrations during the rainy and dry seasons, were not sig-
nificantly different with 2.4 ± 1.6 and 2.0 ± 1.3 mgC L
1
at
Yen Bai; 2.2 ± 1.9 and 2.0 ± 1.8 mgC L
1
at Hoa Binh;
2.0 ± 1.6 and 1.9 ± 1.6 mgC L
1
at Vu Quang; and 1.6 ± 1.8
and 1.7 ± 1.8 mgC L
1
at Hanoi.
Chlorophyll a (Chl-a)
Chl-a concentrations varied between 0.1 to 17.5 μgChl-a L
1
,
with the highest values observed at the Hanoi station (17.5
μgChl-a L
1
) in the main downstream axe in February 2008.
The mean values of Chl-a concentration at Yen Bai and Hanoi
(1.9 ± 2.9 and 2.1 ± 2.5 μgChl-a L
1
, respectively) were higher
than at Hoa Binh and Vu Quang (1.1 ± 0.8 and 1.3 ± 0.9
μgChl-a L
1
, respectively (P<0.05, Table 2). For the whole
Red River system, the mean value of the Chl-a concentration
was low, 1.8 ± 2.2 μgChl-a L
1
.
Fluxes of organic carbon
Annual TOC fluxes (sum of DOC + POC fluxes) at the outlet of
the tributaries Thao, Da, Lo and in the main branch of the Red
River at Hanoi station were calculated from the monthly POC
and DOC concentrations and the daily measurements of river
water discharge at the each station. The mean value of TOC flux
at the outlet of the tributaries Thao, Da, Lo in the period 2008–
2010 were 10.3 × 10
13
± 3.5 × 10
13
, 11.9 × 10
13
± 4.6 × 10
13
,
and 8.6 × 10
13
± 3.1 × 10
13
Mg yr
1
, respectively. This is equiva-
lent to 1681 ± 579, 2326 ± 892 and 2479 ± 883 kgC km
2
yr
1
for the Thao, Da and Lo rivers, respectively, of which POC rep-
resented 59, 34 and 43% of TOC fluxes of the three tributaries,
respectively (Table III).
Table II. POC, DOC, TSS and Chlorophyll amean concentrations (± standard deviation) of the Red River system. Values in brackets are the
minimum and maximum values
Sites Yen Bai Hoa Binh Vu Quang Hanoi
Variables
DOC (mgC L-1) 2.2 ± 1.3 (0.6–8.5) 1.7 ± 1.7 (0.1–5.4) 1.9 ± 1.1 (0.7–6.5) 2.2 ± 1.3 (0.1–7.7)
POC (mgC L-1) 2.6 ± 2.4 (0.1–9.0) 0.5 ± 0.4 (0.2–1.4) 0.8 ± 0.7 (0.7–3.1) 1.6 ± 1.1 (0.1–6.6)
TSS (mg L-1) 195 ± 194 (8–734) 25 ± 34 (0.6–132) 47 ± 48 (3–224) 136 ± 91 (8–1796)
Chl-a (μg L-1) 1.9 ± 2.9 (0.3–16.9) 1.1 ± 0.8 (0.1–3.3) 1.3 ± 0.9 (0.1–4.0) 2.1 ± 2.5 (0.1–17.5)
*
LE T. P. Q. ET AL.
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
Even considering the lower POC concentrations, the contri-
bution of TOC flux from the Da River (10 × 10
13
Mg yr
1
)to
the downstream main branch of the Red River system was
higher than that of the Thao (11 × 10
13
Mg yr
1
) and Lo
(7.9 × 10
13
Mg yr
1
) tributaries (Table 5) because of the higher
water discharge of the Da River.
The TOC flux at the outlet of the total delta area of the whole
Red River was in the range 27.9 × 10
13
–35.8 × 10
13
MgC yr
1
(mean value of 31.5 × 10
13
± 4.0 × 10
13
MgC yr
1
, of which
16.5 × 10
13
± 1.2 × 10
13
MgC yr
1
was DOC and
14.9 × 10
13
± 3.0 × 10
13
MgC yr
1
POC). The Red River there-
fore had a TOC flux of 2012 ± 255 kgC km
2
yr
1
of which
Figure 2. Seasonal variation of water discharge and DOC and POC concentration at the four gauging stations: Yen Bai (Thao river), Vu Quang (Lo
River), Hoa Binh (Da River) and Hanoi (downstream main axe of the Red River) during period from January 2008 to December 2010. Note difference
in scale on the left-hand y-axis for the lower panel (Hanoi).
Table III. Organic carbon flux (10
13
MgC yr
1
) at the outlets of the Thao, Da, Lo tributaries and in the main branch at Hanoi station and at Red River
estuary for the period 2008–2010
2008 10
13
MgC yr
1
2009 10
13
MgC yr
1
2010 10
13
MgC yr
1
Mean value (2008–2010) 10
13
MgC yr
1
DOC POC DOC POC DOC POC DOC POC TOC
Thao 5.7 8.1 3.1 3.6 3.8 6.6 4.2 6.1 10.3
Da 10.4 6.8 6.0 3.9 7.5 1.3 7.9 4.0 11.9
Lo 5.5 6.6 4.2 2.8 5.0 1.6 4.9 3.7 8.6
Hanoi 15.3 15.4 13.5 12.8 13.7 10.2 14.2 12.8 27
Total RR at estuary 17.8 17.9 15.8 14.9 16.0 12.0 16.5 14.9 31.5
TOC FLUXES OF THE RED RIVER
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
955 ± 191 kgC km
2
yr
1
was POC and 1057 ± 74 kgC km
2
yr
1
DOC over the period 2008–2010.
Discussion
Variations of DOC and POC concentrations
Comparison DOC, POC contents of the Red River with other
rivers
The DOC concentrations in the Red River (0.1–8.5 mgC L
1
)
are within the range cited by Tao (1998) who proposed that
DOC concentrations in most non-contaminated rivers are rela-
tively stable varying from 1 to 10 mgC L
1
. It is also close to
those of some Asian rivers such as the Pearl (1.9–3.5 mgC L
1
),
the Ayeyarwady-Thanlwin (1.2–2.9 mgC L
1
) and the Yangtze
Rivers (1.6–3.3 mgC L
1
) (Table IV). However, the concentra-
tions were lower than those of the Yichun (2.9–16.7 mgC L
1
)
and Ashop Rivers (13.6–27.5 mgC L
1
) (Table IV). The mean
value DOC concentration (2.1 mgC L
1
) of the whole Red
River was also lower than the global mean value of 8.0 mgC L
1
found by Meybeck (1988).
POC concentrations in the Red River (0.1–9.0 mgC L
1
,
mean 1.5 mgC L
1
) were also in the range of some other large
rivers. Indeed, the values were similar to those of the
Luodingjiang (0.1–6.3 mgC L
1
), Yangtze, Pearl River (1–3.8
mgC L
1
) and Ayeyarwady–Thanlwin Rivers (1.9–7.6 mgC L
1
)
(Table IV). They were, however, lower than the global mean
value of 5 mgC L
1
provided by Meybeck (2013). The Red
River values were also much lower than in the Yellow River
(116 mgC L
1
) (Hu et al. 2015), where 90% of the TOC loading
is transported as POC despite the existence of numerous reser-
voir impoundments (Ran et al., 2013) (Table IV).
Correlation between DOC and POC and water discharge
Ran et al. (2013) observed that although POC concentrations
are closely tied to river flows and TSS concentrations, DOC
concentration was less dependent on water flow. During the
study period, POC concentrations in the main axe of the Red
River varied simultaneously with river flow and were highest
during the rainy season with highest river discharge. This is sim-
ilar to what has been observed in other large river systems such
as the Yichun River (Tao, 1998), the Zhujiang River (Gao et al.
2000), the Congo River (Coynel et al. 2005); the Yangtze (Wu
et al. 2007) and the upper Yellow River (Ran et al., 2013; Zhang
et al. 2013). The higher POC and TSS concentrations during the
rainy season can be attributed to the higher rainfall amounts
and intensities that accelerate erosion and the leaching of or-
ganic carbon from soils in the basin. This results in a substantial
increase in the amount of POC from allochthonous sources en-
tering the river. Similarly to the Red River, Van Maren and
Hoekstra (2004) found that in tropical river systems, most of
the suspended sediment load is transported during the rainy
season and high river discharge. Positive relationship between
POC concentration and TSS concentration (Figure 3) with high
coefficients of determination along the main upstream of the
Red River and in the Lo River (R
2
= 0.86 and 0.90, respectively)
were observed. In contrast, the coefficient of determination was
lower in the Da River (R
2
= 0.44). This is probably due to the re-
gional differences in the characteristics of lithology in the up-
stream tributaries.
We did not find a clear relationship between DOC and river
discharge either as a whole or at each of the four gauging sta-
tions. This is consistent with other studies on headwater rivers
(Pawson et al., 2012; Dawson et al. 2001). However, this result
is in contrast to the Yichun (China) River where DOC concen-
trations fluctuated in response to precipitation and river flow
during the summer season (Tao, 1998). In the Yichun River,
during the summer season, riverine DOC comes from surface
runoff that carries soluble organic carbon leached from upland
soils and alluvial wetlands and from sediment resuspension
during high river flow. For the Red River, the lack of clear rela-
tionship between DOC concentration and river discharge may
reflect the complex sources of DOC in the river basin, includ-
ing domestic and industrial point sources as well as diffuse ter-
restrial sources.
DOC:POC ratio
The ratio of DOC to POC (DOC:POC ratio) reflects the environ-
mental and hydrological characteristics of a river basin
(Meybeck, 1982). The global mean DOC:POC ratio is 1.5
(Meybeck, 1982), however, this value varies with river system.
In general, although the DOC:POC ratio is often very high
(>20) in highly polluted rivers it can be less than 1 for moun-
tain streams or for rivers that run through pastureland (Malcolm
and Durum, 1976). In temperate river basins in Europe, the ra-
tio of DOC:POC is close to 10 (Schlesinger and Melack, 1980),
whereas it is 4.4 in the Mississippi river, USA (Dagg et al. 2004).
Higher DOC:POC ratios have been reported in the Nanaimo
River estuary (Canada) (35.2), the Imnavait Creek (54.1)
(Oswood et al., 1989), or in two North Alaskan rivers (from
9.8 to 30.7) (Naiman and Sibert, 1978). In contrast, in monsoon
regions like in Asia, the DOC:POC ratio is generally low
(around 1). For example, Ni et al. (2008) reported a value of
1.1 for the Pearl River and Wang et al.(2012) found a value
of 1.0 in the Chanjiang River. Values from the Ganges River
are even lower (0.3; Zhang et al. 1992),as in the Yellow River
(0.08–0.16; Ran et al., 2013; Hu et al. 2015) and Lanyang His
Rivers (0.16; Kao and Liu 1997). The low values are due to
the high riverine POC fluxes (70%) in this region. Widely fluc-
tuating DOC:POC ratios may also be due to the geological fea-
tures of the river basin (Zhang et al. 2013). The DOC:POC ratio
in the Tana River ranged from 0.02 to 16.4 (Tamooh et al.,
2012), where the higher value was observed in the small moun-
tainous streams in the Tana headwaters and the lower value
was recorded in plains in the lower main Tana River (from
0.02). The lower DOC:POC ratios were noted when POC dom-
inated in the main Tana River which is typical for highly erosive
and turbid systems and the higher values of DOC:POC were
found when DOC typically dominated in the tributaries. This
is similar to our study, where the upstream part of the Red River
system is dominated by the mountain areas whereas the down-
stream area is characterized by plains, plus with four reservoirs
dammed in the Da and Lo rivers resulting in a DOC:POC ratio
that varied over a large range (0.2–22.5) with a mean of
2.7 ±2.9 for the whole system. The mean values at four stations
Yen Bai, Hoa Binh, Vu Quang, and Hanoi were 1.7 ± 2.0;
3.5 ± 2.3; 2.9 ± 2.6 and 2.7 ± 3.4, respectively (Table V).
The DOC:POC ratio also varied seasonally (Figure 4). At Yen
Bai and Hanoi, the DOC:POC ratio was higher during the dry
season when the discharge and TSS were lower and when do-
mestic and industrial effluents of these urbanized zone were
less diluted. Interestingly, the highest DOC:POC values were
observed at Hoa Binh and Vu Quang on the Da and Lo rivers,
respectively. These high values are in part a consequence of
the low TSS concentrations in the Da and Lo Rivers (cf.
Table V). Whereas these lower concentrations are probably a
consequence of land use (forest and industrial plants) in the
Da and Lo river basins, the presence of four dams (Hoa Binh,
Son La, Tuyen Quang and Thac Ba dams, Figure 1) is known
to reduce TSS concentrations compared with non-dammed
rivers (Vörösmarty et al., 2003; Le et al. 2007). Reservoir im-
poundments decrease the sediment fluxes to lower reaches
and estuaries (Teodoru and Wehrli, 2005; Dai et al. 2009)
LE T. P. Q. ET AL.
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
Table IV. DOC, POC concentrations and loading of some rivers in the world
River name
River basin
area, km
2
Yearly mean
river discharge
m
3
s
1
DOC conc,
mgC L
1
POC conc,
mgC L
1
DOC specific
loading, kgC
km
2
yr
1
POC specific
loading, kgC
km
2
yr
1
TOC specific
loading, kgC
km
2
yr
1
Year of study Reference
Pearl river
(China) 0.5 × 10
6
8878 1.9–3.5 1–3.8 800 1200 2000 2005–2006 Ni et al., 2008
Ayeyarwady
river (Myanmar) 4.13 × 10
5
13033 1.2–2.9 1.2–5.2 2160 5500–10400 7660–12560 2005–2006
Bird et al.
2008
Thanlwin river
(Myanmar) 2.71 × 10
5
6691 1–1.3 1.9–7.6 830 8800–12400 9630–13230 2005–2006
Yangtze river
(China) 1.94 × 10
6
11600–40000 1.6–3.3 814.4 783.5 1597.9 2009
Wang et al.,
2012
Godavari river
(India) 3 × 10
5
2830 1–3.3 2–20 25–220 8.7–2066.7 34–2287
November 1998;
March, August
1999;
August 2000
Sarin et al.,
2002
Amazon river 6 × 10
6
3.6
Richey et al.,
1990
209000 4500 833.3 5450 1994–2000
Moreira-Turcq
et al., 2003
Caura river
(Venezuela) 4.75 × 10
4
3500 2.1–4.9 (mean 3.2)
0.17–1.89 (mean
0.77) 5072 1348 6421
September 2007–
August 2008
Mora et al.,
2014
Apure river
(Venezuela) 1.67 × 10
5
100–1500 3.8 1.5–6.8 1.33 0.33–2.4 2350 640 2946
Orinoco river
(Venezuela) 1 × 10
6
36000 2.92 0.91 3280 990 4270
35000 3.52–6.96 5296 1810 7106
Lopéz et al.,
2012
Yellow river (China)
7.52 × 10
5
475
3.6 at Toudaoguai;
4.1 at Tongguan;
3.3 at Lijin station 79.8 545.2 553
July 2011–July 2012
for the upstream;
August 2008 –
July 2012 for the
downstream
Ran et al.,
2013
7.95 × 10
5
22 (2–116) 918.2
September 2012 –
October 2013
Hu et al.
2015
Luodingjiang
river (China) 3.164 × 10
3
84.7 1.01–3.77 0.14–6.33 1240 1060 2030
January 2005 –
December 2005
Zhang et al.
2009
Congo/Zaire
river 3.7 × 10
6
42000
2.6–3.5 in low flow
period; 5.3–6.1 in
high-water period 1.6–2.3 3351.2 540.5 3892 1990–1996
Coney et al.,
2005
Mississippi
river (USA) 3 × 10
6
17366 16.9 1033.3 310 1333
April –September
2003; January –
September 2004
Bianchi et al.
2007
Ashop river
(England) 49.5 7.02 13.6–27.5
1.5–4.7 (mean low
flow); 1220
(stormflow) 7520–27570 13440–77850 20960–106000
December 2005 –
January 2007
Pawson et al.,
2012
Red River (Vietnam) 1.5 ×10
6
2640 0.1–8.5 0.1–9.0 1041 940 1981
January 2008 –
December 2010 This study
TOC FLUXES OF THE RED RIVER
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
leading to POC being deposited and trapped in these reservoirs
(Wen 1989; Wang et al., 2012).
Spatially, the DOC:POC ratio showed a significant, negative
relationship with TSS as shown by the fitted curve in Figure 5,
with coefficient of determination R
2
= 0.45–0.98. The only ex-
ception was at Hoa Binh where a much lower coefficient of de-
termination was observed (R2 <0.1) where two large reservoirs
have been built on the river main course (Figure 5). Conversely,
other studies have found that the general trend of DOC:POC ra-
tio decreased with increasing TSS is typical for systems that are
highly erosive and turbid (Ittekkot and Laane 1991; Bouillon
et al. 2009). Our results imply that for the main axe of the
Red River (Yen Bai to Hanoi City) both POC and DOC contrib-
uted to the TOC flux (POC mostly in rainy seasons and DOC in
dry ones) whereas in the two tributaries (Da and Lo Rivers)
where numerous reservoirs are impounded, DOC contributes
a relatively larger proportion than POC to TOC fluxes at the
river outlets.
Fluxes of organic carbon
TOC fluxes are affected by many factors (Kortelainen and
Saukkonen 1998). One of the most important is climate change
(Arvola et al. 2004), including the El Niño Southern Oscillation
(ENSO) (Sharma et al., 2014). However, little work has been
carried out on ENSO effects in the Red River basin, especially
for TOC and suspended loading. Gao et al. (2015) revealed that
El Niño and La Niña events are likely to induce extreme low
and extreme high discharge, respectively, from the Red River
into the Tonkin Gulf over the period 1956–2009. Pham
(2015) studied the minimum water level in the Red River at
Hanoi station in the end of March and the beginning of April
in 2010 due to the abnormality of the weather, specifically
the La Niña phenomenon which happens with a cycle of 2 to
7 years. In our study, TOC fluxes in the Da and Lo Rivers in
2010 were lower (8.8 × 10
13
Mg yr
1
and 6.6 × 10
13
Mg yr
1
,
respectively) than in 2009 (9.8 × 10
13
Mg yr
1
and
Figure 3. Relationship between POC and TSS at the four gauging stations in the Red River. Note difference in scale for the lower panels. [Colour
figure can be viewed at wileyonlinelibrary.com]
Figure 4. Seasonal variation of DOC:POC ratio at four gauging stations in the Red River.
Table V. Mean ± standard deviation and range (maximum–minimum) of water discharge, DOC:POC, POC:Chl-a, and C:N ratios at the four gauging
stations during the period from January 2008 to December 2010
Yen Bai Vu Quang Hoa Binh Hanoi
Water discharge (m
3
s
1
) 632 ± 691 (82–10100) 855 ± 759 (238–5630) 1531 ± 1408 (94–8020) 2109 ± 1722 (207–10600)
DOC:POC 1.7 ± 2.0 (0.3–9.2) 2.9 ± 2.6 (0.5–8.9) 3.5 ± 2.3 (0.4–8.6) 2.7 ± 3.4 (0.2–22.5)
POC:Chl-a (mgC mgChl-a
1
) 2419 ± 2865 (96–9413 ) 1089 ± 809 (73–2455) 1385 ± 1371 (57–7263 ) 1447 ± 1555 (23–8604)
C:N ratio 7.9 ± 1.3 (6.0–10.6) 8.1 ± 1.5 (5.4–10.2) 6.3 ± 1.5 (4.9–9.3) 6.8 ± 1.6 (2.4–11.2)
LE T. P. Q. ET AL.
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
7.0 × 10
13
Mg yr
1
, respectively) and in 2008 (17.2 × 10
13
Mg yr
1
and 12.1 × 10
13
Mg yr
1
, respectively) may partly reflect the
impact of La Niña phenomenon in 2010 and El Niño phenome-
non in 2008 and 2009 to TOC flux of the two main upstream
tributaries, leading to the change in TOC flux of the whole
Red River outlet.
The second factor affecting TOC flux of the Red River is the
impoundment of reservoirs and dams. Indeed, sediment load
from the Red River to the ocean has more than halved over
the past two decades (from ~1300 × 10
13
Mg yr
1
in 1990 to
500 × 10
13
Mg yr
1
in 2010) (Milliman and Syvitski, 1992;
Wang et al., 2011). In the Da River, sediment was almost en-
tirely trapped by the Hoa Binh Reservoir, the mean load at
Hoa Binh from 600 × 10
13
Mg yr
1
(1958–1985) decreased to
70× 10
13
Mg yr
1
(1989–2001) (Wang et al., 2011). Prior to
2008, significant sediment deposition in the Hoa Binh and
Thac Ba dams of the Red River tributaries had previously been
observed, about 50% and 30% suspended loading, respec-
tively (Le et al. 2007). This was also found in our study, in
the Da and Lo Rivers, as mentioned above, TOC fluxes in
2010 were much lower than in 2009 (and in 2008 reflecting
the high sediment deposition and associated organic carbon
in the two new dams: the Son La (in 2010) and the Tuyen
Quang (in 2009).
The impact of damming on river sediment and organic car-
bon fluxes has been observed in several other large Asian rivers
such as the Yellow River (Walling, 2006; Miao et al., 2011; Hu
et al. 2015) and the Yangtze River (Chiangjiang) (Wu et al.
2007). In the past, these rivers transported significant quantities
of TSS but dam impoundment reduced TSS and TOC fluxes by
more than half. The same is likely to be true for the Red River
where impoundment of the large dams resulted in a clear de-
crease of TSS for the period 1960–2008 (Dang et al. 2010).
An increase in the number of reservoirs within these river sys-
tems, as is already planned both by Chinese and Vietnamese
authorities, will further alter the transport of sediment and asso-
ciated organic carbon in the Red River system. Moreover, it is
probable that the superposed effects of these reservoirs may
well be more complicated than the effect caused by a single
reservoir, as has been already observed in the Minjiang River
(Xu and Yan 2010) and the Upper Mekong River (Kummu
et al., 2007). When additional reservoirs (Lai Chau, Huoi
Quang) are in operation, sediment and TOC export of the Red
River system to the coastal ocean will probably be reduced
even further. The superposed effects of these reservoirs need
to be investigated more fully in the future.
We observed that the sum of the annual TOC fluxes of the
three tributary rivers (input to the Delta area) was higher than
the mean annual load of the Hanoi site (output). This is due
to the complexity of the hydrological network in the Delta area
where some distributaries flow out from the main branch of the
Red River (Duong, Day, and Nhue Rivers) (Luu et al., 2010).
With a mean TOC flux of 2012 ± 255 kgC km
2
yr
1
of which
955 ± 191 kgC km
2
yr
1
was POC and 1057 ± 74 kgC km
2
yr
1
DOC over the period 2008–2010, the pattern of TOC flux in
the Red River basin was consistent with that of some monsoon
drainage basins, such as the Yangtze (Changjiang) and the Pearl
River deltas (1598 kgC km
2
yr
1
; 2000 kgC km
2
yr
1
, respec-
tively) (see Table IV). However, the values in the Red River
basin were lower than those of other large river systems such
as the Ayeyarwady, Amazon and Ashop Rivers (12560 kgC
km
2
yr
1
; 5450 kgC km
2
yr
1
; 106000 kgC km
2
yr
1
, respec-
tively) (Table IV). This difference is probably a consequence
of different patterns of soil erosion in river basins.
Source of organic carbon
POC:Chl-a ratios
DOC and POC in aquatic systems originates from two main
sources: in situ primary production (autochthonous source)
and inputs from terrestrial organic matter (allochthonous
source). Previous authors have proposed that the POC:Chl-a
Figure 5. Relationship between DOC:POC ratio and TSS at four gauging stations in the Red River. Note difference in scale for the lower panels.
Figure 6. Seasonal variation of POC:Chl-a (mgC mgChl-a) ratio at
four gauging stations in the Red River over the period 2008–2010.
TOC FLUXES OF THE RED RIVER
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
ratio can be an indicator of organic carbon sources (Cifuentes
et al. 1988; Abril et al. 2002; Bouillon et al. 2009). Abril et al.
(2002) proposed that when the POC:Chl-a ratio is in the range
of 30–100 mgC mgChl-a
1
that POC is mainly of phytoplank-
ton origin. Bouillon et al. (2009) further noted that if the POC:
Chl-a ratio is higher than the range of values for typical phyto-
plankton development in a river system (30–100 mgC mgChl-
a
1
), then POC probably originates from leaching and erosion
in the basin (e.g. from solid wastes and surface soil). Therefore,
a low POC:Chl-a ratio occurs when POC concentration is low
with a high contribution of phytoplankton biomass. In contrast,
a low total Chl-a that leads to a high POC/Chl-a ratio suggests
that the contribution of phytoplankton biomass to POC levels
is minimal. Cifuentes et al. (1988) proposed the value of 200
mgC mgChl-a
1
for the POC:Chl-a ratio above which organic
matter would be retained in the system and possibly
decomposed whereas if the POC:Chl-a ratio is below 200
mgC mgChl-a
1
, phytoplankton community development
could be observed. In particular, a very wide range of POC:
Chl-a was found in the tropical river Tana River, Kenya (from
75 to 40781 mgC mgChl-a
1
) (Tamooh et al., 2012) or in the
Yellow River, China (from 50 to 22520 mgC mgChl-a
1
) (Zang
et al., 2013). The group author suggested that POC was pre-
dominantly of terrestrial origin. In our work, a similar range
found of the POC:Chl-a ratio from 23 to 9413 mgC mgChl-a
1
(Table V). The highest mean value at Yen Bai was 2419 ± 2865
mgC mgChl-a
1
and the lowest mean value was recorded close
to the reservoirs at Vu Quang (1089 ± 809 mgC mgChl-a
1
).
The mean value of the POC:Chl-a ratio (1585 ± 870 mgC
mgChl-a
1
) for the whole Red River system was much greater
than that reported for phytoplankton development in river wa-
ter. This can be explained by low Chl-a concentrations ob-
served (0.1 to 17.5 μgChl-a L
1
), values that are much lower
than is commonly observed in eutrophic rivers (e.g. 100
μgChl-a L
1
and above, Garnier et al. 1995, 2005; Vardaka
et al., 2005) demonstrating that the contribution of phytoplank-
ton biomass to POC concentrations in the river water was not
significant. Furthermore, a mean POC:Chl-a ratio of
1585 ± 870 mgC mgChl-a
1
also reflects the low contribution
of reservoir phytoplankton from the four large reservoirs (Hoa
Binh, Lai Chau, Thac Ba and Tuyen Quang) located in the up-
stream part of the Red River system. This further supports the
hypothesis that POC in the water of the Red River is mainly de-
rived from soil organic leaching and erosion in the basin.
The POC:Chl-a ratio is, by definition, also dependent upon
the environmental conditions controlling phytoplankton bio-
mass, such as light, temperature, nutrients and turbidity as well
as on phytoplankton species composition. The POC:Chl-a ratio
of the Red River shows a clear seasonal variation with higher
values during the rainy season when POC concentrations
peaked co-incident with the low Chl-a values (May to October)
and the lower values during the dry season (November to
March; Figure 6). Indeed, heavy rainfall during the rainy season
results in higher soil leaching and erosion processes, increasing
turbidity, reducing the light availability and thus restricting the
phytoplankton growth and, thereby resulting in an increase of
the POC:Chl-a ratio (Stringfellow et al., 2006; Bilotta and
Brazier 2008; Yeh et al. 2011; Hu et al. 2015). This is also what
is observed in numerous rivers in the world (Bouillon et al.
2009; Tamooh et al., 2012; Zhang et al. 2013).
Correlation between organic carbon and TSS
High C:N ratios in TSS (>12) are indicative of terrestrial organic
matter rather than phytoplankton material (Wu et al. 2007).
High C:N ratios of TSS have previously been observed in rivers
where terrestrial organic matter is considered a major organic
matter source, e.g. the Danube (13.1), rivers in USA (11.2)
and St. Lawrence (12.1) (Onstad et al., 2000) and in the turbid
Ayeyarwady and Thanlwin rivers (6–10 and 9–14, respec-
tively). Lower C:N values (7.2 to 9.3) have also been observed
for different productive branches of the Pearl River (Ni et al.,
2008).
The annual mean C:N ratios of TSS at the outlets of the Thao,
Da, Lo and in the main branch Red River at Hanoi station were
7.9 ± 1.3; 6.3 ± 1.5; 8.1 ± 1.5 and 6.8 ± 1.6, respectively, and a
mean value of 7.3 ± 0.1 for the whole Red River system. Again,
this confirms that soil organic matter is an important organic
matter source in the Red River and that algal-derived carbon
is a minor component of TOC, as a consequence of the turbid
conditions, leading to light limitation.
The percentages of POC in TSS at the four gauging stations
varied from 0.6 to 6.1% with a mean value of 1.5 ± 0.6% for
the whole Red River system and are similar to those of other
rivers in Asia such as the Changjiang (1.16%) (Wu et al.
2007), the Xijiang (1.2%) (Sun et al., 2010), the Brahmaputra
(0.50%), and the lower Yellow (0.75%) (Hu et al. 2015), the
Pearl River Delta (1.5–2.5%) (Ni et al., 2008) but are lower than
in the Congo River (3.9–11.5%) (Coynel et al. 2005).
In many river systems, the proportion of organic carbon in
TSS decreases logarithmically (Ludwig et al. 1996; Gao et al.
2000, 2002, 2007; Balakrishna and Probst 2005; Ran et al.,
2013). This relationship is due to several factors such as: (i)
the increase of TSS that reduces light penetration in the river
thereby limiting phytoplankton primary production and hence
autochthonous POC production;(ii) strong erosion in the catch-
ment or resuspension of TSS from the river bed which reduces
the percentage of POC in TSS; and (iii) mechanical erosion
which leads to the export of lower soil horizons with lower or-
ganic matter contents (Hu et al. 2015). In our work, the rela-
tively low percentage of POC in TSS provides further
evidence that soil organic material was the dominant POC
source this system.
Errors in estimation of TOC fluxes
The daily TOC flux at each tributary outlet as mentioned in the
method was calculated from daily water river discharge with
monthly organic carbon concentration which was used as the
value of daily OC concentration. This was the first error in esti-
mating the TOC flux of three tributaries. In addition, the TOC
flux of the whole Red River was calculated by extrapolating
the flux measured at the Hanoi gauging station taking into ac-
count the respective Delta area in the watershed may give the
second cause of error in determining TOC flux. For these rea-
sons, we estimated the order of 25–40% confidence level of
our figures, which must be taken with caution. Nevertheless,
the resulting fluxes of the whole Red River appeared quite con-
sistent with other monsoon rivers.
A 3-year period of data observations may be too short for
assessing impacts beyond large-scale human interference with
the river system, and even then may be bit short for dealing
with the response time required by the system to adjust to a ma-
jor disturbance such as a dam. However, this study provided
some positive signs of different relationship between different
variables (TSS, water river discharge, DOC and POC). This will
encourage further studies on the calculation of TOC fluxes over
historical chronicles using daily TSS and river discharge values
without DOC and POC data.
Further, for a full budget of the Red River system, the inputs
to the watershed and the different carbon transformation pro-
cesses such as carbon degradation, deposition and outgassing
could be taken into account in addition to output TOC fluxes.
LE T. P. Q. ET AL.
Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017)
Conclusions
DOC and POC concentrations were analyzed every month at
four gauging stations in the Red River from 2008 to 2010. No
clear spatial or seasonal variation was observed for DOC con-
centrations whereas POC varied significantly both spatially
and between the dry and rainy seasons.
The calculated TOC flux at the outlet of the total delta area of
the whole Red River ranged between 27.9 × 10
13
and
35.8 × 10
13
MgC yr
1
(mean 31.5 × 10
13
± 4.0 × 10
13
MgC
yr
1) and the TOC flux was 2012 ± 255 kgC km
2
yr
1
, both
of which are consistent with data from other monsoon rivers.
Within its three main tributaries, the Da contributed a higher
proportion of the total TOC flux of the main branch Red River
due to its higher river water discharge. The TOC flux of the
Red River in 2010 was lower than in 2008 and 2009, probably
reflecting: (i) the impact of meteorological–hydrological char-
acteristics; and (ii) the important influence of two new reser-
voirs that were impounded in 2010. The reservoirs presently
in operation have reduced downstream river suspended sedi-
ment and TOC fluxes and the additional construction of reser-
voirs (e.g. Lai Chau, Huoi Quang) will probably further
reduce sediment and TOC export to the coastal ocean. This
clearly needs to be investigated more fully in the future.
The mean POC:Chl-a ratio (1585 ± 870 mgC mgChl-a
1
) for
the whole Red River system was high due to light limitation of
phytoplankton development in river water. The percentage of
POC in TSS, with a mean value of 1.4 ±0.6% is rather low, and
the C:N ratios, with mean value of 7.3 ± 0.1 (from 2.4 to 11.2),
all indicate that organic carbon in the Red River water is mainly
derived from soil organic leaching and erosion in the basin.
Finally, the DOC:POC ratios and POC concentrations
showed significant relationships with TSS concentrations,
opening up the way for further studies on the calculation of
TOC fluxes over longer timescales using daily TSS and river dis-
charge values.
Acknowledgements—The authors would like to thank the International
Foundation for Science (IFS W4210-2 project), the Ministry of Educa-
tion of Singapore (R-109-000-086-646 project), the Asia-Pacific Net-
work for Global Change Research - the National Science Foundation
(NSF) (APN ARCP2014-03CMY-Quynh/ARCP2013-06CMY-Quynh/
ARCP2012-11NMY-Quynh project), and the Vietnam National Founda-
tion for Science and Technology Development (Vietnam-NAFOSTED
105.09-2012.10) for their financial supports.
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