Riverine input and air– sea CO2 exchanges near the Changjiang

ARTICLE IN PRESS
Continental Shelf Research 28 (2008) 1476– 1482
Contents lists available at ScienceDirect
Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr
Riverine input and air– sea CO2 exchanges near the Changjiang
(Yangtze River) Estuary: Status quo and implication on possible
future changes in metabolic status
Chen-Tung Arthur Chen a,, Weidong Zhai b, Minhan Dai b
a
b
Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, ROC
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China
a r t i c l e in f o
a b s t r a c t
Article history:
Received 30 June 2006
Received in revised form
18 July 2007
Accepted 4 October 2007
Available online 13 March 2008
Due to anthropogenic activities, the nutrient loadings of the Changjiang (Yangtze River) are strickly on
the rise. The high nutrient concentrations notwithstanding, river water was pCO2 supersaturated in the
inner estuary during summer 2003 but decreased quickly in the mid-estuary due to mixing with low
pCO2 waters from offshore. In addition, settling of particles in the estuary resulted in better light
conditions so that phytoplankton bloomed, driving down pCO2 to 200 matm. In the outer estuary and
outside of the bloom area, pCO2 increased again to near or just below saturation. Literature data also
reveal that the mainstream of the Changjiang is always supersaturated with respect to CO2 probably
because the decomposition of terrestrial organic matter overwhelms the consumption of CO2 due to
biological production.
Because the Changjiang outflow accounts for 90% of the total river flow to the East China Sea (ECS),
any variation in the Changjiang could have significant implications for the ECS. For instance, completion
of the Three Gorge’s Dam could change the metabolic status of the estuary by cutting off 70% of the
downstream transport of organic carbon-containing particles. This would reduce the extent of organic
carbon decomposition, producing better light conditions and enhancing autotrophy. As a result, the
estuary could become a smaller source of CO2 to the atmosphere. On the other hand, if the Three Gorge’s
Dam reduced freshwater output, especially in summer, upwelling of nutrient-rich offshore waters
would be reduced resulting in a reduction in autotrophy in the much wider ECS shelves. This effect
could outweigh the reduced heterotrophy in the estuary and the ECS as a whole could become a smaller
CO2 sink.
& 2008 Elsevier Ltd. All rights reserved.
Keywords:
Changjiang
Air–sea exchange
pCO2
Metabolic status
Three Gorge’s Dam
East China Sea
1. Introduction
Human intervention in the carbon cycle over most of the last
two centuries has given rise to anthropogenic carbon fluxes that
are comparable in magnitude to major natural fluxes in the global
carbon cycle (Global Carbon Project, 2003). On a global scale,
approximately 40% of all freshwater and particulate matter
entering the oceans is transported by the 10 largest rivers in the
form of buoyant plumes on the open shelves. Changes in large
river systems contribute to the anthropogenic impacts. The
construction of dams and irrigation systems in river basins has
had a large impact on riverine inputs of freshwater, sediments,
nutrients and carbon to the oceans. There has been a seven-fold
increase in the number of large dams since 1950, and within the
Corresponding author. Tel.: +886 7 525 5146; fax: +886 7 525 5346.
E-mail address: [email protected] (C.-T.A. Chen).
0278-4343/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2007.10.013
next few decades more than 20% of all global river flow to the seas
may be dammed or diverted. This could lead to further changes in
coastal ecosystems and associated community structure (Chen
et al., 2003a b; Li et al., 2003). As residence times of waters behind
dams increase, less carbon and nutrients flow downstream and
the release of greenhouse gases such as CO2, CH4 and N2O
increase. Export of carbon to the atmosphere and oceans from
fluvial systems will be affected (Chen, 2002; Sabine et al., 2004).
Coastal systems may be heterotrophic because of the input of
terrestrial organic material (Smith and Hollibaugh, 1993), or
autotrophic because of the input of nutrients including nutrients
from the upwelling of subsurface waters from offshore (Walsh
et al., 1981; Chen, 2003). This issue is complex because of
temporal and spatial variations within each system. For example,
inner estuary waters in many systems (e.g., Dagg et al., 2004;
Green et al., 2006; Dai et al., 2008) are heterotrophic because of
the large input of terrestrial particulate organic carbon (POC). In
these systems, the water is always highly supersaturated with
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respect to CO2 where the partial pressure of CO2 (pCO2) is
frequently higher than 1500 matm and even exceeds 4000 matm in
polluted tributaries. Outside the river mouth, water is less
heterotrophic due to a reduced amount of available terrestrial
POC for decomposition, and increased biological productivity
because of better light conditions after the suspended particles
settle (Breed et al., 2004). Further, because there is mixing with
ambient seawater which has a lower pCO2, the pCO2 of surface
water quickly drops to near or slightly below saturation. Farther
afield, the surface waters may become net autotrophic as CO2
is further consumed by in situ biological production on the
continental shelf.
Supersaturated surface water in a heterotrophic system may
become undersaturated with a reduction of temperature while
maintaining its heterotrophic state. Conversely, undersaturated
water in winter may become supersaturated in spring due only to
rising temperature, regardless of the metabolic state. An area may
also be autotrophic and undersaturated with respect to CO2 in the
surface layer but heterotrophic and supersaturated in the subsurface layer. Clearly, whether a shelf is a source or sinks for
atmospheric CO2 depends on many factors affecting surface water
super or undersaturation.
Most of the nutrient supply to the East China Sea (ECS) shelf is
supported by the upwelling of nutrient-rich subsurface waters
from offshore (Chen et al., 1996; Fang, 2004). This process may be
diminished as the Changjiang (Yangtze) river outflow is reduced
by the Three Gorge’s Dam and associated changes in evaporation,
groundwater seepage, water consumption in the watershed and
diversion of water to northern China (Chen 2000). Furthermore,
biological productivity will likely be reduced, augmenting pCO2 on
the shelf. The seasonal pattern of freshwater outflow is also
changed by the Three Gorge’s Dam as less is be discharged in the
wet season while more will enter the ECS in the dry season. Lastly,
there will be a reduced amount of POC outflow. Consequently,
pCO2 will likely be reduced near the estuary. With few exceptions
(e.g., Zhai et al., 2007), background pCO2 information for the
Changjiang Estuary is non-existent.
It is not known if the Changjiang is supersaturated within the
estuary, and declines to near or below saturation outside of the
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estuary. The purpose of this paper is to answer this question.
Further, as the Changjiang accounts for 90% of riverine outflow to
the ECS, whether changing riverine characteristics would have
some consequences on the metabolic status and air–sea CO2
exchanges in this system is discussed.
2. Background on Changjiang
The Changjiang is the longest and largest river in China (6380 km
long, ranked third in the world after the Nile and Amazon Rivers;
960 109 m3 yr1 in discharge, ranked fourth in the world after the
Amazon, Zaire and the Orinoco Rivers) and originates on the icy
summit of Yigeladanshu (6621 m) of the Tanggula Range on the
Tibetan Plateau. Before entering ECS, it drains the area between
241300 –351450 N and 901330 –1221250 E which includes nine provinces: Qinghai, Tibet, Sichuan, Yunnan, Hubei, Hunan, Jiangxi,
Anhui and Jiangsu, as well as the mega city of Shanghai. The total
area of the drainage basin is 1,808,500 km2, or nearly 20% of the
total area of China, and it sustains the life of 420 million people, 40%
of the population of China (Sun et al., 2002).
The Changjiang Estuary, about 120 km long and more than
90 km wide at its outer limit, is mesotidal, partially mixed and
characterized by complex morphology commonly associated with
multi-step bifurcations (Li and Chen, 1998; Shen and Pan, 2001).
The influence of fresh water extends hundreds of kilometers
offshore. The enormous discharge of freshwater and sediments
along with the associated particulate and dissolved organic and
inorganic carbon, nitrogen and phosphorus all greatly influence
the biological and geochemical processes in the river plume, the
estuary, and the ECS (Chen and Wang, 1999; Gong et al., 2000;
Chen, 2003). Furthermore, the saline interface shows considerable
variability in depth and width determined by interactions
between river discharge and marine driving forces (Chen et al.,
1999a, b; Shen, 2001; Shen et al., 2003). Complicated biophysical
and geochemical processes determine the direction of CO2
exchange between the ocean and the atmosphere.
Modern hydrographical data have been collected at Datong
since October 1922 (Fig. 1). Datong is 624 km from the river mouth
Fig. 1. Map of the Changjiang Estuary along with one of its downstream tributaries, the Huangpujiang (HPJ). The August–September 2003 transect is also indicated by the
thicker dark line.
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Monthly water discharge (m3 s-1)
60000
50000
Long-term average
Aug.2003
40000
30000
20000
10000
0
Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Fig. 2. Monthly water discharge at Datong Station (data taken from Hydrological Information Centre of China, http://sqqx.hydroinfo.gov.cn/websq/).
with a drainage area of 170.5 104 km2, or 94.7% of the drainage
area of the Changjiang). The maximum discharge is in July and is
about five to six times higher than the January minimum (Fig. 2).
In the warm, wet season (May–October), the flux amounts to
70.7% of the annual discharge. The annual load of the dissolved
inorganic carbon (DIC) is almost 2 1012 mol, which is either the
largest in the world (Carbon Cycle Research Unit, 1982) or the
second largest (Cai et al., 2008). Systematic measurements of
water chemistry began in 1963 and in 1981 measurements that
can be used to calculate pCO2 began. Nevertheless, direct
measurements of riverine or inner estuarine pCO2 have not been
conducted prior to this study.
3. Material and methods
3.1. Field survey
Between 25 August and 2 September 2003, surface pCO2 and
dissolved oxygen (DO) were measured in the environs of the
Changjiang Estuary and the neighboring Huangpujiang outlet (Fig.
1) using an underway pumping system, as described in Zhai et al.
(2005). During the survey, a Li-6252 was used to measure xCO2 in
the equilibrator and in the air. CO2 gas standards with the xCO2
values of 197 106, 400 106, 700 106, 2.19 103 and
4.96 103 mol/mol were applied for calibration in order to better
fit the wide range of pCO2 in the study area. The uncertainty of
these standards is o1%, which represents the maximum level of
uncertainty during the period of extensive measuring of pCO2 and
the data processing (see details in Zhai et al., 2005). To transform
the xCO2 data into pCO2, a set of meteorological sensors (R.M.
Young Company, USA) was used to measure air pressure,
temperature and humidity. Most of the properties used for
support such as salinity, temperature, DO, pH and fluorometric
Chl a were continuously measured with an YSI 6600 sonde.
Discrete samples for Winkler DO, NIST-traceable pH and titrated
salinity were also collected for calibration purposes.
3.2. Literature data
Literature data from the Datong and Nanjing Stations have
been used to extend our knowledge on biogeochemical variation
in the Changjiang, and to compare with our field data. Datong
Station is the last downstream station without tidal influence
(Chen et al., 1999a), and as such, it has provided the best water
flux data. Nanjing Station, slightly downstream, is subject to some
tidal movement. Since numerous chemical measurements have
been taken at the Nanjing Station, for discussion, we combine the
data from these two stations. More specifically, we have used
temperature, pH and DIC data from January 1963 to December
1984 (Carbon Cycle Research Unit, 1982; Gan et al., 1983; Shen,
2000a, b, 2001; Shen et al., 2001).
4. Results
On average, August has the second highest monthly river
discharge (Fig. 2). However, during our survey in August 2003,
discharge was 16% less than the long-term average, and it was
less than that in the preceding and following months (Hydrological Information Centre of China, http://sqqx.hydroinfo.gov.cn/
websq/). This may be because the Three Gorge’s Dam was in the
first filling stage that began in June 2003. Around the mouth of the
estuary, a significant salinity front was observed. Salinity
increased from 0.2 at 10–25 km upstream of the river mouth to
between 25 and 30 at 50–60 km outside the river mouth (Fig. 3a).
Upstream of the salinity front, surface pCO2 mostly ranged
between 1000 and 1440 matm, which is 1.5–3 times higher than
pCO2 in the air. On the other hand, DO ranged from 180 to
220 mmol O2 kg1, i.e., only 73–93% of the saturation level (Fig. 3c).
In the frontal area, surface pCO2 and DO varied a great deal.
Overall, pCO2 dropped to 400–600 matm and DO decreased
initially but then increased rapidly to above 300 mmol O2 kg1,
while salinity jump to 25–30.
During the survey, the highest pCO2 values (4000–4600 matm)
along with the lowest pH (as low as 7.4; Fig. 4b) and DO values
(60 mmol O2 kg1, not shown) were measured in the Huangpujiang which is a downstream tributary of the Changjiang (Fig. 1)
that runs through the city of Shanghai and receives a large load of
sewage (Xu and Yin, 2003). The influence of the Huangpujiang on
the mainstream of the Changjiang is largely limited to the ebb tide
period. During two ebb tide surveys in the mainstream of the
Changjiang, pCO2 values of 1900 matm were observed near the
Huangpujiang outlet (Fig. 3b). On the other hand, during a flood
tide survey, pCO2 around the Huangpujiang outlet was only
1050 matm, which is consistent with the corresponding values in
the Changjiang (Fig. 3b).
In the Huangpujiang outlet, excess CO2 is linearly correlated
with oxygen depletion with a DCO2:(DO2) ratio of 0.7 (Fig. 4a).
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210
25
180
Excess CO2 (µmol kg-1)
Salinity
C.-T.A. Chen et al. / Continental Shelf Research 28 (2008) 1476–1482
20
15
10
5
0
2000
Huangpujiang
120
0.62
90
S < 10
60
30
-30
aqueous pCO2
pCO2 (µatm)
0.90
150
0
Huangpujiang River
plume
1500
1479
-60
1000
0
60
120
180
Oxygen depletion (µmol O2 kg-1)
500
-60
0
60
120
180
8.8
400
8.6
300
air pCO2
Changjiang plume
out of the mouth
8.4
200
pH
8.2
DO (µmol O2 kg-1)
400
8
7.8
300
S < 10
7.6
Saturated DO
7.4
200
Huangpujiang River
7.2
100
-300
-200
0
-100
Distance from the mouth (km)
100
200
Fig. 3. (a) Salinity, (b) air–water pCO2, and (c) dissolved oxygen along the transect
surveyed in the Changjiang (Yangtze River) Estuary in August–September 2003.
Data for the Huangpujiang (HPJ) are not included.
This is within the range of Redfield respiration (see Zhai
et al., 2005). pH is also linearly correlated with oxygen depletion
(Fig. 4b).
5. Discussion
The pCO2 upstream of the salinity front was supersaturated
(Fig. 3b). In fact, pH data collected at the Nanjing Station and
therefore the computed pCO2 (Fig. 5) are consistent with our
surveyed results. The highest values of pCO2 are in summer and
supersaturation is observed year-round because of the decomposition of particulate organic matter and discharge of high pCO2
groundwater (Sarin et al., 1989).
The pCO2 supersaturation found in the inner estuary is not
unusual as estuaries are generally highly heterotrophic ecosystems where organic carbon carried by rivers is partially remineralized. As the sea-to-air exchange of CO2 is relatively slow,
estuaries tend to become a source of CO2 to the atmosphere
throughout the year (Gattuso et al., 1998; Frankignoulle et al.,
1998; Crossland et al., 2005; Fig. 3b). The above assessment is
consistent with the findings for DO which was undersaturated
(Fig. 3c). Consumption of DO due to mineralization of organic
Fig. 4. (a) Excess CO2 vs. surface oxygen depletion, and (b) pH vs. oxygen depletion
in the Changjiang (Yangtze River) Estuary and downstream from the Huangpujiang
during the August–September 2003 survey. The dashed circles in the two panels
give prominence to inner estuarine data with salinity o10. The two dashed lines in
panel (a) show the upper limit (0.90) and the lower limit (0.62) for the
stoichiometric DCO2:(DO2) ratio of aerobic biological respiration in the environment with an abundance of HCO
3 (see Zhai et al., 2005).
matter seems to be higher than the air-to-sea oxygen transport
and caused the undersaturation.
Abril et al. (2000), however, have pointed out that although
river waters may be supersaturated they contribute to only a
small percentage (10%) of the CO2 emissions in an estuary compared with heterotrophic activities which are the major contributors. Cai and Wang (1998) have also put forth the view that
the combined effects of pelagic and benthic respiration, photodegradation and the mixing of seawater and river water are not
sufficient to sustain such high pCO2 values, which might explain
the high water-to-air fluxes in estuaries in Georgia. They have
suggested that CO2 input from organic carbon respiration in
tidally flooded salt marshes controls CO2 supersaturation. Salt
marshes are not as widespread around the Changjiang Estuary as
they are in Georgia, but the effects should not be ignored.
Similar to what was reported in the Pearl River Estuary (Zhai
et al., 2005; Dai et al., 2006), our survey data also suggest the
domination of strong aerobic respiration in the Huangpujiang
outlet. However, in the mainstream of the inner Changjiang
Estuary, many data points of excess CO2 are significantly less than
the theoretical lower limit of 0.62 (Fig. 4a). The high pH of 7.7–8.0
suggests a large buffer capacity in the Changjiang and its inner
estuary (Figs. 4b and 5a), especially when compared to the pH
levels of 6.89–7.71 in areas upstream of the Pearl River Estuary
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8.2
pH
8.1
8.0
7.9
7.8
7.7
0
60
120
180
Julian Day
240
300
360
0
60
120
180
Julian Day
240
300
360
pCO2 (µ atm)
2000
1600
1200
800
400
Fig. 5. Mean monthly (a) pH and (b) pCO2 at Datong and Nanjing Stations (data
taken from (J) the Carbon Cycle Research Unit, 1982, (K) Gan et al., 1983 and (r)
Shen, 2001).
3000
rivers
Lindingyang shelves
2500
pCO2 (µatm)
2000
1500
1000
500
0
-800
-600
-400
-200
200
0
Distance from the river mouth (km)
400
Fig. 6. The pCO2 vs. distance form the river mouth of the Pearl River, its main
estuary (Lindingyang) and the adjacent shelves in September 2003. The horizontal
dashed line shows the equilibrium value (data taken from Chen et al., 2008).
(Zhai et al., 2005). As argued by Cai et al. (2008), buffering
capacity is an important influencing factor on riverine/estuarine
CO2 outgassing. In addition, other CO2 consumption processes,
such as the dissolution of calcite and dolomite particles, are
worthy of further investigation.
Most of the decrease in pCO2 at the strong salinity front is
probably a result of dilution. For example, water at the end of our
transect (pCO2 of about 340 matm and salinity of about 30) would
result by mixing river water with a salinity of 0.2 and pCO2 of
about 1000 matm (Fig. 5b) with the average ECS water with a
salinity of 33.1 and pCO2 of 300 matm (Chen et al., 1995). This
mixed water would contain 9.4% of the original river water. ECS
shelf water is generally undersaturated with respect to atmospheric CO2 (Wang et al., 2000). A 10-fold dilution with the shelf
water would reduce the pCO2 of the river water to near saturation.
Furthermore, sinking suspended sediment in the nutrient-rich
discharge plume often results in high biological productivity. This
is most significant in an area 80–160 km away from the
river mouth, where DO of 100–190% saturation and pCO2 of
200–340 matm were simultaneously observed in the salinity range
of 25–30 (Fig. 3). This region frequently has algal blooms in
summer (Ye et al., 2004; Zhou et al., 2004; Zhu et al., 2005). The
pH values in the plume area, as high as 8.4 (Fig. 4b) compared to
the values of around 7.8 in the river (Fig. 5a) and the inner estuary
(Fig. 4b), are consistent with the suggestion of high biological
productivity. Tan et al. (2004) also reported that pCO2 values were
reduced to between 117 and 617 matm in the outer Changjiang
Estuary in July–August 2001. Significant river plume-induced CO2
sinks have also been reported in the Amazon, Mississippi and
Pearl River plumes (Ternon et al., 2000; Körtzinger, 2003; Dai
et al., 2008; Lohrenz and Cai, 2006).
An issue worth noting is the metabolic status which plays an
important role in controlling the observed pCO2. Apparently, the
high nutrient concentrations not withstanding, biological consumption in the Changjiang and the inner estuary cannot drive
down the pCO2 to below saturation. This may be partly because
the N/P ratio of 100 or more in the Changjiang (Shen, 2001; Li
et al., 2007) is much greater than the Redfield ratio of 16 (Redfield
et al., 1963). Particles collected in the ECS show a similar ratio
(Chen et al., 1996). Thus, it follows that phytoplankton growth in
the Changjiang must be limited by the availability of PO4. In fact,
DIN can still be detected in the Changjiang plume in the ECS
several hundred kilometers from the river month, whereas PO4
cannot since it is quickly reduced to below the detection limit
(Wang and Chen, 1998; Wong et al., 1998). It takes the upwelling
of the subsurface Kuroshio (Wong et al., 1991) and South China
Sea Intermediate Waters to support most of the PO4 needed for
new production in the ECS (Chen, 1996, 2005; Chen and Wang,
1999).
Since phosphate is the limiting factor relative to nitrate for
photosynthesis, confining the water behind the newly constructed
Three Gorge’s Dam can be expected to decrease the amount of
phosphate, and hence, further increase the N/P ratio of the
outflow (Liu et al., 2003; Zhang and Zhang, 2003; Gao and Wang,
2008). In other words, an external source of P much greater than
that of riverine input is required to sustain new production in the
ECS in whole.
It may be worthwhile pointing out that Tsunogai et al. (1999)
proposed the concept of continental shelf pump. They have,
however, inaccurately concluded that the subsurface ECS shelf
waters are transported away from the shelf and into the deeper
layers of Kuroshio. The truth is, of course, that subsurface
Kuroshio waters upwell onto the ECS shelves almost year-round
(Liu et al., 1992; Ito et al., 1994; Chen, 1996, 2005) and supports
most of new production of the ECS. Further, DIC is thus
transported onto the shelf while DOC is exported offshelf (Chen,
2004), thus lowering the pCO2 of shelf waters.
Important here is that the completion of the Three Gorge’s
Dam, and other dams on the tributaries of Changjiang, will block
the transport of a large percentage (70%) of fine grained
sediment, which may, in turn, lead to a clearer water column in
the estuary and a deeper euphotic zone (Chen et al., 2003a, b; Gao
and Wang, 2008). This will result in even higher productivity and
more severe eutrophication off the Changjiang Estuary. Over the
ECS continental shelves, however, productivity is expected to
decline as a result of a reduction in the upwelling of the nutrientrich subsurface waters from offshore (Chen, 2000).
Along with the reduced sediment supply, there should also be
a much smaller quantity of particulate organic matter flowing into
the estuary and the ECS. Recall that the mainstream of the
Changjiang and the inner estuary are heterotrophic with high
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pCO2 (Figs. 3b and 5b). On the other hand, the outer estuary and
beyond is autotrophic with low pCO2 (Fig. 3b). This is by no means
unique. For instance, the second largest river in China, namely,
the Pearl River, also had a high pCO2 supersaturation in the
mainstream of the river and its main estuary, the Lindingyang,
in the summer of 2003 (Fig. 6). In the outer estuary and
beyond, however, the pCO2 quickly reduced to near saturation.
Reduced sediment outflow should ultimately lead to even less
heterotrophy.
As a final note, flood control of the Three Gorge’s Dam would
reduce water outflow in summer when there is sufficient light,
but it would increase water outflow in winter when there is less
light. As a result, increased buoyancy in winter may not be
conducive to enough biological productivity to compensate for
reduced summer productivity given the reduced upwelling.
Reduced autotrophy due to a smaller buoyancy effect would
probably affect a much larger area on the ECS shelf, whereas
reduced heterotrophy due to dams would affect a much smaller
area near the estuary. Whether the combined effects would
reduce the capability of the ECS to absorb CO2 remains to be seen.
6. Summary and concluding remarks
Both our field survey near the Changjiang Estuary in the
summer of 2003 and seasonal variations in the mainstream based
on data in the literature indicate that Changjiang water is always
supersaturated with respect to CO2. Degassing of CO2 must have
occurred year-round, especially in summer. When organic matter,
especially particles, gets transported to the estuary, it tends to
settle down and decompose there, which probably sustains the
high pCO2 observed in the inner estuary in this study. However, a
clearer water column coupled with sufficient nutrients at only tens
of kilometers from the river mouth resulted in enhanced biological
productivity, which drove the observed pCO2 to well below
saturation while DO became saturated or even supersaturated in
the surface layer. As a result, the plume quickly changed from a CO2
source to the atmosphere to a CO2 sink. So, whether the ECS is a
source or sink of CO2 to the atmosphere is neither always nor
directly affected by the high pCO2 Changjiang discharge.
Construction of the Three Gorge’s Dam could make the estuary a
smaller source of CO2 to the atmosphere. On the other hand, it is
suspected that because of the much wider shelf area, the reduced
autotrophy on the shelf due to reduced upwelling would outweigh
the reduced heterotrophy in the estuary. The outcome could, ultimately be that the ECS as a whole would become a smaller CO2 sink.
Acknowledgments
This research was partially supported by the National Science
Council of Taiwan (NSC 94-2621-Z-110-001, 95-2611-M-110-001
and Aim for the top university plan, 95C 0312) and the Natural
Science Foundation of China through Grant nos. 40406023 and
90211020, respectively. We thank Pinghe Cai for assistance with
data collection, and Baoshan Chen for pH measurements during
the August–September 2003 survey. M. Dagg and two anonymous
reviewers provided constructive and valuable comments which
strengthened the manuscript.
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