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Continental Shelf Research 28 (2008) 1527– 1537
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Continental Shelf Research
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Comparison of hypoxia among four river-dominated ocean margins:
The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers
C. Rabouille a,, D.J. Conley b, M.H. Dai c, W.-J. Cai d, C.T.A. Chen e, B. Lansard a, R. Green f,1, K. Yin g,i,
P.J. Harrison g, M. Dagg f, B. McKee h
a
Laboratoire des Sciences du Climat et de l’Environnement, UMR CEA-CNRS-UVSQ and IPSL, Av. de la Terrasse, 91198 Gif sur Yvette, France
GeoBiosphere Science Centre, Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden
c
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China
d
Department of Marine Sciences, The University of Georgia, Athens, Georgia 30602, USA
e
Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
f
Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USA
g
Atmospheric, Marine and Coastal Environment Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
h
Department of Marine Sciences, University of North Carolina, Chapel Hill, NC 25599, USA
i
Australian Rivers Institute, Griffith University (Nathan Campus), Brisbane, Qld. 4111, Australia
b
a r t i c l e in fo
abstract
Available online 13 March 2008
We examined the occurrence of seasonal hypoxia (O2o2 mg l1) in the bottom waters of four riverdominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the
processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to
biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river
plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment
by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi
and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl
rivers systems are less affected, although nutrient concentrations in the river water are very similar in
the four systems. Spring and summer primary production is high overall for the shelves adjacent to the
Mississippi, Changjiang and Pearl (1–10 g C m2 d1), and lower off the Rhône River (o1 g C m2 d1),
which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of
the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a
long residence time and frequent occurrences of hypoxia during summer over very large spatial scales,
whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence
time of the bottom water. Physical stratification plays an important role with both the Changjiang and
Mississippi shelf showing strong thermohaline stratification during summer over extended periods of
time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind
effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate
depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low
residence time such as the Pearl River estuary during the summer wet season when mixing and flushing
are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no
hypoxia.
& 2008 Elsevier Ltd. All rights reserved.
Keyword:
Oxygen
Hypoxia
Carbon cycle
Coastal oceanography
Estuaries
River plumes
Nutrients
Continental shelves
1. Introduction
Regional index terms: China, Changjiang (Yangtze River), East China Sea; USA,
Mississippi River, Gulf of Mexico; China, Pearl River, South China Sea; France,
Rhône River, Gulf of Lion.
Corresponding author.
E-mail addresses: [email protected] (C. Rabouille), [email protected]
(D.J. Conley), [email protected] (M.H. Dai), [email protected]
(C.T.A. Chen), [email protected] (R. Green), k.yin@griffith.edu.au
(K. Yin), [email protected] (P.J. Harrison), [email protected] (M. Dagg),
[email protected] (B. McKee).
1
Current address: Ocean Optics Section, Naval Research Laboratory, Stennis
Space Center, MS 39529, USA.
0278-4343/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2008.01.020
Eutrophication and its consequences are an increasing threat to
coastal ecosystems through harmful algal blooms, alteration of
plankton community structure, and hypoxia in bottom waters
(Cloern, 2001). These impacts are believed to be associated with
high nutrient loads delivered by rivers to estuaries and ultimately
to the coastal waters. Although the presence of hypoxia and anoxia
has existed through geologic time, the occurrence in coastal waters
has been accelerated by human activities (Diaz, 2001).
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Large rivers exert a strong influence on the continental
shelves over which they discharge, forming stratified and turbid
plumes in their estuaries and outside on the shelves throughout
the year and especially during the high discharge periods (Dagg
et al., 2004; Mckee et al., 2004; Dai et al., this issue). In addition,
elevated nutrient concentrations in river waters often trigger
high primary production and algal blooms in the river plume,
and the adjacent shelf water (Lohrenz et al., 1997; Dagg and
Breed, 2003). A portion of the organic matter produced in the
plume, along with terrestrial inputs, settles to the seafloor
where it is decomposed by benthic fauna and bacteria using
oxygen. When vertical exchange of the water is limited (i.e.
stratification), the oxidation of organic matter may reduce oxygen
below 2 mg l1 (or 62.5 mmol l1) producing a state of hypoxia
(Rabalais et al., 2002) where benthic fauna become stressed or die
due to the lack of oxygen (Diaz and Rosenberg, 1995). Hypoxia
may ultimately lead to anoxia where oxygen completely disappears and sulfide may diffuse from the sediment to the bottom
water, both causing mass mortality of organisms (Smetacek et al.,
1991).
Several estuaries and continental shelves of the world coastal
ocean undergo severe hypoxia including the Chesapeake Bay, USA
(Hagy et al., 2005), the Danish Straits (Conley et al., 2007), and the
Mississippi River outlet in the northern Gulf of Mexico which is
one of the largest areas of the world with seasonal hypoxia
(Rabalais et al., 2002). Other estuaries or river-dominated margins
such as the Changjiang in China (Zhou and Shen, this issue;
Wei et al., 2007) and the St. Lawrence River in Canada (Gilbert
et al., 2005), exhibit hypoxia in the inner or outer part of their
estuaries.
The decrease of oxygen from saturation (250–300 mmoll1)
to hypoxic levels occurs when both thermal and haline stratification of the water due to freshwater discharge and summer
warming, nutrient utilization by phytoplankton, and subsequent settling of phytoplankton to bottom waters, occur in the
right sequence. Yet, some shelves adjacent to large rivers do
not exhibit hypoxia and large variations in the volume of
hypoxic waters are known to occur within hypoxic systems
(Hetland and DiMarco, 2007) indicating that there are complex
interactions between physical, chemical, and biological processes
causing and maintaining hypoxia in river-dominated ocean
margins.
In this paper, we compare the physical, chemical, and
biological processes leading to the formation, occurrence, and
intensity of hypoxia in four river-dominated ocean margins which
receive large amounts of river-derived organic loading and
dissolved nutrients: the Changjiang (Yangtze River), Mississippi,
Pearl (or Zhujiang), and Rhône Rivers. These margins are all
impacted by their river inputs, but only some of these margins
exhibit hypoxia. We will compare the occurrence of hypoxia in
these four estuaries/shelves with their nutrient/organic inputs
and their physical settings (tides, margin shape, shelf currents,
water residence times, and wind regimes) in order to identify
general principles about the development and maintenance of
coastal hypoxia.
2. Locations and characteristics of the four estuaries/shelves
2.1. Geographical location and discharge
All four river systems are located in the northern hemisphere
and three of them discharge at latitudes between 201N and 321N:
the Pearl, Changjiang, and Mississippi Rivers (Table 1). The Pearl
and Changjiang watersheds are dominated by the monsoon but
the Mississippi watersheds is mostly temperate. Only the Rhône
River is located north of 431N and is thus a completely temperate
river system (Fig. 1). On a global basis, the Mississippi and the
Changjiang are large rivers, while the Pearl is somewhat
intermediate and the Rhône River is significantly smaller in
discharge (Table 1). The seasonality of the discharge is similar for
the Pearl and Changjiang with high flows in summer associated
with the monsoon rain, whereas the Mississippi and Rhône show
low water discharge in summer and higher water fluxes in
autumn and spring, due to autumn rain and spring snowmelt
(Fig. 2).
The Changjiang is the longest river in China (6200 km) and
drains a large portion of central continental China (Table 1). It
discharges into the East China Sea (ECS) mainly through its south
branch which discharges more than 95% of the runoff (Cai et al.,
this issue). The annual freshwater discharge of the Changjiang
ranked fifth in the world and fourth for sediment discharge
(Milliman and Meade, 1983; Meade, 1996) before the Three
Gorges Dam was built. This discharge is highly seasonal with 75%
of the river runoff occurring during the flood/rainy season
between May and October. According to Yang et al. (2007), the
building of the Three Gorges Dam has resulted in a decrease
(by 30%) in sediment discharge.
The Mississippi–Atchafalaya River basin drains approximately
41% of the USA and carries approximately 65% of all the suspended
solids and dissolved solutes that enter the ocean from the USA. It
effectively injects these materials onto the continental shelf as
two point sources in the northern Gulf of Mexico. The Mississippi–Atchafalaya River system has the third largest drainage
basin among the world major rivers (Table 1; Milliman and
Meade, 1983; Meade, 1996). This river system also ranks seventh
in freshwater discharge and fifth in sediment discharge. About
two-thirds of the total flow passes through the birdfoot delta of
the Mississippi River and discharges into the outer shelf region,
with the remainder flowing into Atchafalaya Bay before entering
the Gulf of Mexico on the coast (Wiseman and Garvine, 1995).
The Pearl River is the 13th largest river in the world in terms of
freshwater discharge and has an annual sediment delivery of
80 Mt (Table 1; Zhao, 1990). It discharges into the sub-tropical,
oligotrophic South China Sea (SCS). Together with the Mekong
River, they provide the largest freshwater input to the SCS, which
is the largest marginal sea in the north Pacific. The Pearl River
estuary is a complex estuarine system (Cai et al., 2004; Harrison
et al., this issue) which discharges into the SCS through three subestuaries, Lingdingyang, Maodaomen, and Huangmaohai via eight
main outlets, or distributaries. It is fed mainly by three river
tributaries, namely the Xi-jiang (West River), the Bei-jiang (North
Table 1
Major characteristics of the four river systems
Changjiang
Mississippi
Pearl
Rhône
Length (km)
Latitude of river
mouth (1N)
Drainage basin
(103 km2)
Freshwater discharge
(109 m3 yr1)
Sediment discharge
(Mt yr1)
6200
6300
2200
800
311300 N
291000 N
221300 N
421500 N
1800
3210
450
100
924
530
330
53
486
210
80
10
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1529
Fig. 1. World map with the location of the four river-dominated ocean margins systems presented in this study.
Monthly discharge (m3/s)
60000
Table 2
Annual nutrient and carbon discharge from the four river systems
Pearl River
Changjiang River
Mississippi River
Rhone River *5
50000
Freshwater
discharge
(109 m3 yr1)
40000
Nutrient Input (109 mol yr1)
DIN
30000
a
Changjiang
Mississippib
Pearlc
Rhôned
20000
924
530
330
53
a
116
68
28
8.8
DIP
a
0.3
1.3
0.3
0.08
DSi
100
72
46
4.8
a
DOC
POC
–
150
–
11
–
317
–
16
10000
a
Shen et al. (2006).
Goolsby et al. (1999).
c
Cai et al. (2004).
d
De Madron et al. (2003).
b
0
0
2
4
6
Month
8
10
12
Fig. 2. Monthly discharge of the four river systems. Rhône River discharge is
multiplied by 5 to match the scale.
2.2. Nutrient discharge from the rivers
River), and the Dong-jiang (East River). The West River has the
greatest freshwater discharge (78% of the total discharge).
The Rhône River is located in the south of France and is the
largest freshwater input to the Mediterranean Sea since the
damming of the Nile River in Assouan. It receives water from a
drainage basin which spans temperate and Mediterranean
climatic regions (Table 1; Pont et al., 2002). The Rhône delta is
divided into two outlets, the Petit Rhône and the Grand Rhône,
corresponding to 20% and 80% of the water flow, respectively.
Discharge is characterized by the occurrence of autumn and
winter floods which may be 20 times higher than the remainder of
the year and reach 410,000 m3 s1. Inputs from these episodic
floods are not well quantified; giving rise to uncertainties on the
particle discharge at the mouth of the river (1075 Mt y1).
The amount of dissolved inorganic nitrogen (DIN) discharged
by the four rivers is clearly linked to the water discharge volume
(Table 2). The Changjiang has the largest input of DIN and the
Mississippi, Pearl, and Rhône Rivers have lower values. In these
human impacted systems, a large fraction of the nitrogen inputs
are due to agricultural practices as well as sewage and industrial
sources. All four basins are dominated by agriculture strongly
affecting the relationship between freshwater and nutrient
discharge. This is reflected in the high DIN concentrations of the
rivers: 20–200 mM for the Mississippi (Dagg et al., 2004),
75–100 mM for the Pearl (Cai et al., 2004), and 60–160 mM in the
Changjiang (Shen, 2004) and the Rhône (De Madron et al., 2003).
Dissolved inorganic phosphorus (DIP) fluxes are linked to the
geology of the river watershed and its weathering in addition to
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the fertilizer input. Therefore, the load of the rivers is not strictly
related to water discharge and DIP loadings for Changjiang and
Pearl are similar, although the Changjiang discharge is three times
larger (Table 2). The DIP concentrations are low in all seasons in
the Pearl (seldom 4 1 mM), while the other three rivers have a
considerable range and larger average concentrations.
DSi concentrations range from 90 mM in the Rhone to 140 mM in
the Pearl and are below the world average DSi concentration of
150 mM (Treguer et al., 1995).
DOC and particulate organic carbon (POC) inputs have been
reported only for the Rhône (Sempéré et al., 2000) and Mississippi
(Trefry et al., 1994).
2.3. Dynamics of shelf bottom waters
Tidal range is a key determining characteristic for the
development of deltas (Syvitski and Saito, 2007) because it
greatly affects the rate of exchange with the coastal sea. The tidal
range for the four deltas/estuaries is mostly microtidal (Table 3)
with the exception of the Changjiang, which slightly exceeds 2 m
and is mesotidal. The Pearl tidal range is 0.86–1.89 m at the main
outlet. Among the four systems considered in this paper, building
of the Rhône and Pearl deltas is significantly affected by waves,
whereas the Changjiang and Mississippi deltas are mostly affected
by river sediment discharge (Syvitski and Saito, 2007).
Thermal stratification is strong for the Mississippi, Pearl, and
Changjiang margins during summer (Table 3). While discharge is
low, there is sufficient freshwater inflow from the Mississippi,
coupled to calm winds, to further strengthen the pycnocline and
Table 3
Tidal range, summer stratification, and bottom current speed
Changjianga
Mississippib
Pearlc
Rhôned
Average tidal
range (m)
Stratification
Bottom
current
(cm s1)
Wind speed
in summer
(m s1)
2.5
1
1.4
0.4
Strong
Strong
Strong
Medium
25–30
2–5
25–50
10–15
4
2.5
10
a
Zhu et al. (2004).
Zavala-Hidalgo et al. (2003); Wiseman et al. (2003).
c
Yin et al. (2000).
d
Lansard, (2004).
b
support the spreading of the plume. Freshwater discharge peaks
in summer for the Changjiang and Pearl and also promotes
density stratification. In the ECS close to the Changjiang mouth,
the interaction of freshwater discharge and the Taiwan Warm
Current (TWC) with high salinity and high temperature water at
depths near 30 m contributes to stratification (Li et al., 2002; Wei
et al., 2007; Zhu et al., 2004). In the SCS near the Pearl River
mouth, the water is stratified during spring–summer when the
estuarine plume forms. There is little or no stratification in winter
due to wind mixing. Average winter wind speeds (10 m s1) are
four times higher than summer (2.5 m s1; Yin, 2002). Stratification is less established in summer for the Rhône margin, because
frequent high winds disrupt the density structure (Estournel et al.,
2001).
Summer bottom currents range from weak (0.5–5 cm s1) to
strong (50 cm s1) among the four margins (Table 3). Summer
surface currents are 5–10 cm s1 beyond the immediate plume of
the Mississippi (Zavala-Hidalgo et al., 2003) and bottom currents
at 20 m in an area of frequent hypoxia are lower (0.5–2 cm s1;
Wiseman et al., 2003). The current reversal occurring at the end of
spring creates a temporary low in the transport rate in June, July
and August (Smith and Jacobs, 2005). Due to the TWC water,
speeds off the Changjiang are about 30 cm s1 throughout the year
(Li et al., 2002; Zhu et al., 2004). Bottom currents on the Rhône
prodelta show intermediate speeds of 10–15 cm s1 during late
spring at 16 m depth (Lansard, 2004) which are consistent with
current velocities measured at the shelf edge at 240 m during
summer (5–10 cm s1; Flexas et al., 2002). Current speeds are the
highest in the Pearl River estuary reaching values of 50 cm s1,
which results in a residence time (volume of the estuary divided
by the freshwater flow) of days during the wet season (March–
October; Yin et al., 2004a), and a couple of weeks in the dry
season (Dai, unpublished data).
For the Rhône, Changjiang, and Mississippi margins, residence
time of the bottom water can be calculated using current
velocities and an estimation of the extent of the zone of higher
oxygen demand due to river inputs (Fig. 3). The size of this zone
was estimated from the zone of river-plume primary production
(Table 4) because primary production is the main source of
organic matter for fueling hypoxia (Green et al., 2006). The
residence time of the water is calculated as the ratio between the
volume of water in this box and the lateral flux of water through
the box:
tres ¼ V hypoxic =F water ,
Surface water
River input
Depth = 0 m
Upper shelf
Depth = 30-50 m
bottomwater
water(pot.
(pot. hypoxic)
bottom
hypoxic)
sion
xten
L : E system
e
of th
S: surface
S:
surfacearea
area
nt ω
ore
Lower shelf
re
cur
gsh
n
Alo
Depth = 100 m
Fig. 3. Schematic diagram of alongshore circulation in bottom waters. The upper box represents surface water which receives the river input while the lower box represents
bottom waters which may become hypoxic. L is the alongshore extension of the system, S is the cross-section of the system through which the alongshore current runs and
o is the speed of the alongshore current.
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0
Table 4
Current speed in intermediate and deep waters near the river mouths, the
extension of the mixing system, and the calculated residence time
10
River
30
Pearl
Rhône
a
Taiwan warm current 30
Louisiana bottom current
2–5b,c
Estuarine mixing
Lingdingyan
Ligurian bottom current
(20–40 m) 10e
Extension of the
system—L (km)
300
400
Residence
time tres (d)
11
95
d
50
3–5
50
6
Pearl River
20
Depth (m)
Changjiang
Mississippi
Alongshore current speed
o (cm s1)
Rhône
shelf
40
50
60
Rhône
prodelta
70
80
0
V hypoxic ¼ Sbox L
and
F water ¼ Sbox o,
where Sbox being the lateral surface area of the box considered, o
the current speed (cm s1), and L the extension of the system
alongshore (km). The residence time can thus be expressed by
tres ¼ L=o.
Residence times (Table 4) show a large range from 6 to 95 days.
The largest values occur on the Mississippi shelf due to a
combination of low currents and a large extent of the system.
The Changjiang is intermediate with a value of 11 days and Rhône
and Pearl are the lowest.
2.4. Bathymetry of the shelf close to the river mouth
Mississippi, Changjiang, and Rhone freshwater plumes extend
far onto the continental shelf and mix with seawater of the
continental margin. The Pearl River has a seasonal plume that
stretches onto the shelf only during high discharge (Yin et al.,
2001), but is otherwise a classic estuarine mixing zone. A broad
continental shelf characterizes the Pearl, Changjiang, and Mississippi (Fig. 4), while the Rhône shelf reaches depths 4100 m
within 40 km of the river mouth. The subaqueous deltas of the
Rhône and Mississippi are prograding onto the shelf, which results
in steep slopes to depths of 100 m within 10–20 km of the river
mouths (Fig. 4).
Mississippi
shelf
100
20
40
60
80
100
Distance (km)
b
where tres is the residence time of the bottom water in the
hypoxic part of the shelf with regards to lateral current, Vhypoxic is
the volume of potential hypoxic waters, and Fwater the flux of
water through that volume.
If the geometry is rectangular, this problem can be simplified
since
Changjiang
Mississippi
delta
90
a
Zhu et al. (2004).
Zavala-Hidalgo et al. (2003).
c
Wiseman et al. (2003).
d
Yin et al. (2000).
e
Lansard, (2004); Flexas et al. (2002).
1531
Fig. 4. Bathymetry of the four coastal systems at the river outlet. The four thick
lines represent the very shallow estuary of the Pearl River, the shelf of the
Changjiang, the Louisiana shelf west of the Mississippi and the shelf west of
the Rhône. Thinner lines (Rhône and Mississippi) display bathymetric features at
the river mouth, which are characterized by prodeltas and steep slopes.
concentration measured in the core of this hypoxic region was
lower in 1999 (Li et al., 2002) than in the 1980s, suggesting that
summer hypoxia has increased over time. Recent studies (Chen
et al., 2005; Wei et al., 2007) suggested that the spreading of the
summer hypoxic zone continued in 2003 (20,000 km2 with
o3 mg l1 of oxygen). These studies identified August as the
maximum hypoxia period.
3.2. Mississippi
The shelf region of the northern Gulf of Mexico that is
impacted by the Mississippi and Atchafalaya River systems is
the best documented example of hypoxia (Rabalais and Turner,
2001; Rabalais et al., 2002; Dagg et al., 2007). Since 1993, an
extensive zone of bottom water hypoxia, typically 415,000 km2,
develops during most years (Rabalais et al., 2002).
In the northern Gulf of Mexico, hypoxia is seasonal. It starts in
late spring and reaches its maximum in late July before fall wind
mixing and destratification. Hypoxia occurs in shallow bottom
waters (5–40 m) along the coast of Louisiana in regions affected by
nutrient inputs from the Mississippi/Atchafalaya River discharge.
The oxygen concentration in bottom waters in the center of this
hypoxic zone is frequently o1 mg l1 of oxygen (or 31 mmol l1).
Generally, there is a strong correlation between the volume
of river discharge in spring and summer and the extent of
hypoxic waters. During two severe droughts of the Mississippi
River in 1988 and 2000, the area of hypoxic bottom water
was o5000 km2 (Rabalais et al., 2002; Rabalais and Turner,
2006). In contrast, a much larger hypoxic zone occurred from
1993 to 1997 (415,000 km2), and in 1999 and 2001 when it was
420,000 km2.
3.3. Pearl River
3. Occurrence of hypoxia
3.1. Changjiang
Early hydrological surveys off the Changjiang River in summer
reported the occurrence of hypoxic waters about 100 km from the
river mouth along 1231E at 35 m depth with oxygen concentrations of 1.5–2 mg l1 (Tian et al., 1993). Subsequent surveys
identified a zone of low oxygen concentration during the summer
(o 3 mg l1) extending over 13,000 km2 with a thickness of 20 m
at a depth between 30 and 50 m (Li et al., 2002). The oxygen
Hypoxic conditions rarely occur in the lower Pearl River
estuary or on the shelf (Yin et al., 2004a). In the summer of
1999 (high river flow), a survey of the estuary showed oxygendepleted waters (around 3–4 mg l1) occurring in several areas of
the estuary, but hypoxic waters (o2 mg l1) occurred only around
the turbidity maximum at one station (Yin et al., 2004a). A
monthly, 10-year (1990–1999) time series around Hong Kong
Island (in the eastern estuary and the nearby shelf) also showed
low oxygen waters (o3 mg l1), but no hypoxia in the bottom
waters (Yin et al., 2004a).
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3.4. Rhône River
On the continental shelf near the Rhône, near saturation values
of oxygen in bottom waters have been reported throughout the
year (Denis and Grenz, 2003). In the same area, the data sets for
the summer of 2005 and 2006 showed oxygenated water down to
1 m above the seabed (Lansard, pers. comm.). In August 2006,
oxygen contents in the Bay of Beauduc located west of the Rhône
were near-saturation under windy conditions (Lansard, pers.
comm.). Hence, the continental shelf near the Rhône River does
not appear to exhibit significant hypoxia.
4. Discussion
The occurrence of hypoxic conditions in coastal regions
connected to large rivers is due to the interplay between water
column production, which supplies organic matter to bottom
water and sediment thereby generating oxygen consumption, and
physical forcings (stratification, the circulation pattern in the
region and residence time of the water in the mixing zone) which
limit oxygenation of bottom waters. In spite of large inputs of
organic matter to the bottom, the interplay between the physical
and biogeochemical forcings for each margin may or may not lead
to hypoxia as exemplified by the Changjiang and Mississippi
margins in which hypoxia occurs, and the Pearl and Rhône
margins where hypoxia is absent. In this section, we draw a
comparison of the physical and biogeochemical factors between
the four river/margin systems.
4.1. Physical forcings
Various physical factors, including the physiography of the
margin, water residence time on the shelf, summer stratification
and its relation to the wind regime and currents, and tidal
magnitude influence the occurrence and intensity of hypoxia.
Hypoxia of the coastal shelf near river mouths occurs when
oxygen consumption from the decomposition of organic inputs to
the bottom waters exceeds the replenishment of oxygen in the
bottom waters. Thus, conditions affecting the rate of oxygen
replenishment to sub-pycnocline waters by either horizontal
advection of water through circulation on the shelf (water
residence time, shape of the margin), turbulent vertical mixing
by surface cooling and wind/tidal mixing or diffusion across the
pycnocline (thermal stratification, tidal regime) are key to
determining the development, duration, and extent of bottom
water hypoxia (Rowe, 2001).
4.1.1. Shape of the margin
The different shapes of the continental shelves near the four
rivers influence the occurrence of hypoxia. Among the four
systems, shallow (30–50 m) and broad shelves such as Mississippi
and Changjiang are linked to the occurrence of hypoxia. Shallow
shelves are more subject to summer warming and stronger
stratification. They may also experience more intense pelagic–
benthic coupling and higher deposition of labile organic matter to
the sediments which enhance the local sediment oxygen demand.
On the contrary, very shallow depths (5–20 m) such as the Pearl
River estuary can facilitate wind-driven vertical mixing of the
surface and bottom waters, thereby tending to reduce hypoxia. In
contrast, the Rhône shelf which is over 100 m deep, displays no
hypoxia.
Another feature, which may influence the development or
intensity of hypoxia, is the existence of a prodelta (as in the
Mississippi and Rhône), which carries water, nutrients, and
particles beyond the coastal and shallow shelf. In the current
physical configuration of an intensively leveed Mississippi River, it
appears that direct sedimentation of terrestrial organic matter
from this plume has a relatively small contribution to the
development of hypoxia on the shelf (Green et al., 2006), because
most of this material is deposited and buried (around 60%) on the
deeper shelf adjacent to the river (Corbett et al., 2004, 2006).
Rather, hypoxia may be related to the recycled nutrients from the
plume that are transported over the broad shallow shelf to the
west (Green et al., 2006), or to nutrients discharged by the
Atchafalaya at the coast (Green et al., 2006), and supplemented by
organic matter inputs from coastal bays and marshes (Dagg et al.,
2008). Physical conditions contributing to the transport of
Mississippi River water from its discharge site near the shelf
break to the coastal region are key factors for hypoxia development. In the case of the Rhône prodelta, the main plume is
diverted to the south-west over the deeper shelf and rarely
penetrates into a shallower coastal bay, thus preventing its
contribution to hypoxia.
4.1.2. Residence time of the bottom water
Residence time of bottom waters is a very important factor for
hypoxia development, because it directly affects the duration of
oxygen consumption during organic matter decomposition and
thus the degree of O2 depletion. A long residence time allows
complete decomposition of organic matter and large oxygen
consumption. Residence time of bottom waters is controlled by
horizontal advection of the water under the influence of
intermediate and bottom shelf currents.
The results from Table 4 show that hypoxia is related to the
residence time. The Mississippi system, with the longest bottom
water residence time, shows the strongest and largest hypoxia,
while the Rhône with a short residence time and the Pearl inside
the lower estuary displays no hypoxia. In the ECS/Changjiang
zone, hypoxia is less recurrent and temporally shorter which may
be partly related to the lower residence time of its bottom water.
4.1.3. Tidal regime, summer stratification
Tidal mixing of water can disrupt density stratification which
promotes the isolation of bottom water from the atmosphere and
the subsequent occurrence of hypoxia (Wiseman et al., 1997). The
tidal regime of the four estuaries varies from microtidal (Rhône,
Mississippi, about 2 m for the Pearl Rivers) to meso-tidal for the
Changjiang (Table 3). There is no correlation between the tidal
regime and the occurrence of hypoxia since the Mississippi and
the Changjiang margins are at the lower and higher ends of the
tidal range yet both undergo seasonal hypoxia. The tidal range in
the four systems examined is rather narrow and tidal mixing may
not be significant especially for waters at 30–50 m, which are
hypoxic during summer.
The strength of summer stratification is another variable which
may promote hypoxia since it restricts oxygenation of the water
by limiting exchanges between the bottom water and the
atmosphere. For the Mississippi shelf, a correlation between
summer stratification and hypoxia has been proposed (Wiseman
et al., 1997; Rowe and Chapman, 2002). Stratification of the
Mississippi is shown on Fig. 5. A large decrease in temperature
and an increase in salinity of the bottom waters produce high
density bottom waters and strong stratification in August.
A higher surface temperature is due to heat exchange with the
atmosphere and a lower surface salinity is related to the input of
freshwater from the river in the Louisiana current. If wind and
tidal mixing are low, then hypoxia can occur over a large area.
A similar feature occurs in the Changjiang where permanent
salinity stratification combines with thermal stratification in late
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0
0
5
5
5
10
10
10
15
15
15
20
20
20
25
25
25
30
30
24
26
30
28
32
34
36
38
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30
22
26
24
28
Temperature (°C)
Salinity (ppt)
30
32
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Oxygen (mg l-1)
Fig. 5. Typical profiles of T, S, and oxygen during hypoxic conditions in the Mississippi system showing strong thermohaline stratification (August 12, 2004, at 28132.860 N,
90154.620 W; M. Dagg unpublished).
Table 5
Average summer nutrient concentrations (mM) in four rivers and the nutrient
atomic ratio (see reference in the conclusions table)
River
DIN conc DIP conc DSi conc PIP conc DIN/DIP DIN/TP DIN/DSI
Changjiang 125
Mississippi 128
Pearl
85
Rhône
118
0.3
2.5
1
1.2
110
136
140
71
5.8
2.1
417
51
85
98
15
36
1.13
0.9
0.6
1.7
summer (Li et al., 2002). The degree of stratification correlates
well with Chl-a concentration and hypoxia occurrence in late
summer at the Changjiang mouth (Wei et al., 2007), despite the
strong current transporting water from the south in the ECS. The
TWC which spreads at intermediate depths further limits oxygen
exchange with the surface waters (Li et al., 2002; Zhu et al., 2004).
These observations on the Mississippi and Changjiang where
hypoxia is recurrent suggest that stratification plays a key role in
hypoxia development. Indeed, oxygen depletion is very sensitive
to cross-pycnocline exchange (Rowe, 2001) and O2 replenishment,
either vertically through the pycnocline or laterally, must be
properly quantified.
The stratification situation is completely different for the Pearl
River for which the highest discharge occurs during summer (July)
together with warmest water temperatures. Stratification during
the summer cannot develop over long periods because of short
water residence time (3–5 days; Yin et al., 2000), monsoon winds
and occasional typhoons (Yin, 2002, 2003; Yin et al., 2004c;
Harrison et al., this issue) which contribute to the mixing of the
estuary.
For the Rhône, the occurrence in summer of strong northwestern winds (410 m s1) and frequent wind reversals promote
small scale circulation and inertial waves. This breaks summer
stratification and provides oxygen to the bottom waters in the
zone located around the plume area (Estournel et al., 1997, 2001).
Therefore in the Rhône coastal region, stratification occurs during
summer warming, but it is counteracted by secondary windinduced circulation, which maintains high oxygen levels.
4.2. Nutrient and organic matter discharge
As hypoxia is the result of the interplay between oxygen supply
from surface water and atmosphere, and oxygen consumption
from degrading organic matter, nutrient inputs which generate
algal production and subsequent settling to the bottom waters are
important forcing factors on the system. River estuaries and deltas
are regions of very high productivity sustained to a great degree
by nutrients of terrestrial origin. In this section, we examine
nutrient discharge, and N:P and N:Si nutrient ratios (where
available). We also examine the delivery of particulate and
dissolved organic matter and their role in oxygen consumption.
4.2.1. Nutrient inputs and nutrient ratios of the river discharge
In all four systems, the nutrient concentration in the river
input is high during all seasons (Table 5). Indeed, DIN concentrations are around 100 mM on average for all systems, while DIP
concentrations are much more variable. DIN and DIP input fluxes
(Table 2) are related to water discharge and are maximal for the
Mississippi and Changjiang and lower for the Pearl and Rhône
Rivers.
Riverine nutrient input fuels high primary production in the
adjacent coastal zone. The nutrient load is generally associated
with impacts in the estuary and surrounding waters as it is the
total quantity of nutrients delivered, and its utilization by
the ecosystem, which ultimately controls the total production of
the delta and coastal system. Positive correlations exist between
DIN load and primary productivity for the Mississippi (Lohrenz
et al., this issue). Therefore, for determining the magnitude of
potential hypoxia, the total flux of nutrients is a primary driver of
the algal biomass formed and the potential oxygen consumption
in bottom waters. However, differences in the timing of nutrient
delivery, the nutrient concentrations, and nutrient ratios can
significantly affect the amount and pattern of primary production.
We therefore use nutrient concentrations and nutrient ratios to
compare the formation of hypoxia in these four river-coastal
systems with different discharge volumes.
Data from a suite of studies assessing nutrient limitation of
phytoplankton production (summarized in Rabalais et al., 2002)
indicate that, in the case of the Mississippi, either N, P, or Si can
limit phytoplankton growth, depending on the physical and
biogeochemical conditions. In fact, N limitation appears most
common during low flow periods and at higher salinities,
particularly during late summer and autumn. Nitrogen is
generally believed to be the main limiting nutrient, due to
efficient recycling of P and loss of N through denitrification
(Rabalais et al., 2002). During spring and early summer, P may
play an important role in limiting primary production, especially
in low and intermediate salinity regions of the plume. Evidence
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from nutrient ratios and bioassays show that P limitation can
occur during high flow and at intermediate salinities (Smith and
Hitchcock, 1994; Lohrenz et al., 1997; Lohrenz et al., 1999; Sylvan
et al., 2006). Models (Justic et al., 2002; Scavia et al., 2003; Turner
et al., 2005) have utilized statistical relationships between
hypoxia and riverine nutrient inputs to the shelf to predict or
hindcast hypoxia. These models reinforce the view that nutrient
stimulated production from the Mississippi and Atchafalaya
Rivers drives the development and maintenance of coastal
hypoxia. Si limitation is more variable and most pronounced in
the spring (Dortch and Whitledge, 1992). Si limitation impacts
diatoms (Nelson and Dortch, 1996; Lohrenz et al., 1999; Rabalais
et al., 1999), which are an important fraction of the biomass of
sinking particles (Redalje et al., 1994). Hence, Si dynamics are
believed to be especially relevant for hypoxia formation (Turner
et al., 1998; Dortch et al., 2001).
In the Pearl River during the summer, nutrient loads are high
due to inputs from the river (DIN 100 mM, DIP 1 mM, and DSi
100 mM) and rain (DIN 50 mM, DIP 1 mM, and DSi 50 mM) (Yin
and Harrison, this issue; Yin et al., 2000, 2004b). The nutrient
ratio of 100N:100Si:1P is about 7 times the Redfield ratio required
for phytoplankton growth and hence P potentially limits biomass
production in the summer (Yin et al., 2000, 2004b; Yin 2002,
2003). The Pearl is similar to the Mississippi since Si limitation
may occur before the river discharge begins to increase in May
and potential N limitation may occur in the winter dry season due
to the monsoon-driven invasion of coastal water. This is also the
case for the Rhône and Changjiang Rivers where the DIN:DIP ratio
is also clearly higher than the utilization ratio (16:1). In contrast,
the DIN:DSi ratio is close to the utilization ratio (1) which
indicates that Si may not be limiting, except in the case of the
Rhône River. However, these patterns, based on annually averaged
concentrations, are only indicative of the potential limiting
nutrient, as concentrations and phytoplankton species vary during
the seasons and might provide different situations for different
seasons. The pattern of nutrient limitation seems to be uncorrelated to the occurrence of hypoxia as Changjiang and Rhône
(for example) are both strongly limited by P but only ECS close to
the Changjiang undergoes summer hypoxia.
Regeneration processes are crucial for transporting nutrients as
part of the buoyant plume that forms from the discharging river
water. In the case of the Mississippi, respiration accounts for 65%
of the fate of the fixed carbon in the plume (Green et al., 2006)
and recycling of plume nitrogen is a major pathway for nitrogen
flow (Cotner and Gardner, 1993; Gardner et al., 1994). This
nitrogen recycling and associated production of organic matter
provides an important mechanism for transporting a portion of
the river nitrogen load and resultant organic matter production to
the shallow regions susceptible to hypoxia development. Because
ratios of N, P, and Si have changed in most rivers during the past
50 years due to changes in land use, and dam building, the
limiting nutrient has likely shifted also. For example, P limitation
is likely more important now because of increased N input from
the Mississippi River over the past 50 years (Turner and Rabalais,
1991). Nitrogen inputs have also increased in the Changjiang
(Zhou and Shen, this issue) and other coastal areas of China
(Harrison et al., 1990) over the last few decades. For the Rhône,
nitrogen has increased during the seventies and stabilized since
the early 1980s (Moutin et al., 1998).
4.2.2. Phytoplankton productivity, community structure, export and
respiration
As the river mixes with the coastal water, light conditions
improve through dilution and sinking of large lithogenic particles
and particulate organic material, and phytoplankton growth
increases dramatically (Dagg and Breed, 2003). In the Mississippi,
phytoplankton growth rates approach the theoretical maxima in
the mid-salinity region, and can be as high as 3 d1 at summer
temperatures of 30 1C (Fahnenstiel et al., 1995). In this region,
primary production can reach 10 g C m2 d1 (Lohrenz et al., 1999)
during early summer. Both grazing and dilution by physical
mixing tend to reduce phytoplankton abundance, but the growth
rates typical of mid-salinity regions are so high that phytoplankton biomass rapidly accumulates in spite of these loss terms
(Lohrenz et al., 1999). These high stocks of phytoplankton are
often visible in satellite images (Lohrenz et al., this issue).
Large phytoplankton cells are subject to lower grazing mortality
in the low- and mid-salinity regions because of the relatively slow
numerical response by their major grazers, the copepods (Liu and
Dagg, 2003). Consequently, large cells, especially diatoms, typically
dominate the phytoplankton community in the mid-salinity region
of the plume where sinking of organic matter is most intense (Bode
and Dortch, 1996; Green et al., 2006).
In the Pearl River estuary during the summer, large chainforming diatoms such as Skeletonema, Thalassiosira and Chaetoceros dominate and Chl a can range from 20 to 4100 mg m2 (Yin
2002, 2003) with the highest concentrations observed at the
seaward edge of the plume. The Pearl River estuary is very
productive with values ranging from 1 to 5 g C m2 d1 due to high
surface temperatures of about 27 1C (Yin et al., 2000). The fate of
this fixed carbon is not well understood in terms of how much
sinks and how much is grazed by zooplankton.
For the Rhône River, a contrasting situation is found. The
primary production encountered in the transition zone between
the near- and mid-fields is much lower (o1 g C m2 d1) than for
the other three river systems, although the structure of the
ecosystem is similar. Within the turbid plume where production is
low, 50% of the chlorophyll a and 80% of the primary production is
attributed to pico- and nanoplankton (Videau and Leveau, 1990;
Pujo-pay et al., 2006). In the transition area between the plume
and the marine system, microplankton represents 50% of the
chlorophyll a and primary production, largely due to diatoms
during spring.
Primary production in the Changjiang-influenced coastal zone
ranges from 1.5 to 4.5 g C m2 d1 (Chen et al., 2004) during
summer. This high primary production is highly seasonal with
values falling to 0.04 g C m2 d1 during winter (Chen et al., 2001).
During summer, primary and new production is largely driven by
diatoms in the coastal zone influenced by the river. In the case of
the Changjiang, coastal upwelling plays an important role in
supplying phosphorus-enriched shelf water to the surface waters
during the summer (Chen et al., 2004). In the turbid area of the
plume, flagellates may photosynthesize and accumulate due to
their ability to swim upward during the day (Chen et al., 2003).
The source of the organic load fueling hypoxia in the Changjiang
coastal zone, whether it is from in situ production or organic
matter advected from outside the plume, is currently under
debate (Wei et al., 2007).
Except in the case of the Rhône River, high primary production
during spring and summer (1–10 g C m2 d1) is due to large fastgrowing diatoms, which can be exported through direct sinking at
the end of the bloom, and contribute to large inputs of labile
organic matter to bottom waters and sediments. Large phytoplankton cells can sink at rates of several meters per day, and
aggregates of cells or mixtures of cells and detritus at rates
of 10–100 m d1. At these rates, direct sinking of phytoplankton
from the most productive layer of the plume, typically only
1–10 m thick, can easily provide a large enough input of organic
matter to the sub-pycnocline layer that fuels hypoxia formation.
Rapidly sinking fecal pellets provide an additional source of
organic matter to the sub-pycnocline layer (Breed et al., 2004).
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If a significant fraction (50%) of the 5 g C m2 d1 reaches the
sediment or bottom waters, this may cause large oxygen
consumption in bottom waters. Up to 200 mmol m2 d1 of
oxygen could be consumed, which may create hypoxia in days
to weeks if the replenishment of oxygen through the pycnocline is
decreased by stratification. As an example, vertical flux of organic
carbon in various parts of the Mississippi River plume is as high as
1.80 g C m2 d1 during spring, but rates are lower during other
seasons (0.29–0.95 g C m2 d1), and even lower away from the
immediate plume (0.18–0.40 g C m2 d1) (Redalje et al., 1994).
This vertical flux is equivalent to approximately 10–25% of
phytoplankton production in mid-salinity regions of the plume
(Breed et al., 2004). Amon and Benner (1998) calculated that
bacterial oxygen consumption alone could reduce the oxygen
concentration to hypoxic levels within 1–7 weeks during the
summer. Chin-Leo and Benner (1992) calculated that 26–56 days
of sub-pycnocline respiration (excluding benthic respiration)
could lead to hypoxia. Large benthic oxygen demands ranging
from 20 to 50 mmol O2 m2 d1 (Morse and Rowe, 1999) indicate
that the sediment may constitute a large sink of oxygen. Dortch
et al. (2004) suggested that bottom water oxygen depletion
through both water and benthic respiration would take about 1
month in the summer. These different calculations indicate that
organic matter mineralization into the sub-pycnocline layer is
sufficient to consume oxygen and create hypoxia if oxygen is not
replenished.
4.2.3. Doc and poc loading: what is their role in generating/
sustaining hypoxia?
The contribution to hypoxia of organic matter directly
discharged by rivers remains poorly quantified, but evidence from
the Mississippi indicates it is small for this river-shelf system
(Green et al., 2006). Most of the POC from the river is deposited
close to the river mouth. Contrary to the paradigm of the
terrigenous organic matter being largely refractory, this organic
matter can be remineralized in the sediment giving rise to large
oxygen demands (Morse and Rowe, 1999; Lansard et al., this
issue). Sediment oxygen demand created by the direct deposition
of organic matter can add to the oxygen demand generated by the
settling of local primary production, reaching a threshold which
might trigger hypoxia in bottom waters.
Even less clear is the role of DOC in generating hypoxia.
A fraction of the DOC from rivers is believed to be degradable, but
largely degraded in estuaries (Raymond and Bauer, 2001)
although Benner and Opsahl (2001) estimated that only 2–4% of
Mississippi DOC is labile and would thus contribute to oxygen
consumption when entrained in coastal bottom waters. Processes
such as adsorption on particles (minerals or organic) in the water
column can contribute to this transfer, but little is known about
these processes near the river mouth. Cross-pycnocline exchanges
will also promote the introduction of labile DOC in bottom waters
together with oxygen, thereby reducing the net oxygenation of the
waters.
1535
our comparison to properly evaluate this process. Physical
constraints such as summer thermohaline stratification and long
residence time are required for maintaining the isolation of
bottom waters for hypoxia to occur. Broad intermediate-depth
shelves (40–50 m) are also a precondition where tidal and wind
mixing are damped and where intense pelagic–benthic coupling
operating in the vertical transport of the algal biomass to depth.
The Mississippi and Rhône Rivers and their shelves are at the
two extremes of this classification: the Mississippi system has a
long residence time, strong summer stratification, a broad
intermediate-depth shelf and high primary production in its
plume. This system currently displays the largest area of seasonal
hypoxia among the four systems compared. Mediterranean waters
close to the Rhône River have short residence time, summer
stratification can be broken by wind reversals, primary production
on the shelf is low and the shelf is deep. Therefore, it shows no
evidence of hypoxia.
The Changjiang and adjacent ECS are intermediate with a
broad shelf, strong summer stratification and high primary
production, but also high advection of water from the southern
shelf, which makes summer hypoxia less persistent and less
recurrent.
The Pearl River estuary is shallow (o15 m) and wind and tidal
mixing can reduce stratification. It has a short residence time and
high flushing during the summer due to the high water discharge,
and hence little hypoxia occurs in the lower estuary.
The importance of the physical constraints on the occurrence
of hypoxia points towards a potential risk of an increase in
hypoxic area in a changing climate. Indeed, warmer summers and
weaker winds, or changes in coastal water circulation, could lead
to enhanced stratification or decreased open-water intrusion on
the shelf. This could lead to an increase vulnerability to hypoxia in
coastal areas, which are, at present, impacted by high nutrient
discharge, but have sufficient summer mixing to prevent complete
oxygen consumption.
Acknowledgments
We thank the organizing committee (our co-authors M. Dagg,
M. Dai, and P. Harrison) of the symposium ‘‘Coastal Ecosystem
Responses to Changing Nutrient Inputs from Large Temperate and
Sub-tropical Rivers’’ held at Xiamen University (China) for inviting
us to the meeting. C.R. acknowledges the funding by the ANRCHACCRA program (ANR-06 VULN-001-01) and the help of B.
Bombled for figure design. M.H.D. acknowledges funding from
NSF-China through Grant # 40576036. P.J.H. and K.Y. acknowledge
funding from UGC (Project no. AoE/P-0404) and RGC (P-04-02)
from the Hong Kong Government and NSFC 40676074 and
40490264. K. Yin and W.-J. Cai acknowledge the project supported
by CAS/SAFEA International Partnership Program for Creative
Research Teams. This is LSCE contribution number 2678.
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