JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C11020, doi:10.1029/2009JC005951, 2010

Nutrient dynamics in the winter thermohaline frontal zoneof the northern shelf region of the South China Sea
Su Mei Liu,1 Xinyu Guo,2 Qi Chen,3 Jing Zhang,1,4 Yan Feng Bi,1 Xin Luo,1and Jian Bing Li1
Received 30 October 2009; revised 23 May 2010; accepted 29 June 2010; published 23 November 2010.

[1] As the first attempt to estimate the nutrient transport across the winter thermohaline
frontal zone on the northern shelf of the South China Sea, the nutrient dynamics around thefront and the effects of cross‐frontal water exchange on nutrient transport were
investigated using wintertime field observations. Both water temperature and salinity
increased from coastal to oceanic waters, showing the presence of a thermohaline front.
The concentrations of dissolved inorganic nutrients decreased oceanward, especiallyacross the thermohaline front, while those of dissolved organic nutrients (i.e., dissolved
organic nitrogen (DON) and dissolved organic phosphorus) showed patchy distributions.
Ammonium was the major constituent of dissolved inorganic nitrogen, and DON was
the main component of total dissolved nitrogen. Molar ratios of PO3−
4 /total dissolved
phosphorus decreased from coastal to oceanic waters, indicating that PO3−
4 was rapidly
removed and/or consumed from the water column and that organic matter degradation−+
increased offshore, replenishing PO3−
4 . Molar ratios of NO3 /(NH4 + DON) were 0.01–0.6,
indicating dominance of regenerated nitrogen. Surface water convergence and bottom
water divergence were identified in the across‐shore velocity field, and the calculated
across‐shore nutrient fluxes suggest that the presence of the winter thermohaline front
promotes the offshore transport of nutrients from coastal waters. The transport path begins
with convergence of surface coastal waters toward the front, followed by the sinking
in the frontal region and the oceanward movement through the bottom layer of the front
offshore side. With an assumption of 500 km as the length of thermohaline front on thenorthern shelf of the South China Sea, the calculated offshore fluxes of nutrients across theentire front are larger than those from the Zhujiang (Pearl River) and the Changjiang(Yangtze River).
Citation: Liu, S. M., X. Guo, Q. Chen, J. Zhang, Y. F. Bi, X. Luo, and J. B. Li (2010), Nutrient dynamics in the winterthermohaline frontal zone of the northern shelf region of the South China Sea, J. Geophys. Res., 115, C11020,
doi:10.1029/2009JC005951.

1. Introduction[2] Fronts are often observed in the ocean, but the characteristics of one front can be completely different from theothers. Yanagi [1987] classified the fronts into coastal water
fronts, shelf fronts, and open ocean fronts and summarizedthat there are four types of fronts in coastal waters. Estuarine
fronts are located near river mouth or along with river
plume, thermal effluent fronts are close to power plants,
1
Key Laboratory of Marine Chemistry Theory and Technology,
Ministry of Education, Ocean University of China, Qingdao, China.
2Center for Marine Environmental Studies, Ehime University, Matsuyama,
Japan.
3
Laboratory of Ocean Dynamic Processes and Satellite Oceanography,
Second Institute of Oceanography, State Oceanic Administration, Hangzhou,
China.
4
State Key Laboratory of Estuarine and Coastal Research, East China
Normal University, Shanghai, China.

Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JC005951

thermohaline fronts are formed in transition zones between
cold coastal waters and warm oceanic waters in winter, andtidal fronts are formed in transition zones between vertical
mixed waters attributed to tidal stirring and stratified watersin summer. If we consider a region far from river mouth anda power plant, only thermohaline fronts are expected incoastal waters in winter.
[3] The formation of thermohaline fronts depends on
wintertime atmospheric cooling and salinity difference
between the coastal area and the offshore area. Because of
atmospheric cooling, the waters are colder in the shallow
coastal area than in the deep offshore area. River discharge
keeps the salinity lower in the coastal area than in the deep
offshore area. If the differences in water temperature andsalinity between the coastal area and the offshore area allow
the presence of a surface convergence zone, thermohaline
fronts with strong horizontal temperature and salinity gradients are generated [Endoh, 1977; Akitomo et al., 1990].
The density difference across the thermohaline fronts is

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small, but a weak density peak caused by the cabbeling
effect can be found in the frontal region [Endoh, 1977;
Akitomo et al., 1990]. These fronts have been identified inmany western Pacific coastal areas such as the Kii Channel[Yoshioka, 1971], Tokyo Bay [Yanagi and Sanuki, 1991],
and Ise Bay [Yanagi et al., 1997] based on hydrographic
observations. Studies using satellite data and numericalmodels also suggested the presence of thermohaline fronts
in the South China Sea (SCS) in winter [Wang et al., 2001;
Chu and Wang, 2003].
[4] The SCS is an epi‐continental marginal sea of thewestern Pacific Ocean, with a surface area of 350 × 104 km2
and an average water depth of 1212 m. The SCS is a
Mediterranean‐type basin dominated by monsoon circulation, which travels northeasterly from October to April andsouthwesterly from May to September. The northern part of
the SCS has a wide shallow shelf, and several rivers empty
into this region, including the Zhujiang (Pearl River).
[5] The coastal zone in the northern SCS is China’s mostpopulated area and has the most developed economy in
China. Consequently, eutrophication is a serious problem,and harmful algal blooms occur frequently [Song et al.,
2004; Yin and Harrison, 2007]. On the other hand, thebasin‐wide SCS is devoid of nutrients in the surface layer.
The concentration of nutrients increases with depth [Chenet al., 2001], and the deepwater maximum of nutrients isshallow and smooth because of its rapid ventilation andshort flushing time relative to the open ocean [Zhang andSu, 2006].
[6] The primary production in the SCS is higher in winterthan in summer [Liu et al., 2002; Ning et al., 2004; Lee
Chen, 2005; Chen et al., 2006; Lee Chen and Chen, 2006].
This seasonal variation of phytoplankton depends closely on
the wind surface mixing that is stronger in winter than insummer and therefore brings more nutrients to surface layer
in winter. In addition, the material transport across thethermohaline front is also a possible mechanism for thenutrient sources in the SCS in winter because water transport across fronts plays a significant role in nondiffusive
biogeochemical material transport processes [Read et al.,
2000; Naveiragarabato et al., 2002; Li et al., 2003], which
influence the distributions of nutrients, phytoplankton, andzooplankton and hence affect the stability of the ecosystem[Kachel et al., 2002].
[7] Although a thermal front in the SCS has been identified with wintertime satellite data [Wang et al., 2001], there
has not been a field report on salinity and nutrient distributions around the front yet. Furthermore, nutrient transport in relation to the front in the SCS is also unknown. On
the other hand, quite different picoplankton group dynamics
have been observed across the thermohaline front in thenorthern SCS shelf [Pan et al., 2005]. Therefore, it is necessary to clarify nutrient distributions around the thermohaline front and to determine the role of the front on nutrienttransport.

2. Materials and Methods[8] Field observations were performed on the northern
SCS shelf from 27 January to 6 February 2004. Hydrographic surveys and water sampling occurred at stations
along sections A, C, F, and G (Figure 1). Only sections A

C11020and G are presented here to examine the influence of thethermohaline front on nutrient transports by comparing thenutrient fluxes in the presence (such as section A) andabsence (such as section G) of the thermohaline front.
Anchor stations were maintained at stations A3 and A6 forover 25 h, during which measurements were performed
every 3 h for chemical parameters and each hour forhydrographical parameters and current profiles.
[9] Vertical temperature, salinity, and current profiles
were measured with a SBE25‐CTD and a Lowered Acoustic
Doppler Current Profiler (LADCP; Teledyne RDI 1200 kHz).
The current data measured by LADCP at each station were
processed with the Teledyne RDI standard software. The
current data are available from 3 to 30 m depth with avertical interval of 1 m. Assuming that the current did notchange from sea surface to 3 m depth and was zero at sea
bottom, we extrapolated the current data to the layer shallower than 3 m and the layer deeper than 30 m depth.
[10] Discrete water samples were collected in 10 L Niskinbottles attached to a conductivity‐temperature‐depth (CTD)
rosette, and sampling depths were determined by assessinghydrographic parameters. After water collection, samples
were immediately filtered through precleaned 0.45 mm porecellulose acetate filters, and the filtrates were poisoned withsaturated HgCl2.
[11] Nutrient concentrations (NO−3 , NO−2 , NH+4 , PO3−
4 , and
Si(OH)4) were quantified using an autoanalyzer (Skalar
SANplus) [Liu et al., 2005]. The analytical precision of NO−2 ,NO−3 , NH+4 , PO3−
4 , and Si(OH)4 was 0.01, 0.06, 0.09, 0.03,
and 0.15 mM, respectively. The dissolved inorganic nitrogen(DIN) concentration is the sum of NO−3 , NO−2 , and NH+4 .
Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) were treated with boracic acid‐persulfate
oxidation solution (K2S2O8 and H3BO3 in NaOH solution)in an autoclave and hydrolyzed at 120°C for 30 min. Then,
TDN and TDP were decomposed to NO−3 and PO3−
4 ,respectively, and measured using an autoanalyzer[Grasshoff et al., 1999]. The analytical precision of TDN
and TDP was 0.68 and 0.02 mM, respectively. Dissolved
organic nitrogen (DON) and dissolved organic phosphorus(DOP) concentrations were estimated as the difference
between TDN and DIN or TDP and PO3−
4 , respectively.
Duplicate samples were run, and the precision for DON and
DOP was 5% and 15%, respectively.

3. Results
3.1. Hydrographic Characteristics[12] At section A, both water temperature and salinity
increased from coastal (<15.5°C and <33.45 salinity) tooceanic waters (>20.5°C and >34.6 salinity) (Figure 2). A
strong thermohaline front was observed between stations
A4 and A5, with temperature and salinity gradients of
0.75°C km−1 and 0.25 km−1, respectively. On the front
onshore side, inverse temperature layers (17°C) were foundat the bottom layer of station A3; on the front offshore side,
low‐salinity layers (<34.3) were found along the bottom
layer. Above the inverse temperature and low‐salinity
layers, the water column was vertically well mixed. The
water density in the surface layer was highest at the front
region and decreased toward its offshore and onshore sides.
In the vertical, the increase in water density from the surface

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C11020Figure 1. (top) Advanced very high resolution radiometer infrared image of the northern part of theSouth China Sea, taken on 24 January 2004 (courtesy of J.‐C. Chen, Center for Astrodynamics Research,
Hong Kong University of Science and Technology). (bottom) Map of stations along legs A and C (i.e.,
stations A1–A11 and C16–C20) and legs F and G (i.e., stations F1–F9 and G1–G8). Only sections A and
G are presented in this study to examine the influence of the thermohaline front on nutrient transports bycomparing the nutrient fluxes in the presence (such as section A) and absence (such as section G) of thethermohaline front.

layer toward the bottom layer occurred at both the front
region and its two sides. If we consider the convergence of
two water masses with different water temperature andsalinity but similar density (oceanic water with high watertemperature and high salinity, coastal water with low watertemperature and low salinity) toward the front, the spatial
structure of density at section A (Figure 2) can be explained
partly by the cabbelling effect.

[13] At section G (Figure 2), which was surveyed 3 days
after section A, both water temperature and salinity
increased oceanward. The density was largest near thebottom layer, above which the density was high at station
G8 and decreased oceanward. No similar frontal region asthat observed at section A can be identified at section G.
Since the thermohaline properties of section G were similar

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

Figure 2. Vertical distributions of water temperature, salinity, and density along sections A and G in
winter 2004. Note that there were no data at station A1 along section A because of a CTD malfunction.
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to those observed on the front offshore side at section A, wesuggest that there was an onshore movement of the front.
[14] With the sea surface temperature (SST) image fromsatellite, we can confirm the onshore and offshore movements of the front (Figure 3a). In all the available satellite
images, an apparent SST front parallel to the coast can be
identified. The front is not in a straight line, and themeandering of the front is found in the images. In our
survey area, the front was sometimes far from the coast (e.g.,
29 December 2003) or sometimes close to the coast (e.g.,
6 January 2004). If all the front positions are overlapped in
one single image (Figure 3b), the onshore and offshore
movements of the front become more apparent. Although
the satellite images in Figure 3 were not from the same day
as our hydrographic observations, the season was the same.
Therefore, the onshore movement of the front deduced from
Figure 2 is reasonable.
3.2. Spatial Variation of Nutrients[15] The distributions of inorganic and organic nutrientsfor the two studied sections are presented in Figure 4. For
section A, the thermohaline front was also clearly observedon the inorganic nutrient distributions between stations A3
and A5. In general, concentrations of dissolved inorganic
nutrients decreased from coastal (approximately 3.5 mMNO−3 , 0.25 mM PO3−
4 , 7.0 mM Si(OH)4) to oceanic(approximately 0.1–0.2 mM NO−3 , 0.08 mM PO3−
4 , 1.6 mM Si(OH)4) waters with 20‐fold differences for NO−3 and NO−2
and fourfold differences for PO3−
4 and Si(OH)4. The concentrations of NO−3 , NO−2 , PO3−
4 , and Si(OH)4 were lower inthe bottom layer than in the upper layer of the front onshoreside, while they were higher in the bottom layer than in theupper layer of the front offshore side. The concentrations of
NH+4 , DON, and DOP do not have such a spatial distribution. One or more patches with high or low concentrations
were found for these variables.
[16] The inorganic nutrient concentrations at section G
were generally lower than those at section A, except for
NH+4 . The concentrations of NO−3 and NH+4 decreased
oceanward, and they increased from surface to bottom witha fivefold vertical difference for NO−3 , a threefold difference
for NO−2 , and a onefold to twofold difference for PO3−
4 and
Si(OH)4. The DON and DOP concentrations showed patchy
distributions with concentrations of 2–8 mM for DON and
0.04–0.1 mM for DOP.
3.3. Daily Nutrient Variations[17] To examine daily nutrient variations, 25 h observations were performed at stations A3 and A6. At both stations, high temperatures (>19°C) and salinities (>34.5) wererecorded, which were characteristic of the waters on thefront offshore side (Figure 2). Here we show the data at
station A6 to confirm the daily variations in nutrient concentrations (Figure 5). The data at station A3 were similar to
those at station A6 and therefore not presented here.
[18] At station A6, NO−3 , NO−2 , and DOP varied daily byapproximately tenfold, and NH+4 , PO3−
4 , Si(OH)4, and DON
varied twofold to sixfold. Nutrient concentrations showedvertically well‐mixed distributions, with the exception ofdissolved silicate, which was higher in the bottom layer thanin the upper layer. NH+4 accounted for 70%–90% of DIN,
with an average of 80%. DON composed 70%–90% of

C11020

TDN, with an average of ∼80%. PO3−
4 represented 35%–
90% of TDP, with an average of 60%.

4. Discussion
4.1. Nutrient Composition[19] Nutrient concentrations in the study region, except
for that of ammonium, were quite low compared to those inother Chinese coastal areas (i.e., the Bohai Sea, Yellow Sea,and East China Sea) [Zhang and Su, 2006]. Dissolved
inorganic nutrients decreased oceanward, while DON and
DOP showed patchy distributions, indicating a stronger,
controlled by biological activity. High NH+4 and NO−2 concentrations at the coastal stations are most likely related todecomposition of organic matter [Cai et al., 2004] and river
inputs. The proportion of PO3−
4 to TDP decreased fromcoastal (65–90%) to oceanic (30–75%) waters alongsection A, from 20% to 75% along section G, and ∼50% at
anchor station A6, indicating that PO3−
4 was rapidly removed
and/or consumed within the oceanic waters and organic
matter degradation increased oceanward, replenishingphosphate. The molar ratios of NO−3 /(NH+4 + DON) also
decreased from coastal (0.6) to oceanic (<0.05) waters alongsection A, 0.01–0.05 along section G, and <0.08 at anchor
station A6, indicating the importance of regenerated nitrogen (NH+4 + DON) in sustaining primary production. With
respect to nutrient composition, NH+4 was the major component of DIN, and the highest contribution of TDN corresponded to DON.
[20] There are remarkable correlations among the nutrient
concentrations with a correlation coefficient (R) for DIN
versus PO3−
4 of 0.76, R for Si(OH)4 versus DIN of 0.87, andR for Si(OH)4 versus PO3−
4 of 0.85 at p = 0.001, n = 123.
The mean molar ratio from all the data was 19 for DIN/PO3−
4 ,0.91 for Si(OH)4/DIN, and 23 for Si(OH)4/PO3−
4 , respectively. Nitrogen and/or silicon limitation was reported in thenorthern SCS [Lee Chen and Chen, 2006; Zhang et al.,1999]. However, the mean molar ratios from our study
indicate that phosphorus may be the limiting nutrient forphytoplankton growth with the condition that phosphateconcentrations are under their limiting values [Ning et al.,
2004; Xu et al., 2008]. Previous incubation experiments
indicated that phosphate was the limiting element in the
Zhujiang riverine effluent plume [Zhang et al., 1999]. It wasreported that the Zhujiang river has a very high DIN input,
resulting from fertilizer usage, and high dissolved silicate
concentrations, reflecting the drainage basin mineralogy, but
it has a relatively low phosphate input [Cai et al., 2004;
Zhang et al., 1999].
[21] Spatially, as water temperature increased, NO−3 ,NO−2 , PO3−
4 , and Si(OH)4 concentrations decreased significantly, that of NH+4 decreased slightly, and those of DON
and DOP had no trends (Figure 6a). Apparently, there werehigh dissolved inorganic nutrient levels in coastal waters withtemperature <16°C. Their concentrations decreased significantly as water temperature increased to 18°C, reflecting thedominant role of the thermohaline front along section A. In
addition, the biological activities also affect the variation ofnutrients with temperature.
[22] Plotting nutrient concentrations versus salinity indicated that dissolved inorganic nutrient concentrations
decreased as salinity increased (Figure 6b). The linear cor-

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Figure 3. (a) Advanced very high resolution radiometer infrared images of the northern part of the South
China Sea between 22 December 2003 and 8 January 2004 (courtesy of J.‐C. Chen, Center for Astrodynamics Research, Hong Kong University of Science and Technology). The dotted line in each panel denotes
position of front. (b) Composite infrared image overlapping all the front positions between 22 December
2003 and 8 January 2004.
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Figure 4. Vertical nutrient distributions (NO−3 , NO−2 , NH+4 , PO3−
4 , Si(OH)4, DON, and DOP) for sections(a) A and (b) G in winter 2004. The red dots denote the depths of water samples for nutrient analysis.
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Figure 4. (continued)
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Figure 5. Temporal variation of temperature, salinity, and nutrients (NO−3 , NO−2 , NH+4 , PO3−
4 , Si(OH)4,
DON, and DOP) at anchor station A6 in winter 2004. The red dots denote the depths of water samples fornutrient analysis.
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C11020Figure 6. Plots of nutrient concentrations (mM) versus (a) water temperature and (b) salinity; shown are
all data for all stations in the four sections in winter 2004.
relations are significant at the 95% confidence level, and thecorrelation coefficients range from 0.41 to 0.90 (n = 74),
suggesting that terrestrial input (nonpoint discharge) is the
major cause of high dissolved inorganic nutrient levels incoastal waters. On the other hand, the DON and DOP
concentrations were not correlated with salinity, pointing toa stronger biological control on the organic nutrient levels.
4.2. Nutrient Transport in the Thermohaline
Frontal Zone[23] Fronts play an important role in water exchange and,
consequently, in the exchange of dissolved and suspended
matters [Jakobsen, 1997; Brandini et al., 2000]. Since current
data are available for the two sections along which nutrientswere measured, nutrient fluxes can be obtained by multiplying the velocity and the nutrient concentrations. With thecalculated nutrient fluxes, we can examine the influence ofthe thermohaline front on nutrient transports by comparing
the nutrient fluxes in the presence (such as section A) andabsence (such as section G) of the thermohaline front.
[24] The thermohaline front in the SCS is reported to beoriented along the coast [Wang et al., 2001], generally

parallel to the isobaths (Figure 3). To examine nutrienttransport along and across the front, we calculated along‐isobath component and across‐isobath component of thevelocity and nutrient flux. The positive direction for along‐isobath component is 30°T (clockwise 30° from north), or
northeastward along the coast, while that for across‐isobathcomponent is 120°T (clockwise 120° from north), or
southeastward perpendicular to the coast.
[25] Before we discuss nutrient transport along the two
sections (A and G), we need to examine the contamination
attributed to tidal currents. Tidal currents are periodic oscillations and do not contribute to the nutrient fluxes withtime scales longer than the tidal period (∼1 day or longer).
However, nutrient transport caused by tidal currents is
actually included in any snapshot observation of currents
like ours along these two sections in the SCS. If tidal
currents are very weak like they are in the deep open ocean,
their influence on the flux estimate from snapshot current
data can be neglected; if the tidal currents are of the sameorder as or stronger than the subtidal currents, their influence
cannot be neglected.

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

Figure 6. (continued)[26] Removing tidal influences from snapshot current data
is not easy. A numerical tidal current model [Foreman andFreeland, 1991] or fitting tidal currents with a mathematical
function [Candela et al., 1992] provides possibilities for
detiding such snapshot current data. However, validating
tidal currents predicted by a numerical model is difficult,and the precision of the functional fit of tidal currents is stillquestionable [Carrillo et al., 2005]. Here, instead of detiding the snapshot current data, we examine the extent of tidal
current contamination on the nutrient fluxes estimated fromthe snapshot data in our study area.
[27] At the two anchor stations, currents were measured
every hour for 25 h. Such current data allow a direct harmonic analysis of the tidal currents by assuminguðt Þ ¼ u0 þ u1 cosð!1 t  1 Þ þ u2 cosð!2 t  2 Þ þ "ðtÞ;ð1Þ

where u is eastward or northward velocity; t is time; u0 isconstant during the anchor period (hereafter called subtidal
current); u1, w1, and a1 are the amplitude, frequency, andphase of the diurnal tidal constituent represented by the K1
constituent; u2, w2, and a2 are the amplitude, frequency, andphase of the semidiurnal tidal constituent represented by theM2 constituent; and " denotes the residual. With leastsquares method to minimize total "2 from all the hourly datameasured at one anchor station, u0, u1, a1, u2, and a2, can beobtained.
[28] Figure 7 shows the vertical profiles of the across‐ andalong‐isobath components of the subtidal current and the
profiles of the diurnal and semidiurnal tidal current amplitudes at stations A3 and A6. The subtidal currents, both theacross‐isobath component and the along‐isobath component,
are larger than the tidal current amplitudes at the two stations.
Within the tidal currents, the diurnal tidal constituent isstronger than the semidiurnal one at station A3 except for thebottom 3 m, but they are comparable at station A6.
[29] The nutrient fluxes caused by the subtidal currents at
stations A3 and A6 and the ratio of fluxes attributed to thetidal currents to those resulting from the subtidal currents arepresented in Table 1. The ratio can be considered an index
indicating the contamination from the tidal currents in theflux estimate caused by the subtidal currents using a snapNP
jVT ðti ÞC ðti Þj
shot current field. This index is calculated by N1
jV0 C ðti Þj ,11 of 18i¼1

C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

Figure 7. Subtidal current (arrows), its across‐isobathcomponent (thick red line) and along‐isobath component(thick blue line), amplitudes of diurnal tidal (thin blue line)
and semidiurnal tidal (thin red line) currents, and their tidal
ellipses at anchor stations (a) A3 and (b) A6. The tidal
ellipse colors correspond to the tidal current amplitudes.
where i is the number of data, N is the total number of data
for the anchor period, ti is the time of measurement, C(ti) is
the concentration of nutrient species at time ti, VT (ti ) is thetidal current predicted by VT (ti ) = u1 cos (w1 ti − a1 ) +u2 cos (w2 ti − a2 ), and V0 is the subtidal current. The valueof this index varies within a range of 0.2–0.5 (Table 1),
indicating that the uncertainty in the fluxes estimated fromthe snapshot current field is acceptable in our study area,
where the tidal currents are not strong enough to change thedirection of nutrient fluxes because of the subtidal currents.

C11020

Therefore, the calculated nutrient fluxes from the snapshot
current data along sections A and G can be considered to bequalitatively accurate and quantitatively acceptable with anuncertainty of 20%–50% in the values.
[30] At section A, the cross‐isobath velocity component in
the upper layer (shallower than 20 m) was positive on thethermohaline front onshore side (stations A3 and A4) andnegative on the front offshore side (Figure 8a), indicating
convergence of surface waters toward the thermohaline front
between stations A4 and A5 (Figure 2). In the lower layer,
the cross‐isobath velocity component was positive for boththe onshore side and offshore side of the front. Such a flow
structure suggests that the downwelled waters and material
attributed to surface water convergence at the front weretransported to the lower layer of the front offshore side,
forming a pathway for offshore transport of coastal material.
[31] The across‐isobath velocity at section G was either
positive or negative in the vertical direction, showing a
possibility of convergence in the entire water column(Figure 8b). Since no clear thermohaline front can be
identified along section G (Figure 2) and the across‐isobath
velocity is almost homogeneous in the vertical direction(Figure 8b), this convergence may be attributed to theonshore movement of the front.
[32] The along‐isobath velocity component is negative(i.e., southwestward) over almost the entire area of section A(Figure 8c). Its vertical variation is small, showing a strong
barotropic character. Horizontally, it is strong on the shallow
onshore side (∼0.2 m/s) and weak on the deep offshore side(∼0.05 m/s). The along‐isobath velocity at section G was
also generally negative, but positive and negative along‐isobath velocity components coexist in the upper layer of theoffshore side (Figure 8d).
[33] The southwestward coastal current appearing as anegative along‐isobath component at sections A and G(Figures 8c and 8d) is consistent with the South China Sea
coastal current that is dominated by the northeasterly monsoon [Su, 1998]. The relaxation or intensification of thenortheasterly wind naturally modifies the current’s strengthand location, which in turn results in the onshore or offshore
movement of the thermohaline front (Figure 3). The spatial
distribution of along‐isobath component at sections A and G(Figures 8c and 8d) suggests that this current is stronger atthe onshore side than at the offshore side.
[34] The direction of the nutrient fluxes depends onthe flow direction, while the magnitude of nutrient fluxes
depends not only on the flow speed but also on the nutrient
concentrations. As an example, the across‐ and along‐isobath nitrate fluxes at sections A and G are presented in
Figure 9. As expected from the nitrate concentrations(Figure 4) and the across‐ and along‐isobath flow components (Figure 8), the nitrate flux is large at the onshore sideof two sections and small at the offshore side, where thenitrate concentration is low. The nitrate fluxes are relatively
larger at section A.
[35] At section A, the positive across‐isobath fluxes at thefront onshore side and in the bottom layer of the front offshore side are significantly larger than the negative across‐isobath fluxes in the surface layer of the front offshore side
(Figure 9a). This suggests that the nitrate transported
oceanward in the bottom layer of the front offshore side

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONTTable 1. Vertically Integrated Flux Across and Along Isobaths at the Two Anchor Stations A3 and A6a
Vertically Integrated Flux Across Isobath

Station A3
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4
Station A6
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4

Vertically Integrated Flux Along Isobath

Flux Causedby the Subtidal
Current

Ratio of Flux Causedby the Tidal Current
to That Caused by the
Subtidal Current

Flux Causedby the Subtidal
Current

Ratio of Flux Causedby the Tidal Current
to That Caused by the
Subtidal Current−1.510−0.194−0.064−2.503−6.197−0.099−0.105−2.214

0.293
0.318
0.515
0.295
0.292
0.387
0.263
0.389

−2.813−0.394−0.137−4.730−11.458−0.198−0.196−4.496

0.270
0.267
0.266
0.273
0.275
0.271
0.282
0.271

−2.622−0.475−0.333−3.428−15.587−0.213−0.160−4.013

0.464
0.520
0.464
0.464
0.484
0.497
0.444
0.523

−3.215−0.647−0.401−4.243−20.068−0.295−0.184−5.764

0.210
0.189
0.212
0.208
0.202
0.196
0.219
0.189

a
The method for calculating the fluxes and the ratio is described in section 4.2. The unit of velocity flux (i.e., volume transport
through a unit width) is m2 s−1, and the unit of other quantities is mM m2 s−1.

originates mainly from the front onshore side (i.e., fromcoastal waters).
[36] For a quantitative discussion on nutrient transports,
the across‐ and along‐isobath fluxes for all nutrients werethen integrated from bottom to surface at all stations along
two sections, and the results are presented in Tables 2 and 3.
The offshore transports of nutrients were larger at stations
A3 and A4 and decreased by 1 order at the offshore stations(Table 2). The offshore transports of nutrients could also befound at some stations along section G, but the magnitude oftransports were smaller than those near the thermohaline
front such as stations A3 and A4 by 1 order (Table 2).
[37] In general, the direction of vertically integrated fluxes
is consistent with that of the vertically integrated velocity,
indicating a vertically homogeneous distribution of nutrientsand current, just like on the shallow shelf in winter (Table 2).
However, this consistency does not hold at the thermohaline
front offshore side (stations A5 and A6) for the across‐isobath nitrate and nitrite fluxes (Table 2). The cause for thisinconsistency is apparent: the two nutrient concentrations
were high in the lower layer and extremely low in the upper
layer while the across‐isobath velocity was opposite in theupper and lower layers (Figures 8 and 9).
[38] As previous numerical studies suggested [Akitomo
et al., 1990], the surface water converges at a thermohaline front, sinks there, and diverges at the bottom layer. As
this idea is extended to material transport, the concentrationsof material on the onshore and offshore sides of the front
become important. In our study, the converging waters fromthe front offshore side contains few nutrients (Figure 4), andtherefore the nutrients inside the diverging waters at thebottom layer must come mainly from the inner shelf, where
the terrigenous input is the major source of nutrients. Consequently, the presence of the thermohaline front in thenorthern SCS in winter promotes the offshore transport of
terrigenous nutrients.

[39] The along‐isobath nutrient transports at all the stations along two sections were mainly southwestward, andtheir values were usually larger than the across‐isobath
nutrient transports by 1 order (Table 3). The Zhujiang riveris a major source of riverine nutrients to the northern shelf of
the SCS [Lee Chen and Chen, 2006; Zhang et al., 1999]. Our
survey area is located northeast of the Zhujiang river mouth.
It is possible for the Zhujiang diluted waters to affect oursurvey area in summer but not in winter, because the strong
southwestward coastal current in winter tends to push the
Zhujiang diluted waters southwestward. On the other hand,
nutrients from the East China Sea transported by southwestward coastal currents in winter through the Taiwan
Strait have recently been suggested to contribute to thenorthern shelf of the SCS, although the exact nutrient flux
through the Taiwan Strait in winter is still unknown [Chen,
2008].
[40] To understand the role of thermohaline front in
nutrient transports on the northern shelf of the SCS, we
compare the offshore fluxes across the thermohaline front in
the SCS with the nutrient fluxes from the Changjiang andZhujiang rivers. The Changjiang river is the largest sourceof terrigenous nutrients in the ECS while the Zhujiang riveris the largest one in the SCS.
[41] The wintertime nutrient fluxes from the Changjiang
river have been estimated to be 765 mol s−1 for nitrate,
23.9 mol s−1 for nitrite, 195 mol s−1 for ammonium,
6.0 mol s−1 for phosphate, and 1366 mol s−1 for dissolved
silicate, respectively [Zhang et al., 2007]. The wintertime
nutrient fluxes from the Zhujiang river into the SCS havebeen estimated to be 199.1 mol s−1 for nitrate, 7.1 mol s−1
for nitrite, 75.3 mol s−1 for ammonium, 2.3 mol s−1 forphosphate, and 363.4 mol s−1 for dissolved silicate,respectively [Liu et al., 2009].
[42] To estimate the offshore flux across the thermohaline
front, we need to know the length of the front. Wang et al.
[2001] reported it to be about 700 km from infrared satellite

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

C11020Figure 8. Across‐ and along‐isobath velocity (m s−1) components along sections (a and c) A and (b and d)
G in winter 2004. Positive values indicate offshore and northeastward velocities; negative values indicate
onshore and southwestward velocities. The red dots denote the depths of water samples, while the reddashed line indicates the deepest place between which and 3 m depth the ADCP data are available with avertical interval of 1 m. The velocities at depths shallower than 3 m and deeper than the red dashed line wereobtained by extrapolation.
image in wintertime. Referring to the values reported inTable 2 (e.g., those at station A11) and their 20%–50%
uncertainty, and assuming 500 km as a low limit of the front
length, it can be easily deduced that the seaward nutrient
fluxes across the front are 178 mol s−1 for nitrate, 60 mol s−1
for nitrite, 1145 mol s−1 for ammonium, 50 mol s−1 forphosphate, and 1518 mol s−1 for dissolved silicate, respectively. These estimates have a large uncertainty since themeandering of and the movement of the front can result in alarge spatial variability in offshore nutrient fluxes. Therefore, further field research using a higher‐resolution sampling
grid with more alongshore sections across the thermohaline
front is necessary to give a more accurate estimate of theacross‐shore transport. These offshore fluxes are larger than
the nutrient fluxes from the Zhujiang river and those from the
Changjiang river except for nitrate. Although station A11 isfar from the front, its offshore flux in the bottom layer
apparently depends on the presence of the front (Figure 9a).
[43] The presence period of the thermohaline front in thenorthern SCS shelf has been reported from December toMarch [Wang et al., 2001]. Such long presence time suggests that the offshore nutrient transports associated with thethermohaline front are important processes supporting highprimary production in offshore waters of SCS in winter. At
the same time, they are also important processes removing
the nutrients accumulated in coastal waters.

5. Summary[44] In the northern shelf of the SCS, the presence of athermohaline front between coastal waters and oceanic
waters was confirmed by a field observation in winter. High

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

C11020Figure 9. As in Figure 8 but for nitrate flux.

levels of dissolved inorganic nutrients were observed in the
coastal waters with low temperature and salinity, while lowlevels of these nutrients were in oceanic waters with hightemperature and salinity. Dissolved organic nutrients werepatchily distributed, indicating a stronger, controlled bybiological activity than by physical fields. There were highcorrelations (R > 0.76) among several major nutrient concentrations, while the mean molar ratios of DIN/PO3−
4 and
Si(OH)4/DIN were 19 and 0.91, respectively.
[45] The observations indicated that the presence of athermohaline front along the continental shelf in winterpromotes the offshore transport of nutrients from coastal
waters. The transport pathway is the convergence of surface
coastal waters to the front, downwelling at the front, andoceanward movement through the bottom layer of the front
offshore side. The nutrient fluxes transported across theentire front in the SCS are estimated to be larger than thosefrom the Zhujiang and the Changjiang rivers. This indicatesthe important role of thermohaline fronts in transporting

riverine nutrients from the shallow inner shelf to the deep
outer shelf in the SCS.
[46] There is an uncertainty of 0.2–0.5 in the nutrient flux
values since tidal currents were not removed from the currents data used for the calculation of nutrient fluxes. However, the conclusion that the presence of a thermohaline
front in the northern shelf of the SCS induces an offshoretransport of nutrients is acceptable because the uncertaintyof 0.2–0.5 does not change the flux direction. To obtain an
estimate on the magnitude of total nutrient transports acrossthe thermohaline front whose length was assumed as 500 km,
we assumed the same offshore nutrient transports occurring
along the entire front. This assumption has a large uncertainty since the meandering of and the movement of the front
can result in a large spatial variability in offshore nutrient
fluxes. Therefore, additional field research using the higher‐
resolution sampling grid with more alongshore sections
across the thermohaline front is necessary to give a moreaccurate estimate of the across‐shore transport. Moreover,

15 of 18

C11020

C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONTTable 2. Vertically Integrated Flux in the Across‐Isobath Direction at All the Stations Along Sections A and Ga
Station
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4

A1−3.035−10.864−3.575−7.790−9.550−0.805−0.215−21.369
A8−2.015−0.005−0.043−2.595−6.512−0.086−0.178−2.408
G6
1.429
0.383
0.150
2.617
7.915
0.115
0.108
2.205

A2
−1.678−4.730−1.576−4.157−5.075−0.355−0.090−9.892A9−1.422−0.250−0.115−1.820−4.902−0.078−0.161−2.385
G5
0.697
0.146
0.067
1.053
3.938
0.061
0.053
0.973

A3
2.040
5.009
1.714
4.741
6.520
0.365
0.074
10.950
A10
1.187
0.338
0.109
1.897
2.693
0.104
0.110
2.316
G4
0.473
0.037
0.025
0.410
2.680
0.041
0.030
0.492

A4
1.631
2.212
0.708
2.900
6.000
0.198
0.089
5.643
A11
1.140
0.355
0.120
2.290
1.965
0.100
0.065
3.036
G3
−1.531−0.104−0.046−1.748−8.057−0.118−0.093−1.585A5−1.3760.297
0.167−1.504−6.700−0.050−0.124−1.006G2−1.741−0.202−0.113−2.366−7.763−0.130−0.121−2.287

A6−0.2760.171
0.064−0.354−1.190−0.007−0.020
0.006
G8
0.883
0.352
0.127
1.757
6.556
0.059
0.060
1.650
G1
−2.193−0.271−0.101−2.800−10.940−0.136−0.194−3.383A7−0.984−0.166−0.024−1.288−3.791−0.063−0.096−1.433
G7
0.097
0.174
0.075
0.181
1.635
0.025
−0.0030.552
F9−1.678−0.237−0.053−2.134−9.459−0.090−0.181−3.225aPositive value means seaward. The unit of velocity flux (i.e., volume transport through a unit width) is m2 s−1, and the unit of other quantities ismM m2 s−1.

Table 3. Vertically Integrated Fluxes in the Along‐Isobath Direction at All the Stations Along Sections A and Ga
Station
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4
Velocity
NO−3
NO−2
NH+4
DON
PO3−
4
DOP
Si(OH)4

A1−6.588−23.604−7.763−17.008−20.821−1.749−0.444−46.399
A8−4.592−1.465−0.377−5.992−14.717−0.391−0.546−8.792
G6−0.918−0.302−0.130−1.749−4.989−0.081−0.076−1.535A2−6.540−18.814−6.279−16.124−19.863−1.406−0.356−39.151
A9−6.278−1.659−0.493−8.394−17.971−0.529−0.743−11.388
G5
0.040
0.023
0.011
−0.011
0.040
0.005
0.001
0.070

A3
−7.999−15.742−5.372−18.328−23.608−1.269−0.342−37.023
A10−3.367−0.830−0.288−5.138−7.940−0.283−0.303−6.216
G4
0.283
0.036
−0.003
0.214
1.688
0.025
−0.0060.275

A4−6.711−8.473−2.719−12.311−22.302−0.816−0.346−22.393
A11−1.060−0.160−0.100−1.948−2.009−0.059−0.080−1.641G3
0.200−0.051−0.020
0.022
1.065
0.009
0.010
−0.070A5−6.281−3.808−0.958−8.725−22.499−0.534−0.372−13.373G2−1.876−0.360−0.184−2.836−8.112−0.152−0.129−3.044A6−6.063−2.586−0.626−8.014−21.358−0.451−0.491−11.497G8−0.790−0.155−0.048−1.697−3.517−0.036−0.064−0.987
G1
0.939
0.095
0.032
1.140
4.589
0.054
0.086
1.316

A7
−6.953−1.872−0.427−8.876−25.498−0.476−0.728−12.301G7−2.233−0.526−0.179−4.367−11.905−0.139−0.167−3.171
F9
1.723
0.250
0.038
2.034
8.831
0.067
0.215
2.807

a
Positive value means northeastward. The unit of velocity flux (i.e., volume transport through a unit width) is m2 s−1, and the unit of other quantities ismM m2 s−1.

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C11020LIU ET AL.: NUTRIENTS IN THE THERMOHALINE FRONT

the combination of field data with a numerical model is also
necessary.
[47] Recently, the contribution of coastal nutrients to theprimary production of the entire ocean has been estimatedby a global model [Giraud et al., 2008]. Because of model
resolution limitations, many coastal physical processes
including the formation of a thermohaline front are notrepresented by the global model. A revised estimate istherefore necessary in the future, with a combination of a
global model and a regional high resolution model that canrepresent the formation of thermohaline fronts and othercoastal processes.
[48] Acknowledgments. This study was funded by NSFC and RGC(40218001, 40925017, and N‐HKUST609/02) and by the Ministry of
Science and Technology of P. R. China (2007DFB20380). Xinyu Guo
acknowledges the support from JSPS KAKENHI (21310012). We thankDr. Lei Wang for discussion; Bing Deng, J. C. Chen, H. Wei, L. X. Dong,and colleagues from the Ocean University of China for their help in field
and laboratory experiments; and Qingji Li for his contribution to Figure 3.
We thank the captain and crew of the R/V ‘Yan Ping 2‘ for their assistanceonboard. We also thank the Associate Editor and the four anonymous
reviewers for their constructive comments on improving the original
manuscript.

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C11020

University of China, 238 Songling Road, Qingdao, 266100, China.
(sumeiliu@ouc.edu.cn)
Q. Chen, Laboratory of Ocean Dynamic Processes and Satellite
Oceanography, Second Institute of Oceanography, State Oceanic
Administration, Xixihexia No. 9, Hangzhou 310012, China.
X. Guo, Center for Marine Environmental Studies, Ehime University,
2‐5 Bunkyo‐Cho, Matsuyama 790‐8577, Japan.

Y. F. Bi, J. B. Li, S. M. Liu, X. Luo, and J. Zhang, Key Laboratory ofMarine Chemistry Theory and Technology, Ministry of Education, Ocean

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