Environmental Pollution 289 (2021) 117915

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Environmental Pollution
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Budget of riverine nitrogen over the East China Sea shelf
Jing Zhang a, Xinyu Guo b, *, Liang Zhao aabCollege of Marine and Environmental Sciences, Tianjin University of Science and Technology, Tianjin, China
Center for Marine Environmental Studies, Ehime University, Matsuyama, JapanA R T I C L E I N F O

A B S T R A C T

Keywords:
Nitrogen budget
The Changjiang River
The East China Sea

Riverine nitrogen loading to the continental shelf sea is important for terrestrial–marine linkage and global
nitrogen cycling and leads to serious marine environmental problems. The budget and cycle of riverine nitrogenover the continental shelf in the East China Sea (ECS) are unknown. Using the tracking technique within aphysical–biological coupled model, we quantified the nitrogen budgets of riverine dissolved inorganic nitrogen(DIN) and particulate organic nitrogen (PON) over seasonal to annual scales in the ECS, especially from the
Changjiang River, which plays a dominant role in riverine nitrogen input. The horizontal distributions of the
Changjiang DIN and PON generally followed the Changjiang diluted water and coastal currents and were affectedby stratification in the vertical direction. Their inventory variations were dominated by biological fluxes andmodulated by physical ones, and changed most dramatically in the inner shelf among three subregions. Less thanhalf of DIN were converted to PON with most of the rest leaving the ECS through lateral transport pathways,
among which the flux through the Tsushima Strait was dominant. With the increasing loading of the Changjiang
DIN flux from the 1980s–2010s, lateral transports rather than PON production increased due to limited primary
production. Approximately 60 % of the produced PON exported to the sediment and 34 % went to the Tsushima
Strait. According to the export production, the DIN from the Changjiang River contributed 12–42 % to the ECS
carbon sequestration.

1. Introduction

largest nitrogen input among all the rivers in the ECS, the nitrogen fromthe Changjiang River is detectable throughout the shelf (Umezawa et al.,2014) and is responsible for environmental issues in coastal and adja­
cent areas (Gao and Song, 2005; Groβe et al., 2020; Zhang et al., 2007).
The nitrogen input from the Changjiang discharge has significantly
increased since the 1970s due to rapid urbanization, increased popula­tion, intensive industrialization, and large-scale chemical fertilizer use
in the Changjiang River Basin (Dai et al., 2011; Liu et al., 2015, 2018;
Wang, 2006). It has been reported that the increased nutrients from the
Changjiang River, especially the dissolved inorganic nitrogen (DIN),
have caused the rapid increase in the frequency and area of harmful
algal blooms (Li et al., 2014), as well as impacted the seafloor oxygen
demand and the intensified coastal bottom hypoxia area (Liu et al.,2015). Therefore, it is of great significance to clarify the riverine ni­
trogen budget and further evaluate its physical and biological processes
in the Changjiang Estuary and adjacent shelf areas.
Changjiang River diluted water (CDW) is the main factor impacting
the shallow estuary and adjacent areas, and it influences the northernareas in June and the southern areas in November (Wang et al., 2017).

Continental shelf seas are located between land and open ocean andplay an important role in global nitrogen cycling. Large rivers are vital
pathways in the terrestrial–marine nitrogen linkage. Riverine loading isa direct nitrogen source for the shelf seas and contributes to the primary
production and carbon sequestration there. Because of nitrogen con­sumption and its biogeochemical cycle in the shelf seas, its outflow flux
to the open ocean is not easily estimated from its riverine loading.
However, the increase in riverine nutrients has been suggested as a causeof frequent outbreaks of harmful algal blooms, eutrophication, andseasonal hypoxia in the shelf seas (Groβe et al., 2020; Howarth, 2008; Liet al., 2014; Lohrenz et al., 1997). Therefore, constructing riverine ni­
trogen budgets over the continental shelf is essential for improving ourunderstanding of global nitrogen recycling, demonstrating theland-derived effect on the marine ecosystem and carbon sequestrationand solving coastal environmental health problems.
The East China Sea (ECS) is characterized by a broad shelf withcomplex shelf hydrodynamics and high primary production. As the* Corresponding author. Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama, 790-8577, Japan.
E-mail address: guoxinyu@sci.ehime-u.ac.jp (X. Guo).
https://doi.org/10.1016/j.envpol.2021.117915
Received 26 January 2021; Received in revised form 14 July 2021; Accepted 4 August 2021
Available online 6 August 2021
0269-7491/© 2021 Elsevier Ltd. All rights reserved.

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Environmental Pollution 289 (2021) 117915

Accordingly, the nutrients from the Changjiang River are largely
restricted to the estuary, while the ecosystem outside the estuary ismainly supported by other nutrient sources (Liu et al., 2009). However,people also argue that the nutrients from the Changjiang River can betransported a long distance offshore, because they are not consumed
effectively by phytoplankton in the estuary due to the high turbidity(Isobe and Matsuno, 2008), which eventually affects the Tsushima Strait
in summer (Isobe et al., 2002; Morimoto et al., 2012) and results inautumn blooms in the broader regions of the Japan Sea (Shibano et al.,2019).
Although riverine dissolved nitrogen content has been studied, its
extent and transport mechanisms from the Changjiang River to the open
ocean and its fate within the shelf area remains unclear. The transportand budget of particulate nitrogen produced by the riverine dissolved
nitrogen over the ECS shelf are also unclear although they are needed forcompleting the overall understanding of nitrogen cycling and dynamics(Yu et al., 2012; Zuo et al., 2016). Consequently, we cannot even answer
the essential problem of how much and in which form the riverine ni­
trogen is exported to sediment and to the open ocean.
The nitrogen budget in the ECS has been estimated based on fieldobservations and numerical models (e.g., Liu et al., 2020; Wang et al.,2019; Zhang et al., 2007), but few studies have focused on the cycling
and budget of riverine nitrogen. Stable isotope measurements candistinguish riverine nitrogen (Umezawa et al., 2014; Yan et al., 2017;
Zhong et al., 2020), although the strong spatial and temporal variationsin the marine environment and nitrogen cycle increases the uncertaintyof those estimations. Using biogeochemical coupled models can provide
a precise nitrogen budget (Estrada-Allis et al., 2020; Fennel et al., 2006;
Wang et al., 2019; Xue et al., 2013; Zhang et al., 2019b) by considering
the nitrogen from all sources rather than a single riverine one. The
technique of tracing nitrogen from an external source in a simulated
ecosystem is an effective way to interrogate the problem (Kawamiya,
2001; Ménesguen et al., 2006) as it can track nutrients from a specificsource and consider both physical and biological processes. Therefore,
we provide such an analysis here.
In this study, we investigate transport and budget of riverine dis­solved nitrogen and its produced particulate nitrogen in the ECS using aphysical–biological coupled model with a tracking module. Further­more, we interpret the riverine nitrogen budget response over the ECS
according to the change in riverine nitrogen input. Finally, we evaluate
the role of riverine nitrogen in export production and carbon
sequestration.

Fig. 1. The model domain and bathymetry. The grey contour lines represent
the isobaths. The dots along the coast denote the positions of inflow from rivers
and the same group of rivers has the same color. The black thick line shows the
positions of the Taiwan Strait (TAS), Tsushima Strait (TUS), 34.7◦ N section(34N), and three designated isobaths sections: 50-m, 100-m, and 200-m iso­baths. Note that the Cheju Strait section is appended to the 100-m isobath forconvenience. With these sections, the ECS is divided into the inner shelf (0–50m), middle shelf (50–100 m), and outer shelf (100–200 m) subregions. (Forinterpretation of the references to color in this figure legend, the reader isreferred to the Web version of this article.)applied here and the target sources were changed to many rivers.
There are ten main rivers loading nutrients into the ECS in our study
area. We divided them into five groups according to location: (1) the
Changjiang (Yangtze River), (2) the Minjiang and Qiantangjiang rivers
in the southern ECS, (3) the Huanghe, Haihe, Luanhe, and Liaohe rivers
emptying into the Bohai Sea, (4) the Huaihe River in the southern Yel­low Sea, and (5) the Yalujiang and Hanjiang rivers in the northern
Yellow Sea. The nitrogen-related state variables, namely DIN, phyto­plankton, and detritus, with a label of five groups were tracked in themodel.
Notably, even though only the nitrogen from five river groups wastracked, the physical–biological model considered the input of all
external nutrient sources, including atmospheric deposition and thosefrom Taiwan Strait and Kuroshio. The biogeochemical processes in themodel are calculated based on the total nutrient concentration, and thenseparated to each source of DIN based on the ratio of one specific DIN
concentration to total DIN concentration. The tracking module isapplied to separate the riverine dissolved and particulate nitrogen
fraction.
In our default calculation, we set the concentrations of DIN, dis­solved inorganic phosphorus (DIP), and silicate in the Changjiang Riverto the 2010 levels, which are 110.2, 1.39, and 107.6 mmol m− 3,
respectively (Liang and Xian, 2018). The nutrient concentrations fromother rivers were taken from published data (Liu et al., 2009; Zhang,1996). At the beginning of the tracking module, the riverine DIN was
pumped into the ECS through the estuaries where its related state

2. Method
The model covers the region from 24.0 to 41.0◦ N and from 117.5 to
131.5◦ E with a resolution of 1/18◦ (5–6 km) and 21 σ layers (Fig. 1).
The 50-m, 100-m, and 200-m isobaths along with the Taiwan Strait
section (TAS), 34.7◦ N section (34N), and the Tsushima Strait section(TUS) separate the ECS into three subregions: the inner (0–50 m),
middle (50–100 m), and outer shelves (100–200 m), which are used in
the nitrogen budget evaluation.
The model consists of two parts. The first part is a physical–biologicalcoupled model based on Zhao and Guo (2011) and Wang et al. (2019),
which is used for calculating the cycles of all nitrogen sources. Biogeo­
chemical processes in the water column in this model include photo­
synthesis, respiration, and mortality of phytoplankton, andremineralization of detritus, while those in the benthic layer include
remineralization and denitrification. Wang et al. (2019) analyzed thenitrogen budget from all ECS sources based on the results from the same
physical–biological coupled model.
The second part is the tracking model following the method inMénesguen et al. (2006), which separately calculates the state variablesfrom different nitrogen sources. We applied it here to calculate the ni­
trogen budget from riverine source. The method in this study is the sameas that used in Zhang et al. (2019a), but only the DIN tracking case was2

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Environmental Pollution 289 (2021) 117915

variables have a value of zero. Then phytoplankton and detritus sup­ported by the riverine DIN are generated over the continental shelf.
Their sum is defined as particulate organic nitrogen (PON) here. It isnoted that the model does not consider the input flux of dissolved
organic nitrogen (DON) and PON from rivers because they only
comprise 18 % and 7 % of the total Changjiang River nitrogen input flux
(Kwon et al., 2018; Zhang et al., 2003).
Under the circumstances, the riverine DIN-tracking case was run forover 5 years until the state variables reached a stationary state and theresults in the final calculation year are used for our analysis. The annual
mean fluxes of DIN directly pumped into the study area from the
Changjiang River (Group 1), Minjiang and Qiantangjiang rivers (Group2), and Huaihe River (Group 4) are 3.22, 0.22, and 0.05 kmol s− 1,
respectively. The model results for the DIN from the Changjiang Riverare mainly presented in the following sections because its input flux
occupies 92 % (3.22/3.49) of all the riverine DIN sources. The DIN fromthe Changjiang River and its supported PON are denoted by DINC and
PONC.
The physical–biological coupled model without the tracking module
has been approved to be able to reproduce the general circulation andthe spatial-temporal variations in biological variables in the ECS (Wanget al., 2019; Zhang et al., 2019a; Zhao and Guo, 2011). Both thespatial-temporal variations of DIN and PON agree well with observa­
tions (Figs. 2–8 in Wang et al., 2019, Fig. 4 in Zhang et al., 2019a), andthe DIN fluxes from external sources are comparable to previous studies(Table 4 in Wang et al., 2019). These factors are critical for the reliabilityof the tracking module.

(Fig. 2e, h). The concentration DINC in the bottom layer in spring was
relatively lower than that in the surface layer (Fig. 2f). The largest dif­ference between the DINC concentration in surface and bottom layers
occurred in summer when stratification became strongest (Li et al.,2006; Quan et al., 2013). The bottom DINC concentration was relatively
high in the Cheju Strait in summer and autumn. The extension of
Changjiang Diluted Water transported high-concentration unexhausted
DINC nearshore to the Cheju Strait.
PONC generally had a lower concentration than DINC (Fig. 3). The
seasonal variation in the PONC in the surface layer was weak, with a highconcentration region lying in the same inner shelf. The surface PONC insummer was mainly confined to the coastal area rather than heading to
the Tsushima Strait like the DINC. The PONC concentration was higher inthe bottom layer than in the surface layer and showed a slightly strong
seasonal variation with a maximum value appearing in summer. Like
DINC, the PONC concentration in the bottom and surface layers showed
the greatest difference in summer.
3.2. Seasonal variations in DINC and PONC fluxes
The inventories of DINC and PONC over a region are controlled byboth physical and biological processes in the model (Wang et al., 2019).
The DINC and PONC are related through biological processes including
photosynthesis, respiration, and remineralization. A negative value ofall these biological fluxes of PONC indicates that their net effect is atransformation of PONC to DINC. Naturally, their biological fluxes
showed an opposite variation. The physical processes for DINC and PONC
consist of river loading (only for DINC), water–sediment flux at the sea
bottom, and horizontal fluxes through the lateral boundaries of the re­gion. The water–sediment flux for DINC resulted from the regenerated
DIN from sediments, and that for PONC was a sum of sinking andresuspension of PON. Time variation terms in DINC and PONC in­ventories are shown in Fig. 4, with the aforementioned physical andbiological fluxes. The positive (negative) value of flux induces the in­crease (decrease) in inventory. The seasonal variations in DINC and
PONC fluxes and the connections among the three subregions are given
below.
The DIN flux loading from the Changjiang River (Riv) peaked insummer as a result of large river discharges in the rainy season (Fig. 4a).
Among all the fluxes in the inner shelf (Fig. 4a), Riv was the largest withan annual value of 3.22 kmol s− 1 (= 142.3 × 104 t yr− 1), which wascomparable to observed values (135.3 × 104 t yr− 1, Guo et al., 2015).
The seasonal variation in DINC water–sediment flux (Bot) was weak, anda slightly higher value appeared in September corresponding to the large
Bot PONC flux. The biological flux (Bio) of DINC was the largest negativeflux in the inner shelf with an annual average of 1.27 kmol s− 1. Here, thenegative value denotes a DINC sink. The consumption of DINC domi­
nated all the biological processes from March to November while

3. Results and discussion
3.1. Horizontal distributions of DINC and PONC
The surface distributions of DINC concentration are shown in
Fig. 2a–d. The DINC concentration was extremely high in the inner shelfin winter, more than 50 mmol m− 3. The DINC headed southward
following the Min-Zhe Coastal Current south of the Changjiang Estuary(Chen, 2008; Wu et al., 2013). In the spring, the DINC north of the
Changjiang Estuary expanded to the northeast while the DINC south ofthe estuary decreased. The CDW brought the DINC to the Tsushima Strait
in the summer (Chang and Isobe, 2003; Isobe and Matsuno, 2008).
Unlike DIP, the DINC could not be fully assimilated locally and wastransported to the Tsushima Strait (Zhang et al., 2019a). Meanwhile, the
DINC concentration from the southern coast further decreased. In
autumn, the northeastward DINC retreated and turned southward along
the coast (Bi et al., 2018). In general, the surface DINC seasonal patterns
followed the CDW and coastal current distributions.
In winter and autumn, the patterns of DINC in the bottom layer weresimilar to those in the surface layer due to the strong vertical mixing

Fig. 2. Seasonal distributions of DIN concentration from the Changjiang River(DINC) at the (a–d) surface layer and (e–h) bottom layer (unit: mmol m− 3). The
concentration outside the shelf area is not shown.

Fig. 3. Seasonal distributions of PON concentration from the Changjiang River(PONC) at the (a–d) surface layer and (e–h) bottom layer (unit: mmol m− 3). The
concentration outside the shelf area is not shown.
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Environmental Pollution 289 (2021) 117915

Fig. 4. The inventory-related fluxes of (a–c) DINC and(d–f) PONC over the inner, middle, and outer shelvesof the ECS. The time variation term of the DINC or
PONC inventory is denoted by Time. The biologicalflux is denoted by Bio. The flux across the water­–sediment interface is denoted by Bot. The fluxes
across lateral sections are indicated by 200m, 100m,
50m, TUS, TAS, and 34N (see Fig. 1 for the position ofeach section). The values in the legend are annual
means of the corresponding fluxes. The positive(negative) value of flux induces the increase(decrease) in inventory.

regeneration occurred in the remaining time. The other large negativeflux of DINC in the inner shelf was through lateral transport across the
50-m isobath section, which had two peak times, one in July due to the
CDW, and the other in winter due to southward coastal currents. The
remaining few fluxes of DINC in the inner shelf headed to the Yellow Sea
through the 34N section. Overall, the inventory of DINC in the innershelf decreased from March to June because of Bio flux and thenincreased with Riv loading.
One difference of the DINC Time term in the middle shelf from that in
the inner shelf was an extra negative peak in summer (Fig. 4b), resulting
from the largest output flux across the 100-m isobath section due to thelarge across-isobath volume transport there (Zhang et al., 2017).
Another exit for the DINC in the middle shelf was the Taiwan Strait(TAS); however, this route was only possible in winter because of thestrong northeasterly winds (Oey et al., 2014). The Bio flux in the middle
shelf shared a similar seasonal variation with that in the inner shelfalthough its magnitude was smaller than the two across-isobath fluxes(50-m and 100-m). The increase in the DINC in the middle shelf was
attributed to the lateral flux across the 50-m isobath from the inner shelfas the other positive flux was the release of DIN from the sediment,
which was substantially smaller than that in the inner shelf.
The tendency of DINC in the outer shelf (Fig. 4c) was unlike that in
the inner and middle shelves (Fig. 4a and b). The DINC increased inspring and summer but decreased in autumn and winter, indicating thesmall effect of the biological fluxes. All the fluxes were weak in winter.
As the temperature rose, the biological consumption of DINC increased
and reached the maximum in summer rather than in spring. The fluxes
across the 100-m isobath and through the Tsushima Strait (TUS) werecomparable and shared similar seasonal patterns, peaking in August.
Only a small portion of DINC left the shelf across the 200-m isobath.
The PONC fluxes are shown in the lower panel in Fig. 4. In contrast to
the DINC, the biological processes increased the PONC rapidly over the
inner shelf in spring (Fig. 4d). Subsequently, the loss of PONC to thesediment (the combined effect of sinking and resuspension) was stron­
gest in August and September, and was the main export pathway of
PONC in the inner shelf. The lateral fluxes of PONC were significantly
smaller than the water–sediment and biological fluxes. The PONC flux
across the 50-m isobath showed a positive value in summer, rather thana CDW-related negative value. Previous study has shown that the vol­ume transport across the 50-m isobath in summer was in the onshore
direction in the bottom layer (Zhang et al., 2017) where the concen­
tration of PONC was higher than that in the surface layer (Fig. 3g).
The Bio flux was still the largest one among the PONC fluxes in the
middle shelf (Fig. 4e). The strongest offshore PONC flux across the 100m isobath section followed the Bio peak and occurred in June, twomonths earlier than the DINC lateral flux through the same section.
Different peak times of PONC and DINC fluxes are caused by theirdifferent concentrations. The PONC in the middle shelf are produced in
the largest quantities in April, which contributes to its increasing
offshore transport across 100-m isobath to outer shelf in May and June.
When it comes to summer (e.g. August), the DINC fluxes across 50-m
isobath from the inner to the middle shelf peaks following the Chang­
jiang input and the consumption of DINC in the middle shelf weakens.
Their combined effect results in the highest fluxes of DINC across 100-m
isobaths in August.
The time variation term of PONC in the outer shelf was similar to thatof DINC (Fig. 4f), but peaked in May, also two months earlier than the
DINC. The fluxes of PONC and DINC across 100-m isobath contributemost to these two peaks. The positive fluxes, inflow across 100-m iso­bath, and biological flux, were primarily balanced by the flux throughthe Tsushima Strait.
The time variation in DINC over the whole shelf followed the patternin the inner shelf because the biological and physical fluxes of DINC
there were largest, as did the PONC. The time variations of inventories of
DINC and PONC were affected by both biological and physical processes.
3.3. Annual budgets of DINC and PONC
Fig. 5 shows the DINC and PONC budgets for the ECS continental
shelf. The sum of DINC and PONC is referred to as total nitrogen (TNC)
here. The nitrogen budget is given in the three subregions. In the innershelf, the DINC sources are the Changjiang River input (3.22 kmol s− 1)
and the bottom sediment release (0.44 kmol s− 1). Approximately 35 %of the DINC input is converted to PONC through biological processes(1.27 kmol s− 1). Most unexploited DINC leaves the inner shelf to the
Yellow Sea via the 34N section and to the middle shelf across the 50-m
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Environmental Pollution 289 (2021) 117915

important role of export through lateral transport. The PONC contributes
25 % and 28 % of TNC fluxes across 200-m isobath and the TUS section,
respectively. The outflow of DINC through the TUS section (1.00 kmols− 1) is two orders of magnitude larger than that across the 200-m isobath
section (0.03 kmol s− 1).
In general, lateral DINC transport plays a more active role in the DINC
budget. Only a small proportion of DINC transforms to PONC over threesubregions, among which the amount and conversion efficiency arelargest in the inner shelf. The water–sediment flux of PONC also peaks in
the inner shelf and sharply decreases in the middle and outer shelves. By
contrast, the lateral transports of PONC in the middle and outer shelves
increase.
In this estimation, the nitrogen budgets from the five groups show
different patterns according to their different locations (Table .1). The
input from the Changjiang River is the only source of DIN while its
transformation to PONC and lateral fluxes are the sinks. The lateral
fluxes of DINC are an essential way of acting as a sink, especially throughthe Tsushima Strait. Large amounts of produced PONC export to theseabed, while some go to the Tsushima Strait. The Minjiang and Qian­
tangjiang rivers in Group 2 lie south to the Changjiang River and share asimilar nitrogen budget. The DIN and PON from Group 2 are mainlytransported to the TUS and TAS sections. The Huaihe River (Group 4) isto the north of the Changjiang River and near the 34N section. Half of
the input DIN from the Huaihe River is converted to PON, and nearlyanother half goes to the Yellow Sea through the 34N section, along withmost generated PON in Group 4. The rivers of Groups 3 and 5 locatedoutside of the ECS do not pump the nitrogen directly into the ECS but
deliver it through the 34N section. After entering the ECS, the DINs fromGroups 3 and 5 are assimilated or transported to the TUS section, theinput and generated PON have two exits: the sediment and the TUS
section. Among the riverine nitrogen budgets of the five groups, the total
biological and physical fluxes of DINC (− 1.84 and 1.93 kmol N s− 1) and
PONC (1.84 and − 1.81 kmol N s− 1) account for approximately 90 % of
those of all rivers (around 2.00 kmol N s− 1), indicating the dominantrole of the Changjiang River in the riverine nitrogen budget.

Fig. 5. Annual budgets of DINC and PONC over the inner, middle, and outer
shelves of the ECS. The straight blue (red) arrows with numbers denote theannual mean flux of DINC (PONC). The value in each black box denotes the fluxof total nitrogen (TNC). The values inside the three circles denote the trans­
formation flux between DINC and PONC. All values are in kmol N s− 1. (Forinterpretation of the references to color in this figure legend, the reader isreferred to the Web version of this article.)

isobath section, which accounts for over 50 % of the DINC input (1.85kmol s− 1). The PONC fluxes into the Yellow Sea and the middle shelf aresmall and comprise only 14 % (= 0.18/1.27) of the generated PONC.
Most PONC (83 %) is deposited on the sea bottom with an annual mean
flux of 1.05 kmol s− 1; meanwhile, the DINC returns to the water column
with a flux of 0.44 kmol s− 1. Consequently, the TNC is lost to the sedi­ment with a flux of 0.61 (1.05–0.44) kmol s− 1, which is the largest value
among the three subregions. The TNC budget in the inner shelf shows an
input flux of 3.22 kmol s− 1 from the Changjiang River and a total of 3.10kmol s− 1 of output flux that consists of 0.55 kmol s− 1 (0.46 DINC+0.09
PONC) through 34N, 1.94 kmol s− 1 (1.85 DINC +0.09 PONC) across the
50-m isobath and 0.61 kmol s− 1 (1.05 PONC -0.44 DINC) across thewater-sediment interface. The difference between the input flux andtotal output flux is 0.12 kmol s− 1, which equals to the sum of the timevariation terms of DINC (0.08 kmol s− 1) and PONC (0.04 kmol s− 1) in the
inner shelf.
The middle shelf receives TNC from the inner shelf across the 50-m
isobath section, of which DINC and PONC account for 95 % (1.85kmol s− 1) and 5 % (0.09 kmol s− 1), respectively. Adding the bottom
DINC flux (0.09 kmol s− 1), the input flux of DINC reaches 1.94 kmol s− 1in the middle shelf. The generated rate of PONC (0.28 kmol s− 1) repre­
sents 14 % of the input flux of DINC, lower than the transformation ef­
ficiency in the inner shelf (35 %). The ratio (46 % = 0.13/0.28) of PONC
water–sediment flux to the generated PONC in the water column is alsoless than that in the inner shelf (83 %) due to greater water depth. The
aphotic layer is thicker in the middle shelf than in the inner shelf. With aconstant sinking rate, the detritus has more time to decompose in the
middle shelf than in the inner shelf before reaching the sea bottom.
Consequently, fewer PONC reaches the sea bottom of the middle shelfthan that of the inner shelf. The TNC in the middle shelf exports to theouter shelf across the 100-m isobath section, to the South China Sea
across the TAS section and to the sea bottom through water–sedimentinterface. The sea bottom has the lowest export flux with an average of0.04 kmol s− 1 (0.13–0.09), while the former two are more important.
The ratios of PONC to TNC in lateral transport increase from 5 % at the
50-m isobath section to 13–14 % at the 100-m isobath and TAS sections.
The inflow of DINC (1.33 kmol s− 1) in the outer shelf comes from the
middle shelf (1.29 kmol s− 1) and the bottom sediment (0.04 kmol s− 1),
22 % of which is converted to PONC. The generated PONC with the
inflow from the middle shelf (0.19 kmol s− 1) contributes to 0.48 kmols− 1 input flux of PONC in the outer shelf. Only one-sixth of input PONC is
exported to the bottom sediment (0.08 kmol s− 1), indicating the

3.4. DINC and PONC budget comparison between the 1980s and 2010s
The nitrogen concentration in the Changjiang River water has
dramatically increased since the 1970s (Liu et al., 2015). We designed asensitivity experiment in which we changed only the DIN concentrationin Changjiang River from 110.2 mmol m− 3 to 33.5 mmol m− 3, which
was an observed value in the 1980s (Zhang, 1996). Following thischange, the annual Changjiang DINC loading flux reduced to 0.98 kmols− 1. By comparing the DINC and PONC budget in the 1980s and 2010s,
we aim to recall the alterations in the nitrogen cycle and the corre­
sponding environmental effect.
The input flux of DINC was three times larger in the 2010s than in the
1980s (Table 2). Compared to the 1980s, the generated PONC in the2010s increased by 219 %, 298 %, and 322 % over three shelf areas. The
production efficiency of DINC in coastal areas was relatively low (Zhang
et al., 2019a) because of the relatively low concentration of phosphorus.
Increasing of nitrogen to phospate ratio in the Changjiang River has
resulted in an unbalanced ratio between the nitrate concentration andphosphate in coastal water needed by the phytoplankton. The phos­
phorus deficiency occurred initially in the area directly impacted by the
Changjiang River plume (Moon et al., 2020).
With less PONC increase being generated, the export of the residual
DINC by lateral transport notably increased by over 300 % across the
34N and TAS sections, and by more than 400 % across the TUS section,and 50-m, 100-m, and 200-m isobaths. The outflow of DINC and increase
rate across the TUS section were the largest. As a result, more nutrientswere transported to the continental shelf or open ocean instead of being
converted to PONC locally, thus increasing the environmental pressure
there (Liu et al., 2019).
The increasing water–sediment flux rates of PONC to the sea floor
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Table 1
Annual mean of riverine DIN and PON budgets in the ECS. The Phy term is the sum of physical fluxes.
DIN budget

Group 1

Group 2

Group 3

Group 4

Group 5Sum

− 1(kmol N s )Changjiang

Minjiang & QiantangjiangRivers into the Bohai Sea

HuaiheYalujiang & Hanjiang

All rivers

Bio
Bot
200m
TUS
34N
TAS
river
Phy
time-change− 1.84
5.71E-01− 3.39E-02− 9.98E-01− 4.58E-01− 3.67E-01
3.22
1.93
9.41E-02− 1.09E-01
3.96E-02− 1.15E-02− 5.56E-02− 5.84E-03− 7.54E-02
2.18E-01
1.09E-01
3.31E-04− 3.50E-04
1.14E-04− 2.28E-09− 2.40E-04
1.53E-03
0.00
0.00
1.40E-03
1.05E-03− 2.64E-02
3.77E-03− 6.80E-08− 2.30E-03− 2.55E-02− 3.97E-07
5.25E-02
2.85E-02
2.13E-03− 2.25E-02
9.87E-03− 1.14E-07− 1.41E-02
3.88E-02
0.00
0.00
3.46E-02
1.21E-02− 2.006.24E-01− 4.53E-02− 1.07− 4.49E-01− 4.42E-01
3.49
2.11
1.10E-01

PON budget
Bio
Bot
200m
TUS
34N
TAS
Phy
time-change

1.84− 1.26− 7.73E-03− 3.94E-01− 8.92E-02− 5.64E-02− 1.81
3.45E-02

1.09E-01− 6.81E-02− 1.93E-03− 2.84E-02− 1.37E-03− 1.56E-02− 1.15E-01− 6.31E-03

3.50E-04− 2.53E-04− 1.11E-09− 1.76E-04
2.80E-04
0.00− 1.49E-04
2.01E-04

2.64E-02− 9.27E-03− 2.32E-08− 9.80E-04− 1.76E-02− 8.12E-08− 2.79E-02− 1.46E-03

2.25E-02− 1.98E-02− 1.16E-07− 1.21E-02
1.21E-02
0.00− 1.98E-02
2.74E-03

2.00− 1.36− 9.66E-03− 4.35E-01− 9.58E-02− 7.20E-02− 1.97
2.96E-02

the DINC loading into the ECS can be assimilated due to limiting factors
like phosphorus and light. An overestimation is inevitable if the new
production in an area is based on the input flux of DINC (Chen et al.,1999).
Export production is the amount of carbon fixed by photic zone au­
totrophs, and either sinks to deeper water and sediments or is trans­
ported to open oceans. In a steady state, export production balances new
production. In the open ocean, organic carbon exported below the sea­sonal thermocline is isolated from the upper ocean for over one hundredyears. However, in the continental shelf, the carbon and nutrients re­generated below the euphotic layer can easily return to the euphotic
layer and are reused by phytoplankton (Chen, 2003). Thus, the carbon
export to sediment and open ocean rather than below the euphotic layer
in the continental shelf plays an important role in long-term carbon
sequestration (Simpson and Sharples, 2012; Stukel et al., 2015).
Considering the stable ratio of POC to PON (close to the Redfieldratio 6.63) in the ECS (Hung et al., 2000; Zhu et al., 2006), the export
production due to DINC can be referred to as the sum of PONC fluxes toopen ocean and sediment. Treating the ECS shelf area as a whole, theexports of PONC are water–sediment flux (0.69 kmol s− 1), lateral flux to
the Japan Sea through the TUS section (0.39 kmol s− 1), lateral flux to theSouth China Sea through the TAS section (0.06 kmol s− 1), lateral flux
across the 200-m isobath (0.01 kmol s− 1), and lateral flux to the Yellow
Sea through the 34N section (0.09 kmol s− 1). Except for the last flux,
they are all export production induced by the DINC in the ECS, adding up
to 1.15 kmol N s− 1, among which the water–sediment flux and theexport to the Japan Sea account for 60 % and 34 %, respectively.
In our model calculation, the gross primary production supported by
DINC over the whole shelf is 12.71 kmol s− 1. The respiration of phyto­plankton and remineralization of detritus are 7.42 and 3.45 kmol s− 1,
respectively. Their sum (10.87 kmol s− 1) is the recycling flux of DINC,
accounting for 85 % of the gross primary production. This percentage issimilar to the estimation derived from the observation of the total ni­
trogen over the whole shelf (Zuo et al., 2016).
The ratio of export production to primary production (referred to as
the e-ratio) related to the Changjiang River nitrogen is 0.09 (= 1.15/
12.71), suggesting a handful of the primary production is exported. The
e-ratio should be the same as the ratio of new production to primary
production (referred to as the f-ratio). The f-ratio calculated from therelative concentration of nitrate is approximately 0.4 (Chen et al., 2001,
1999; Liu and Chai, 2009; Jiao et al., 1998). Hung et al. (2016) gave an
e-ratio of 0.59 nearshore and 0.16 offshore based on trap-collected POC
fluxes. Chen and Wang (1999) estimated an f-ratio of 0.15 based on theflux of new nutrients in a box model. Our e-ratio derived from DINC isTable 2
Nitrogen budget comparison between the 2010s and 1980s. The DINC and PONC
fluxes lower than 1 are expressed in scientific notation. The values have units ofkmol s− 1.
River input
Bio
DIN
Bot
200m
100m
50m
TUS
34N
TAS
PON
Bot
200m
100m
50m
TUS
34N
TAS

2010s

1980s

2010s/1980s

3.22
1.27
2.83E-01
2.91E-01

9.81E-01
5.81E-01
9.51E-02
9.04E-02

328 %
219 %
298 %
322 %

inner
middleouter

4.41E-01
8.73E-02
4.25E-02
3.39E-02
1.29
1.85
9.98E-01
4.58E-01
3.67E-01

2.11E-01
3.16E-02
1.37E-02
7.76E-03
3.00E-01
4.54E-01
2.14E-01
1.35E-01
9.78E-02

209 %
276 %
310 %
437 %
430 %
407 %
466 %
339 %
375 %

inner
middleouter

1.05
1.31E-01
7.93E-02
7.73E-03
1.88E-01
9.38E-02
3.94E-01
8.92E-02
5.64E-02

4.79E-01
4.59E-02
2.56E-02
2.19E-03
6.61E-02
3.89E-02
1.27E-01
3.71E-02
2.21E-02

219 %
285 %
310 %
353 %
284 %
241 %
310 %
240 %
255 %

inner
middleouterwere related to the generated PONC over the three shelves. As thegenerated PONC increased, the water–sediment fluxes of PONC increased
similarly by 219 %, 285 %, and 310 % over the three shelves. The
increasing rates of lateral transports of PONC were approximately 240 %at the 50-m isobath and 34N section while increasing substantially toover 300 % at the 200-m isobath and TUS section. The higher increases
in lateral transports in the outer shelf correspond to the highest increasein generated PONC.
3.5. New and export productions supported by DINC
Primary production can be divided into new production and regen­erated production based on the nitrogen source (Dugdale and Goering,1967). New production is related to nitrogen from outside the photic
zone, which is also called the new nitrogen. Nitrogen from the rivers,
atmospheric deposition, and open ocean are all external sources for new
production over the ECS continental shelf. As discussed above, not all6

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Environmental Pollution 289 (2021) 117915

lower than the above studies for two reasons: the calculation method isdifferent, and the DIN from the Changjiang River is not assimilated
effectively, indicating a lower e-ratio than other external nitrogensources.
As a carbon sink, the ECS absorbs approximately 6.92–23.30 Tg C
yr− 1 of atmospheric carbon dioxide (Jiao et al., 2018). The export pro­duction estimated here, 1.15 kmol N s− 1, is converted to POC fluxes of
2.89 Tg C yr− 1, accounting for 12–42 % of the carbon sequestration in
the ECS. This percentage is comparable to an estimation of Chen (2000),
33 %).

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4. Conclusions
Using the physical–biological coupled model and a tracking module,
we examined the dissolved and particulate nitrogen budgets over the
shelf area of the ECS, along with the roles of riverine nitrogen in export
production. The horizontal distributions of DINC and PONC generally
follow the CDW and coastal currents and are affected by stratification in
the vertical direction. The surface and bottom distributions of DINC are
similar except for in summer, while the concentration of PONC is larger
in the bottom layer than in the surface layer year round.
The inventories of DINC and PONC are affected by both biological andphysical processes. Both of them changed dramatically in the inner shelf.
Their time variations are dominated by biological fluxes and modulatedby physical ones. The lateral transports of DINC are important exports inthe DINC budget, among which that through the Tsushima Strait is thelargest. With the increasing loading of DINC from the 1980s to the 2010s,
the PONC does not increase proportionally due to limited primary pro­duction. Consequently, residual DINC is transported further from the
ECS, thereby increasing the environmental stress in the adjacent areas.
Most of the produced PONC exports to the sediment, while somemoves to the Tsushima Strait. According to the PONC budget, its totalexport production is 1.15 kmol N s− 1, among which the flux to thesediment accounts for 60 % and the flux through the Tsushima Strait
occupies 34 %. Based on the export production, it is estimated that DINC
contributes approximately 12–42 % to the carbon sequestration of the
ECS. The export production only occupies 0.09 of the total produced
PONC, suggesting a very low export efficiency.
Author statement
Jing Zhang: Software Visualization Writing - Original Draft. Xinyu
Guo: Conceptualization Writing - Review & Editing. Liang Zhao: Meth­odology Writing - Review & Editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
We thank two anonymous reviewers for their valuable comments.
This work was supported by the National Key Research and Develop­
ment Program of China (2016YFC1401602), National Natural Science
Foundation of China (42006018, 41876018), and Grants-in-Aid for
Scientific Research (MEXT KAKENHI grant numbers: 20H04319). L.
Zhao’s participation of this study was supported by the Tianjin Natural
Science Foundation (19JCZDJC40600). J. Zhang’s participation of thisstudy was supported by the Ministry of Education, Culture, Sports,
Science and Technology, Japan (MEXT) under a Joint Usage/ResearchCenter, Leading Academia in Marine and Environment Pollution
Research (LaMer) Project and the Tianjin Municipal Education Com­mission Scientific Research Project (No. 2019KJ219).
7

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