Response of Phosphorus near Reservoirs in South Branch of the

Yangtze Estuary to the Upstream Runoff and Sediment Load

Variations

Xian Zhu

1,2

, Zhenshan Xu

1,2,*

, Ao Chu

1,2

, Hongwei Ding

3

, Min Gan

1,2

and Yongping Chen

1,2

1

State Key Laboratory of Hydrology-Water Resource and Hydraulic Engineering, Hohai University, China

2

College of Harbour, Coastal and Offshore Engineering, Hohai University, China

3

Nanjing Hydraulic Research Institute, China

Keywords: Phosphorus, The Yangtze estuary, Water quality, Numerical model

Abstract: The water quality of reservoirs in the south branch of the Yangtze estuary is strongly influenced by the

phosphorus loadings. The variations of runoff and sediment loads from upstream cause the temporal

variations of phosphorus loadings here, which poses potential risks to the reservoirs. The changes of runoff

and sediment loads from upstream in the future are uncertain because of human activities and global climate

change. An integrated water quality model was established to study the possible changes of phosphorus

loadings in the south branch of the Yangtze estuary in the future in response to variations of runoff and

sediment loads from upstream. The model contains hydrodynamic, sediment dynamics, and some

biochemical processes. The model is well-calibrated and verified based on the related data in 2018.

Different sets of runoff and sediment loads from upstream were then applied in this model to learn the

changes of concentration of total phosphorus (TP), phosphate (PO

4

3-

), and particle phosphorus (PP). Results

show that the TP concentration has no significant change with different conditions of input of runoff and

sediment loads. However, the PO

4

3-

and PP concentrations are significantly influenced by both the changes

of runoff and sediment loads. The annual mean value of PO

4

3-

concentration would increase by 10-25% if

the input of runoff increases by 20% or the sediment loads decrease by 20%. It would decrease by 8-20% if

the input of runoff decreases by 20% or the sediment loads increase by 20%. The annual mean value of PP

concentration would increase by 12-35% if the input of runoff decreases by 20% or the sediment loads

increase by 20%. It would decrease by around 15-30% if the input of runoff increases by 20% or the

sediment loads decrease by 20%. The changes of the PO

4

3-

and PP are caused by the convection and

diffusion of the flow, and the adsorption and desorption of sediments. Notwithstanding, these changes

would reconstitute the component of TP of reservoirs in the south branch of the Yangtze estuary. Although

the PP still takes the main proportion, the proportion of PO

4

3-

would increase significantly and the growth of

plankton would be promoted which may cause a high risk of eutrophication.

1 INTRODUCTION

Phosphorus (P) is one of the most important nutrient

elements determining ecosystem production

(Sanudo-Wilhelmy et al., 2001; Babu & Nath,

2005). However, excessed P loadings in water would

bring about eutrophication following with serious

water quality problems (Hilton et al., 2006). In the

past decades, anthropogenic inputs of P have

increased dramatically in many mega-estuaries, such

as the Yangtze, the Mississippi, and the Nile

(Goolsby et al., 1999; Ludwig et al., 2009; Xu et al.,

2013). The estuarine areas are always densely

populated and economically developed. Thus the

losses caused by eutrophication here are always

huge.

The Yangtze estuary is one of the most urbanized

coastal regions of China. The estuarine environment

has significantly deteriorated in recent decades

(Wang, 2006). The frequency of occurrence of algal

blooms has increased with each decade since 1970.

From 2000 to 2010 over 100 harmful algal blooms

were occurring (Liu et al., 2013). To solve this

problem, the government had made some policies to

reduce the P loadings from fertilizer, sewage

(domestic and industrial), and manure from livestock

Zhu, X., Xu, Z., Chu, A., Ding, H., Gan, M. and Chen, Y.

Response of Phosphorus Near Reservoirs in South Branch of the Yangtze Estuary to the Upstream Runoff and Sediment Load Variations.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 179-188

ISBN: 978-989-758-560-9; ISSN: 1755-1315

Copyright

c

179

in the upstream river basin. The implementation of

these policies has achieved some progress. However,

because of human activities and global climate

change, the input of flow and sediment loadings

from upstream to the Yangtze estuary changes a lot,

which causes the adjustment of P loadings in the

estuary (Tang et al., 2020). The weather change

would change the rainfall in the Yangtze River

basin, which further changes the input of flow to the

estuary. The flow condition controls the diffusion

process, which may change the spatial distribution

of P. After the Three Gorges dam started to work in

2003, the sediment loadings to the estuary show a

sharp decrease (Chen et al., 2010; Ren et al., 2021).

In 2000-2010, the suspended sediment loadings in

the estuary decreased about 20-30% (Li, 2012). The

adsorption and desorption with suspended sediment

are considered as one of the main processes

controlling the P circulation in estuarine areas

(Froelich, 1988; Shen et al., 2008; Xu et al., 2015)

since the P has a high affinity with fine sediment

(Stone & English, 1993; Winterwerp & van

Kesteren, 2004). Thus, the sediment input condition

would change the loadings and constitution of P in

water. For preventing and solving new potential

water quality problems in reservoirs in the Yangtze

estuary, it is necessary to study the P responses to

variations of runoff and sediment loads from

upstream.

Numerical modeling is an effective approach to

study this problem. In the Yangtze estuary, several

researchers have used numerical models to study

water quality problems. However, water quality

processes in these models are mainly based on a

single convection-diffusion equation, which is not

adequate for much more complex estuary systems.

Although some new studies have included some

biochemical processes in their models (Wang et al.,

2016; Zhu et al., 2016; Ge et al., 2020), their results

can only reflect the current situation rather than

study the question imposed in this study.

To study the response of P near reservoirs in the

south branch in the Yangtze estuary to the variations

of runoff and sediment loads from upstream, an

integrated water quality model for the Yangtze

estuary is developed. The model includes the

sediment dynamic processes, the mineralization, and

the adsorption and desorption of sediment to achieve

the following objectives: (1) show the temporal and

spatial distribution of P in the south branch in 2018;

(2) study the changes of P loadings in the south

branch with the change of flow and sediment input;

(3) study the changes of the constitution of P in the

south branch with the change of flow and sediment

input. This study would also bring some

contributions to the knowledge of the geochemical

circulation of P in estuarine areas.

2 MATERIALS AND METHODS

2.1 Study Area

The study area is the Yangtze estuary (from

Xuliujing to the mouth of the estuary).

Hydrodynamic and water quality data were

measured in several observation stations in this area

(red dots in Figure 1). The water level data in

Xuliujing (XLJ) and Shidongkou (SDK) stations are

applied for the calibration and verification of the

hydrodynamic model. In XLJ, SDK, Beigang (BG),

and Nangang (NG) stations, data of temperature,

dissolved oxygen concentration (DO), suspended

sediment concentration (SSC), total phosphorus

concentration (TP), and orthophosphate

concentration (PO

4

3-

) is applied for the calibration

and verification of the water quality model.

Three reservoirs settled in the south branch of the

Yangtze estuary are the main concerns in this

research. They are the Dongfengxisha (DFXS)

reservoir, the Chenhang (CH) reservoir, and the

Qingcaosha (QCS) reservoir (red stars in Figure 1)

Figure 1: Sketch map of the Yangtze estuary and the

distribution of the three reservoirs and the observation

stations used in this study.

WRE 2021 - The International Conference on Water Resource and Environment

180

Figure 2: Model domain of the process-based Yangtze

estuary model (red grid: Datong-Xuliujing; blue grid:

Xuliujing-Sea; green grid: Lucipu-Haining). (Chu, 2019).

2.2 Hydrodynamic Model

The hydrodynamic model for the Yangtze estuary is

set up with the upstream boundary set at Datong, the

tidal limit, including part of the adjacent East China

Sea and the entire Hangzhou Bay based on Delft3D

(Chu, 2019). The hydrodynamic processes are

modeled in the hydrodynamic model. The model

domain is shown in Figure 2, which comprises three

regional models with two-dimensional simulation.

The bathymetry and coastal line measured in 2018

were used in this study. This hydrodynamic model is

well-calibrated and validated (Chu, 2019). More

details about the model domain can be learned in

Chu’s study (2019).

2.3 Water Quality Model

The water quality model is built based on the

hydrodynamic model by including the water quality

module of Delft3D. The water quality model

includes the temperature, the oxygen, the particle

inorganic matter, the dissolved inorganic matter, and

the organic matter. To be specific, they are the water

temperature, the dissolved oxygen, the suspended

and bed sediment, the total phosphorus (TP), the

particle phosphorus (PP) and, the orthophosphate

(PO

4

3-

) both in water and adsorbed onto sediment.

All upstream boundary conditions of all these

matters are set in Datong based on the measured data

in 2018. The data of the sea boundary are set as

constant according to the model calibration. Besides,

there is a point source pollution set as discharge in

the model (LH shown as in Figure 1), and the data

are also set based on the measurement.

Except for the convection-diffusion processes,

the following processes are also modeled: the heat

exchange, the reaeration of oxygen, the resuspension

and sedimentation of sediment, the adsorption and

desorption of sediment, and the mineralization of

organic phosphorus. Based on the literature and the

model calibration, the values of the main parameters

for all these processes are determined and listed in

Table 1.

Table 1: Values of major parameters of the water quality model.

Process Parameter Unit Value Reference

Heat exchange

FactRcHeat - 0.2

Default ZHeatExch ℃/d 0.1

TCKadsP - 1

Oxygen reaeration KLRear - 1 Default

Diffusion

XDisper

m

2

/s

10

Calibrated YDisper 10

VertDisper 0.001

Sediment motion

VSedIM1 m/d 7

Chu, 2019 &

Calibrated

TaucRSIDM N/m

2

0.3

TaucSIM1 N/m

2

0.15

VResDM 1/d 7

Mineralization

Ku_dFdcP20

1/d

0.18

Bowie et al., 1985

Kl_dFdcP20 0.15

Adsorption &

desorption

RcAdPO4AAP 1/d 0.08 Calibrated

RcAAPS1 1/d 0.04 Calibrated

Kads_20 (mol/L) (1-a) 3.8 Default

KdPO4AAP m

3

/g 0.7 Calibrated

Note: the specific meanings of the abbreviations in the table can be learned in the Manual of the Delft3d (D-Water Quality

Processes Library Description (Version 5.01), 2017)

Response of Phosphorus Near Reservoirs in South Branch of the Yangtze Estuary to the Upstream Runoff and Sediment Load Variations

181

Figure 3: Verification of the water levels at (a) XLJ and (b) SDK stations.

The model is calibrated and verified with the

measured data in 2018. The well-calibrated and

verified model is considered as the original case

(Case 1). Then 4 cases based on the original one are

set to change the input of flow and sediment. The

flow condition of the upstream boundary increases

to 120% and decreases to 80% of the original to

learn the change of the flow (Case 2 and 3). The

sediment condition of the upstream boundary

increases to 120% and decreases to 80% of the

original to learn the change of the sediment (Case 4

and 5).

3 RESULTS AND DISCUSSION

3.1 Calibration and Verification

3.1.1 Hydrodynamic Model

An integrated verification of the hydrodynamic

model was carried in Chu’s work (Chu, 2019). In

this research, the following data are collected to

show the brief verification of the hydrodynamic

model, while more details about it can be learned in

Chu (2019). In Figure 3, hourly data of water level

in flood season in 2018 in XLJ and SDK stations are

well verified. The good verification indicates the

hydrodynamic conditions in the water quality model

are accurate and reliable.

2018/7/26 2018/8/2 2018/8/9 2018/8/16 2018/8/23

-1

0

1

2

3

4

5

Water level (m)

model

measure

2018/7/26 2018/8/2 2018/8/9 2018/8/16 2018/8/23

-1

0

1

2

3

4

5

Water level (m)

model

measure

XLJ SDK BG NG

0

5

10

15

20

25

Temperature (℃ )

Station

Measured

Model

WRE 2021 - The International Conference on Water Resource and Environment

182

Figure 4: Comparison of the modeled water quality results

with the measured data in 2018 in the Yangtze estuary.

3.1.2 Water Quality Model

The annual average values of water quality

parameters along the south branch of the Yangtze

estuary were calculated for the comparison with the

model results. The modelled temperature, DO, SSC,

TP, and PO

4

3-

are all generally in good accordance

with the measured data (Figure 4). The average

relative errors for temperature, DO and SSC are

2.8%, 5.9%, and 25.1%, respectively. The average

relative errors for TP and PO

4

3-

are 6.1% and 13.1%,

respectively. The errors of temperature, DO and TP

are below 10%. The maximum error occurs in SSC,

but the errors are below 30% which is acceptable in

the simulation of sediment. The relatively significant

errors of SSC also cause the errors of PO

4

3-

to be

relatively large (over 10%). This is because

sediment is the important carrier of the PO

4

3-

in

water. In general, the model performed well in the

simulation of P concentration in the Yangtze

Estuary.

3.2 P Distribution in 2018

The results of the time series of TP, PP, dissolved

organic phosphorus (DOP), and PO

4

3-

in three

reservoirs in 2018 are shown in Figure 5. It can be

seen that the changing trend and concentration of

different P fractions among the three reservoirs are

very similar, which indicates the spatial distribution

of P in the south branch has little difference. The

concentration of different P fractions in water is in

the order of PP ＞ DOP ＞ PO

4

3-

. The PP takes over

50% proportion of TP. Thus, the changing trend of

PP generally determines the changing trend of TP.

The changing trend of TP, PP, and DOP are nearly

the same. The minimum values are from June to

August, while the maximum values are in October.

In January and October, the TP concentration

exceeds 0.2 mg/L. In the summertime, the TP

concentration reaches the minimum of the whole

year, but still over 0.1 mg/L. The maximum

concentration of the TP can reach around 0.27 mg/L

in October, the concentration of PP and DOP also

reach the maximum of 0.17 and 0.07 mg/L

respectively at this time. The PO

4

3-

concentration is

relatively stable, fluctuating between 0.01 - 0.03

mg/L.

Three reasons can explain the changing trend of P

loadings in the estuary. Firstly, the upstream

boundary conditions of different P fractions play the

main role. Secondly, the diffusion process controlled

by the flow input from upstream affects the P

concentration. In flood season, the flow input

XLJ SDK BG NG

0

3

6

9

12

Measured

Model

DO (mg/L)

Station

XLJ SDK BG NG

0

30

60

90

120

150

180

Measured

Model

SSC (mg/L)

Station

XLJ SDK BG NG

0.00

0.04

0.08

0.12

0.16

Measured

Model

TP (mg/L)

Station

XLJ SDK BG NG

0.00

0.01

0.02

0.03

0.04

Measured

Model

Ortho phosphate (mg/L)

Station

Response of Phosphorus Near Reservoirs in South Branch of the Yangtze Estuary to the Upstream Runoff and Sediment Load Variations

183

increase and the mixing process of P in the estuary

get promoted, which decreases the P concentration

in the south branch. In the dry season, the situation

is just the opposite. Thirdly, the adsorption and

desorption of sediment control the transform

between particle and dissolved P. The sediment

input from upstream determines the suspended

sediment concentration (SSC) in the estuary. The

amount of P adsorbed onto sediment would increase

if the SSC increases, and the PP concentration will

increase and relatively the dissolved P concentration

decrease.

Figure 5: The time-series results of TP, PP, DOP, and

PO

4

3-

in DFXS (a), CH (b), and QCS (c) reservoirs.

In Figure 6, the annual average concentration and

proportion of different P fractions in three reservoirs

are shown. The situation of the three reservoirs does

not show a significant difference. The annual

average concentration of TP is around 0.15 mg/L.

The annual average concentration of PP, DOP, and

PO

4

3-

are 0.08 - 0.095, 0.035 - 0.04 and 0.015 - 0.02

mg/L, respectively. Because of the discharge of the

Liuhe river near the CH reservoir, the P

concentrations in the CH reservoir are a bit higher

than those of the other two reservoirs. The annual

average concentration of P in the south branch in

2018 maintains a relatively low concentration after

2015 compared with those from 2000 to 2010 (Shen

et al., 2008; Hou et al., 2009). The constitution of TP

in three reservoirs is almost the same. The particle

phosphorus accounts for the largest proportion

reaching around 60%, while the phosphate took the

least proportion, with only 12 - 13%. The proportion

of DOP is 25 - 30%.

0.0

0.1

0.2

0.3

Concentration (mg/L)

TP

PP

DOP

PO

4

3-

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

AplMarFeb

Month

Jan

0.0

0.1

0.2

0.3

Concentration (mg/L)

TP

PP

DOP

PO

4

3-

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apl

Mar

Feb

Month

Jan

0.0

0.1

0.2

0.3

Concentration (mg/L)

TP

PP

DOP

PO

4

3-

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

AplMarFeb

Month

Jan

DFXS CH QCS

0.00

0.05

0.10

0.15

0.20

Concentration (mg/L)

TP

PP

DOP

PO

4

3-

(a)

(b)

(c)

WRE 2021 - The International Conference on Water Resource and Environment

184

Figure 6: The annual average concentration (a) and

proportion (b) of different P fractions in the three

reservoirs.

3.3 Effect of Flow and Sediment Input

According to the modeling results in 2018, the P

distribution, concentration and constitution in three

reservoirs are generally the same. This similarity

also shows in the case studies. Thus, the results of

the one-year time series of TP, PP, and PO

4

3-

of the

original case (2018) and the 4 study cases of the

DFXS reservoir are taken as the representative of the

three reservoirs in the following analysis (Figure 7).

In each case, only the upstream boundary conditions

of flow or sediment are changed while other

conditions are consistent with the original case. The

changing trends of TP, PP, and PO

4

3-

of different

cases are generally the same as those of the original

case, which is controlled by the time series of P

concentration conditions of the upstream boundary.

The TP concentration of 5 cases shows little

difference. However, the concentrations of PP and

PO

4

3-

show changes with the various flow and

sediment input conditions.

The changing trends of the PP and PO

4

3-

are

inverse. With the increase of the flow (case2) and

the decrease of sediment input (case3), the PP shows

a decrease while the PO

4

3-

shows an increase. With

the decrease of the flow (case1) and the increase of

sediment input (case4), the PP shows an increase

while the PO

4

3-

shows a decrease. Generally, when

the flow input increase and the sediment input

decrease, the concentration of PP would decrease

and the concentration of PO

4

3-

would increase. On

the contrary, when the flow input decrease and the

sediment input increase, the concentration of PP

would increase and the concentration of PO

4

3-

would

decrease. These changes are affected by the

convection and diffusion, and the adsorption and

desorption processes of sediment. The suspended

sediment concentration (SSC) would decrease

because of the diffusion process with the flow input

increase. Both the increase of the diffusion process

and the decrease of SSC would further decrease the

PP concentration in the south branch. However, the

situation of the PO

4

3-

is relatively complex under

these conditions. The increase of the flow input

promotes the mixing processes of PO

4

3-

, which

decreases the PO

4

3-

concentration in the south

branch. On the contrary, the increase of the flow

input decreases SSC, the amount of carrier of PO

4

3-

,

which increases the PO

4

3-

concentration in the south

branch. It can be concluded that the effect of

adsorption and desorption on the PO

4

3-

concentration

is more significant than that of diffusion because of

the increase of PO

4

3-

in case2 and decrease of it in

case1. In the real nature, the effect of adsorption and

desorption of sediment on the PO

4

3-

concentration

would even become more significant when taking

the release of P during the sediment resuspension

into consideration, like the situation in the MTZ

(Shen et al., 2008; Li et al., 2019).

DFXS CH QCS

0

10

20

30

40

50

60

70

80

90

100

Percentage (%)

PO

4

3-

DOP

PP

0.0

0.1

0.2

0.3

TP (mg/L)

origin

case1

case2

case3

case4

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apl

Mar

Feb

Month

Jan

0.0

0.1

0.2

0.3

PP (mg/L)

origin

case1

case2

case3

case4

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apl

Mar

Feb

Month

Jan

Response of Phosphorus Near Reservoirs in South Branch of the Yangtze Estuary to the Upstream Runoff and Sediment Load Variations

185

Figure 7: The time-series results of (a) TP, (b) PP, and (c)

PO

4

3-

of different cases in the DFXS reservoir.

The annual average concentration and proportion

of different P fractions of different cases in DFXS

reservoirs are shown in Figure 8. The PO

4

3-

concentration increases about 10% and 25%,

respectively when the flow input increase by 20%

and sediment input decrease by 20%. The PO

4

3-

concentration decreases about 8% and 20%,

respectively when the flow input decrease by 20%

and sediment input increase by 20%. The PP

concentration increases about 12% and 35%,

respectively when the flow input increase by 20%

and sediment input decrease by 20%. The PP

concentration decreases about 15% and 30%,

respectively when the flow input decrease by 20%

and sediment input increase by 20%. The amplitude

of variations of cases with changes of sediment input

(case3 and 4) are larger than those with changes of

flow input (case1 and 2). This further verifies that

the effect of adsorption and desorption of sediment

would play more significant roles in the P

distribution than the diffusion process does.

The constant TP concentration and various PP

and PO

4

3-

concentrations between different study

cases indicate a shift of the P fractions in water. The

proportion of DOP is generally stable between 20-

30%. PP always takes the most proportion among all

cases, although it decreases to 45-50% when the

sediment input decreases by 20%. The absolute

difference of the PO

4

3-

of different cases is not big,

but its relative changes are significant. Its proportion

is nearly twice as much as the original when the

sediment input decrease by 20%. This shift of the

constitution of the P would bring some potential

water quality problems. The construction of dams in

the Yangtze basin, especially the Three Gorges dam

completed in 2003, decreased the sediment input to

the estuary from nearly 480 Mt/a in the 1950s to less

than 150 Mt/a in the 2010s (Chen et al., 2010). The

construction of dams will continue during the next

few decades (Chen et al., 2010). This will keep

decreasing the sediment input and increasing the

proportion of PO

4

3-

. The government has taken

action to reduce the nutrients fluxes into the estuary

in recent years. Although the TP concentration has

decreased, PO

4

3-

concentration would stay at a high

level or even increase because of the increase of its

proportion. In the geochemical circulation of P,

PO

4

3-

is the only fraction that can be directly taken

by the food web, like algae (Meybeck, 1982). Thus,

the high concentration of PO

4

3-

would still keep the

water eutrophicated.

Figure 8: The annual average proportion of PP, DOP, and

PO

4

3-

among TP of different cases in DFXS reservoir.

4 CONCLUSION

Because of human activities and global climate

change, the flow and sediment input from upstream

to the Yangtze estuary change a lot, which brings

about the variation of P loadings and some potential

water quality problems. An integrated water quality

model was established with Delft3d to study the P

distribution in 2018 in the south branch of the

Yangtze estuary and its possible changes in the

future in response to variations of runoff and

0.00

0.01

0.02

0.03

0.04

0.05

PO

4

3-

(mg/L)

origin

case1

case2

case3

case4

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

AplMarFeb

Month

Jan

origin case1 case2 case3 case4

0.00

0.04

0.08

0.12

0.16

0.20

Concentration (mg/L)

TP

PP

PO

4

3-

origin case1 case2 case3 case4

0

10

20

30

40

50

60

70

80

90

100

Percentage (%)

PO

4

3-

DOP

PP

WRE 2021 - The International Conference on Water Resource and Environment

186

sediment loads from upstream. The general

conclusions are as follow:

(a) In 2018, the average TP concentration is

around 0.15 mg/L, but it can reach over 0.2 mg/L in

October. The particle P fractions took the largest

proportion (over 60%), while the PO

4

3-

occupied the

least proportion (around 10%). The P concentration

in the south branch in the dry season is higher than

that in the flood season.

(b) Both PP and PO

4

3-

concentrations change

with the changes of flow and sediment input, which

brings about the reconstitution of the component of

TP in the south branch. When the flow input

increases or the sediment input decreases, the PP

concentration would increase and PO

4

3-

concentration would decrease. When the flow input

decreases or the sediment input increases, the PP

concentration would decrease and PO

4

3-

concentration would increase.

(c) The diffusion process and adsorption and

desorption of sediment are the main processes

controlling the response of P to the upstream runoff

and sediment load variations. The adsorption and

desorption would play a more significant role than

the diffusion process does.

ACKNOWLEDGEMENTS

This work was partly supported by the National

Natural Science Foundation of China (51620105005,

51979076) and the Fundamental Research Funds for

the Central Universities of China (Grant No.

B200202057, B200204017).

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