─Journal of the Korean Society of OceanographyVol. 39, No. 1, pp. 14 19 March 2004

Difference of Nutrients Budgets in the Bohai Sea between 1982 and 1992
related to the Decrease of the Yellow River Discharge
MITSURU HAYASHI1, TETSUO YANAGI2*, AND GUO XINYU3
1

Research Center for Inland Seas, Kobe University, Fukae, Higashi-Nada, Kobe 658-0022, Japan
2
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
3
Center for Environmental Management of Enclosed Seas, Ehime University, Bunkyo 3, 790-8577, Japan

Difference of Dissolved Inorganic Phosphorus (DIP), Dissolved Inorganic Nitrogen (DIN) andDissolved Silicate (DSi) budgets in the Bohai Sea between 1982 and 1992 related the decrease ofthe Yellow River discharge is discussed on the basis of observed data. The estuarine circulation inthe Bohai Sea had been weakened from 1982 to 1992 due to the decrease of the Yellow River
discharge and the average residence time of fresh water had become longer. DIN concentration
increased but DIP and DSi concentrations decreased from 1982 to 1992 in the Bohai Sea. Primary
production was regulated mainly by water temperature and DIN concentration in 1982 but it wasregulated mainly by DIP concentration in 1992. Primary production was larger than decomposition
plus bottom release and nitrogen fixation was larger than denitrification in 1982. However,decomposition plus bottom release was larger than primary production and denitrification was larger
than nitrogen fixation in 1992 in the Bohai Sea.
Key words: Nutrient Cycling, the Bohai Sea, the Yellow River, Lower Trophic Level Ecosystem

INTRODUCTION

budgets of the Bohai Sea between 1982 and 1992
using the observed data.

The river discharge of the Yellow River has
decreased from 1970’s due to the overuse of wateron land. Lin et al. (2001) showed that the Sea Surface
Salinity (SSS) in the Bohai Sea has increased by
0.074 psu y−1 (from 28.5 psu in 1960 to 31.0 psu in1997) due to the decrease of the Yellow River discharge. It is interesting to investigate the effect ofsuch decrease of the Yellow River discharge to thematerial cycling and the lower trophic level ecosystem of the Bohai Sea. As for the variation of thelower trophic level ecosystem, Tang et al. (2003) foundthat the primary productivity decreased from 1982
(312 mgC m−2 day−1) to 1992 (216 mgC m−2 day−1) but
the secondary productivity increased from 1992 to1998 in the Bohai Sea. However there is no study
on the variation of the nutrient cycling in the Bohai
Sea related to the decrease of the Yellow River discharge. We investigate the difference of water, salt,Dissolved Inorganic Phosphorus (DIP), Dissolved Inorganic Nitrogen (DIN) and Dissolved Silicate (DSi)

USED DATA
The used data in this study are obtained from thepublished atlas by Tang and Meng (1997) and thepaper by Lu et al. (1999). Their field observations
were carried out in May, August, October 1982 andFebruary 1983 when the Yellow River discharge waslarge. Other field observations were carried out inAugust and October 1992 and February and May
1993 when the Yellow River discharge was small(Fig. 1). Water temperature, salinity, DIP, DIN, DSi,chl.a and primary production in the surface and bottom layers at 82 stations in the Bohai Sea (Fig. 2)
were observed.

ANALYSIS
The concerned box and the adjacent area aredefined as shown in Fig. 2 on the basis of observed
salinity distribution and the data in the box and theadjacent area are averaged.

*Corresponding author: tyanagi@riam.kyushu-u.ac.jp

14Difference of Nutrients Budgets in the Bohai Sea between 1982 and 1992 related to the Decrease of the Yellow

15

Fig. 1. Year-to-year variation of theYellow River discharge at Lijin.

Fig. 2. Definition of the box (inside the full line), the adjacent area (broken line) and observation stations in the Bohai
Sea (a). Water depth in meters of the Bohai Sea (b).

Water and Salt Budgets
At first the water and salt budgets are calculatedon the basis of LOICZ method (Gordon et al., 1996).
Water and salt budgets are written by the followingequations.
dVs/dt = VQ+V P+V E+VG+V R
d(VsSs)/dt = VRS R+Vx(So − Ss)

(1)
(2)

where Vs denotes the volume of the box, VQ the riverdischarge, V P the precipitation, VE the evaporation,
VG the ground water discharge, V R the residual volume transport from the box to the adjacent area, Ss
the average salinity of the box, So the average salinity of the adjacent area, SR = (Ss+So)/2, and Vx thewater exchange volume between the box and theadjacent area.

dVs/dt is estimated from the seasonal variation ofmean sea surface of the Bohai sea, V Q is estimatedfrom Fig. 1, VP and VE are estimated from ECMWF(European Center for the Medium Weather Forecast)data and VG is assumed to be zero because we haveno data of VG. Temporal variations of VQ and V R are
shown in Fig. 3.
Residual volume transport VR decreased by about
1/3 as the river discharge VQ decreased by about 1/3 from 1982 to 1992. Temporal variations of salinity
in the surface and bottom layers of the box are shownin Fig. 4. Salinity in the surface layer increased from1982 to 1992 but salinity in the bottom layer inAugust and February decreased from 1982 to 1992.
This may be due to that the estuarine circulation in
the box had been weakened by the decrease of theYellow River discharge from 1982 to 1992. Salinity
in the bottom layer decreased due to the decrease16

Mitsuru Hayashi, Tetsuo Yanagi and Guo Xinyu

Fig. 3. Temporal variations of river discharge VQ (a) andresidual transport VR (b).
Fig. 4. Temporal variations of salinity in the surface (a) andbottom (b) layers of the box.

of high salinity water flux from the lower layer of
the adjacent area.
The horizontal exchange volume Vx is estimatedfrom Eq. (2) and the result is shown in Fig. 5. Vx
from August to October in 1982 and 1992 was not
obtained due to too large temporal variation of salinity in the box. Vx in 1992 is less than half as thatin 1982 and this is due to the decrease of the strengthof estuarine circulation in the box. Horizontal diffusivity Kh corresponding to Vx is estimated by thefollowing equation,
Kh = Vx∆L/A(3)where ∆L means the length between the center ofthe box and that of the adjacent area and A crosssurface area between the box and the adjacent area.
Estimated Kh is about 8×10 6 cm 2 s−1 in 1982 and2×10 6 cm2 s −1 in 1992.
Average residence time of the box water Tb andthat of fresh water Tf are estimated by the followingequation.

Fig. 5. Temporal variation of horizontal exchange volume Vx.

Tb = Vs/(Vx+VR), Tf = Vs(So-Ss)/VQSo(4)Estimated average residence times are shown in Fig.
6. Tb was 90 days in 1982 and 170 days in 1992. Tfwas 130 days in 1982 and 210 days in 1992. Tb isshorter than Tf due to large Vx.

Difference of Nutrients Budgets in the Bohai Sea between 1982 and 1992 related to the Decrease of the Yellow

17Fig. 6. Temporal variations of average residence time of the box water (a) and the fresh water (b).

Fig. 7. Temporal variations of nutrients concentrations in the box and adjacent area.

Nutrients Budgets
Nutrient budget of the box is written by the following equation.

d (VsCs)/dt = VQCQ+V PCP−VRCs+Vx(Cs−Co) + ∆C
(4)
where Cs denotes the nutrient concentration of the18

Mitsuru Hayashi, Tetsuo Yanagi and Guo Xinyu

box, CQ that of river water, CP that of rain, Co thatof the adjacent area and ∆C the nutrient flux by thebiochemical processes such as photosynthesis, decomposition and release from the bottom in the box. CQ isestimated from the observed data (personal communication of Dr. Mi from the Ocean University ofChina) and CP is assumed to be 0 because we haveno data. In case of DIP budget, positive ∆C meansthat decomposition plus bottom release is larger thanphotosynthesis but negative ∆C means that photosynthesis is larger than decomposition plus bottom
release. When we assume that the main primary producer in the box is phytoplankton, we may estimate
nitrogen fixation (Nfix) minus denitrification (deN)by the following equation (Gordon et al., 1996).
Nfix − deN = ∆DIN − ∆DIP×16(5)Temporal variations of average nutrients concentrations in the box and the adjacent area are shownin Fig. 7. DIN concentrations in the box and the adjacent area increased from 1982 to 1992 but DIP andDSi concentrations there decreased.
Temporal variation of DIN/DIP ratio in the box isshown in Fig. 8. DIN/DIP ratio was smaller than theRedfield ratio of 16 in 1982 but it was larger than
16 in 1992. This suggests that the limiting nutrientof photosynthesis in the Bohai Sea was DIN in 1982
but it changed to DIP in 1992.
Temporal variations of chl.a concentration and primary production in the box are shown in Fig. 9.
Chl.a concentration and primary production decreased
from 1982 to 1992 in the Bohai Sea. They were highin August and October with high water temperatureand low in February with low water temperature in
1982. The correlation of chl.a concentration and primary production to water temperature was not highFig. 8. Temporal variation of DIN/DIP ratio in the box.

Fig. 9. Temporal variations of chl.a concentration (a) andprimary production (b) in the box.

in 1992. This may be due to that the limiting of nutrient concentration to primary production became more
severe and the sensitivity of primary production towater temperature decreased in 1992.
Results of ∆DIP and (Nfix-deN) are shown in Fig.
10. Photosynthesis was larger than decomposition
plus bottom release in 1982 but photosynthesis wasnearly the same as decomposition plus bottom releasein 1992.Average primary production was 320 mgC m-2 day1
in 1982 (Fig. 9) and 0.25 m mol P m-2 day-1 when we
assume the Redfield ratio of 106. On the other hand,average ∆DIP in 1982 was 0.11 m mol P m-2 day-1 because
the sea surface area of the box is 1.77×1010 m2. Thissuggests that the phosphorus decomposition and bottom release was 0.14 m mol P m-2 day -1 in 1982.
Nitrogen fixation was larger than denitrification in1982 but denitirification was larger than nitrogen fixation in 1992.
The changes of physical and biochemical processes
in the Bohai Sea from 1982 to 1992 are summarizedin Table 1.

Difference of Nutrients Budgets in the Bohai Sea between 1982 and 1992 related to the Decrease of the Yellow

19CONCLUSIONSThe estuarine circulation in the Bohai Sea had been
weakened from 1982 to 1992 due to the decrease ofthe Yellow River discharge and the average residence
times of sea water and fresh water in the box had
become longer. DIN concentration increased but DIP
and DSi concentrations decreased from 1982 to 1992
in the Bohai Sea. Primary production was regulated
mainly by water temperature and DIN concentrationin 1982 but it was regulated mainly by DIP in 1992.
Primary production was larger than decomposition
plus bottom release and nitrogen fixation was larger
than denitrification in 1982. However, decomposition
plus bottom release was larger than primary production and denitrification was larger than nitrogen
fixation in 1992 in the Bohai Sea.
We will develop the numerical ecosystem modelof the Bohai Sea in order to investigate the changeof lower trophic level ecosystem there due to thedecrease of the Yellow River discharge in the near
future.

REFERENCES

Fig. 10. Temporal variations of ∆DIP (a) and (Nfix-deN)(b) in the box.
Table 1. Changes of physical and biochemical processes inthe Bohai Sea from 1982 to 1992

Estuarine circulation
Surface salinity
Bottom salinity
Water exchange ratio
Residence time of waterDIN
DIP and DSi
Chl.a and primary production
Limiting nutrient
Nitrogen fixation-Denitrification

1982

1992

stronglowhighhigh
short
low
highhigh
DIN
plusweak
high
low
low
long
high
low
low
DIP
minus

Gordon, D.C. Jr., Boudreau, P.R., Mann, K.H., Ong, J.E., Silvert,W.L., Smith, S.V., Wattayakorn, G., Wulff, F. and Yanagi, T.,1996. LOICZ Biogeochemical Modelling Guidelines. LOICZ
Reports and Studies 5, Texel, The Netherlands, 96 pp.
Lin, C., Su, J., Xu, B. and Tang, Q., 2001. Long-term variations oftemperature and salinity of the Bohai Sea and their influenceon its ecosystem. Progress in Oceanography, 49: 7−19.
Lu, R., Xia B., Li B. and Fei Z., 1999. The fluctuations of primary
productivity in Bohai Sea waters over ten years. J. Oceanography of Huanghai & Bohai Seas. 17(3): 80−86 (in Chinesewith English abstract).
Tang, Q. and Meng T., 1997. Atlas of the ecological environment
and living resources in the Bohai Sea. Qingdao Publishing
House, 242pp. (in Chinese).
Tang, Q., Jin, X., Wang J., Zhuang Z., Cui, Y. and Meng, T., 2003
Decadal-scale variations of ecosystem productivity and control mechanisms in the Bohai Sea. Fisheries Oceanography,
12: 223−233.
Manuscript received October 15, 2003
Revision accepted February 15, 2004
Editorial handling: Chang S. Kim