Simulating the influence of various nutrient sources on hypoxia off the Changjiang River Estuary
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Abstract: Hypoxia is increasingly reported off the Changjiang River Estuary with the confluence of multiple high volume nutrient sources. The Regional Ocean Modeling System coupled with a biological model was used to analyze the effect of different nutrient sources on the development of hypoxia off the Changjiang River Estuary. By comparing to observed data, our model suitably captured the regional dynamics of chlorophyll a, dissolved oxygen, and nutrient concentration. A series of sensitivity experiments were conducted to investigate the hypoxia response to the various nutrient sources, such as loading from the Changjiang River, Kuroshio and Taiwan Warm Current. Our model results indicated that nutrients from different sources significantly influenced the hypoxia off the Changjiang River Estuary, and it was mostly affected by nutrients sourced from the Kuroshio. The nutrients input from the Changjiang River had larger impacts on the hypoxia in the north of 30°N than that in the south of 30°N. The nutrients sourced from the Taiwan Strait had a least influence on the hypoxia off the Changjiang River Estuary.
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Key words:
- hypoxia /
- nutrient /
- different source /
- Changjiang River Estuary
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Figure 1. Model domain and water depth (a), the red two-way arrows indicate the Taiwan Strait boundary and Kuroshio boundary, respectively. In b, Red dots represent station observation in August 2011; blue dots represent observations in August 2011, 2012. The blue and red rectangles indicate Box1 and Box2, respectively.
Figure 2. Monthly averaged sea surface chlorophyll concentration from the model (a) and MODIS data (b) in September 2010.
Figure 3. The
${\rm{PO}}_4^{3-} $ concentration distribution along the section at 30°N in August 2011 from the model (a) and observation (b).
Figure 4. The distribution of DO concentration along the section at 30°N in August 2011 (a, b) and 2012 (c, d) from the model (a, c) and observation (b, d).
Figure 5. The comparison between modeled results (red bars) and observed data (blue bars) of sea surface
${\rm{NO}}_3^- $ (a),${\rm{PO}}_4^{3-} $ (b), and DO (c) at the stations in Fig. 1b in August 2011.
Figure 6. Model monthly mean circulation at 10 m depth in September 2010 (a), model monthly mean circulation at sea bottom in September 2010 (b).
Figure 7. The simulated results of the distribution of hypoxic zone off the Changjiang River Estuary in 2010 (a. July, b. August, c. September, d. October).
Figure 8. The simulated hypoxic area (DO concentration<2 mg/L) between the Base and N-only simulations.
Figure 9. The comparison of the simulated hypoxic area (DO concentration<2 mg/L) among the sensitivity experiments.
Figure 10. The monthly mean
${\rm{NO}}_3^- $ concentration distribution at the sea surface in September 2010 among the different sensitivity experiments (a−d). The total${\rm{NO}}_3^- $ amount in Box1 (e) and Box2 (f) (refer to Fig. 1b) from July to October, respectively.
Figure 11. The monthly mean
${\rm{PO}}_4^{3-} $ concentration distribution at the sea surface in September 2010 among the different sensitivity experiments (a−d); the total${\rm{PO}}_4^{3-} $ amount in Box1 (e) and Box2 (f) from July to October, respectively.
Figure 12. The monthly mean chlorophyll (Chl) distribution at the sea surface in September 2010 among the sensitivity experiments (a−d). The total Chl amount in Box1 (e) and Box2 (f) from July to October, respectively.
Figure 13. The monthly mean chlorophyll concentration distribution at the sea surface in September 2010 in Case4 (a); the monthly mean DO concentration distribution at the sea bottom in September 2010 in Case4 (b).
Figure 14. The monthly mean DO concentration distribution at the bottom in September 2010 among the sensitivity experiments (a−d). The minimum value of DO concentration at the sea bottom water in Box1 (e) and Box2 (f) from July to October, respectively.
Figure 15. The main ocean current pattern in the East China Sea in summer 2010 where the isobaths of 20 m, 50 m, 100 m, 200 m are overlapped. The blue arrows represent the Taiwan Warm Current (TWC); the yellow arrows represent the nearshore Kuroshio branch current (NKBC); and the black arrows represent the Changjiang diluted water (CDW). The upwelling is indicated by the pink cycle.
Table 1. The parameters used in ecosystem model
Parameter Value Unit Light attenuation by chlorophyll 0.024 86 m−1 Light attenuation due to seawater 0.04 (mg·m2)−1 (in terms of chlorophyll) half-saturation for phytoplankton ${\rm{NO}}_3^- $ uptake 0.5 mmol/m3 (in terms of N) half-saturation for phytoplankton ${\rm{NH}}_4^+ $ uptake 0.5 mmol/m3 (in terms of N) half-saturation for phytoplankton ${\rm{PO}}_4^{3-} $ uptake 0.03 mmol/m3 (in terms of P) Phytoplankton, initial slope of P-I curve 0.025 mol/(g·W·m2·d) (in terms of C) Phytoplankton growth rate 0.69 d−1 Zooplankton maximum growth rate 0.6 m3/(mmol·d) (in terms of N) Zooplankton half-saturation constant for ingestion 2 (mmol/m3)2 (in terms of N) Phytoplankton mortality rate 0.15 d−1 Maximum chlorophyll to carbon ratio 0.053 5 mg chlorophyll per mg C Nitrification rate: oxidation of ${\rm{NH}}_4^+ $ to ${\rm{NO}}_3^- $ 0.05 d−1 Half-saturation radiation for nitrification inhibition 0.1 W/m2 Radiation threshold for nitrification inhibition 0.009 5 W/m2 Zooplankton nitrogen assimilation efficiency 0.75 − Zooplankton mortality rate 0.025 d−1 Zooplankton basal metabolism 0.1 d−1 Zooplankton specific excretion rate 0.1 d−1 Large detritus remineralization rate N-fraction 0.1 d−1 Small detritus remineralization rate N-fraction 0.2 d−1 Large detritus remineralization rate C-fraction 0.1 d−1 Small detritus remineralization rate C-fraction 0.2 d−1 Coagulation rate 0.05 d−1 Vertical sinking velocity for phytoplankton 0.5 m/d Vertical sinking velocity for large detritus 5.0 m/d Vertical sinking velocity for small detritus 2.0 m/d Note: − represents no dimension. 
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Table 2. Model sensitivity experiment
Nutrients input source Base Changjiang River +Taiwan Strait + Kuroshio Case1 Changjiang River + Kuroshio Case2 Changjiang River + Taiwan Strait Case3 Changjiang River Case4 Kuroshio + Taiwan Strait
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Table 3. The total area of hypoxic area under different thresholds of DO concentration
Experiment Integrated hypoxic area/(103 km2) <2 mg/L (hypoxia) <3 mg/L (Low DO) Base 250.1 1 021.6 Case1 55.4 (−77.8%) 499.2 (−51.1%) Case2 0.7 (−99.7%) 142.1 (−86.1%) Case3 0 (−100.0%) 3.3 (−99.7%) Case4 37.0 (−85.2%) 274.5 (−73.1%) Note: Low DO indicates dissolved oxygen concentration between 2 mg/L and 3 mg/L.
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Table 4. The volume-averaged chlorophyll concentration (mg/L) in sensitivity experiments in September 2010
Region Base Case1 Case2 Case3 Case4 Box1
(southern region)1.54(100%) 1.37(89%) 0.48(31%) 0.18(12%) 1.36(88%) Box2
(northern region)3.24(100%) 3.12(96%) 2.48(77%) 2.20(68%) 1.89(58%)
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Table 5. Integrated Low DO area (103 km2) in Box1 and Box2 in each experiment in 2010
Base Case1 Case2 Case3 Case4 Box1
(southern region)415(100%) 227(55%) 41(10%) 0(0%) 178(43%) Box2
(northern region)555(100%) 264(48%) 90(16%) 2(0%) 90(16%) Note: Low DO indicates dissolved oxygen concentration between 2 mg/L and 3 mg/L.
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