Fluxes of riverine nutrient to the Zhujiang River Estuary and its potential eutrophication effect
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Abstract: The Zhujiang River Estuary is becoming eutrophic due to the impact of anthropogenic activities in the past decades. To understand nutrient dynamics and fluxes to the Lingdingyang water via four outlets (Humen, Jiaomen, Hongqimen and Hengmen), we investigated the spatial distribution and seasonal variation of dissolved nutrients in the Zhujiang River Estuary, based on fourteen cruises conducted from March 2015 to October 2017, covering both wet (April to September) and dry (October to March next year) seasons. Our results showed that riverine fluxes of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) into the Lingdingyang water through four outlets varied seasonally due to the influence of river discharge, with the highest in spring and the lowest in winter. However, riverine flux of phosphate exhibited little significant seasonal variability. Riverine nutrients into the Lingdingyang water most resulted through Humen Outlet. The estuarine export fluxes of DIN out of the Zhujiang River Estuary derived from a box model were higher than fluxes of riverine nutrients in May, likely due to the influence of local sewage, while lower than riverine flux in August. The export fluxes of phosphate were higher than the fluxes of riverine phosphate in May and August. In contrast, large amounts of DSi were buried in the estuary in May and August. Although excess DIN was delivered into the Zhujiang River Estuary, eutrophication effect was not as severe as expected in the Zhujiang River Estuary, since the light limitation restricted the utilization of nutrients by phytoplankton.
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Key words:
- riverine nutrient /
- flux /
- Lingdingyang /
- Zhujiang River Estuary /
- eutrophication
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Figure 1. Location of sampling stations in the Zhujiang River Estuary during March 2015–October 2017. The solid circles denoted the sampling stations 1–20 during March 2015–October 2017; triangles, sampling stations 21–34 in May and August 2015. HM, JM, HQM and HeM represented the Humen, Jiaomen, Hongqimen and Hengmen, respectively. Dashed Line A was the boundary of riverine flux to the Lingdingyang water via four outlets; Line B showed the boundary of the budget systems as previous study (Liu et al., 2009).
Figure 2. Total Nutrient flux (residual flux (VRCR) and mixing exchange flux (VXCX)) from the Zhujiang River Estuary to the South China Sea derived from box model and riverine input (VQCQ) to the estuary.
Figure 3. Spatial and temporal variations in the surface salinity and temperature in the Zhujiang River Estuary during March 2015–October 2017. Vertical bars denoted standard deviation errors. HM, JM and HQM represented the Humen, Jiaomen and Hongqimen, respectively.
Figure 4. Spatial and temporal variations in the surface
${\rm{NO}}_3^- $ ,${\rm{NO}}_2^- $ ,${\rm{NH}}_4^+ $ , DIN, DIP and DSi concentrations in the Zhujiang River Estuary during March 2015–October 2017. Vertical bars denoted standard deviation errors. HM, JM and HQM represented the Humen, Jiaomen and Hongqimen, respectively.
Figure 5. Monthly average concentration ratios of DIN:DIP, DSi:DIN and DSi:DIP at the sea surface at Stations 1−20 during March 2015–October 2017. Vertical bars denoted standard deviation errors. HM, JM and HQM represented the Humen, Jiaomen and Hongqimen, respectively.
Figure 6. Variations in the concentrations of
${\rm{NO}}_3^- $ , DIN, DSi and DIP along a salinity gradient in the Zhujiang River Estuary at the sea surface and the sea bottom at Stations 21–34 in May and August 2015.
Figure 7. Spatial and temporal variations in chlorophyll a (Chl a) (a) and dissolved oxygen (DO) (b) concentrations at the sea surface during March 2015–October 2017. The solid line in b represented the value of DO equalled to 2 mg/L. Vertical bars denoted standard errors. HM, JM and HQM represented the Humen, Jiaomen and Hongqimen, respectively.
Figure 8. Relationship between riverine nutrients fluxes to the Lingdingyang water via four outlets and the river discharge at the sea surface.
Figure 9. The relationship of riverine nutrient fluxes at the sea surface at Stations 1–20 during March 2015–October 2017.
Figure 10. Relationship between
${\rm{NO}}_3^- $ and DIN and relationship between${\rm{NH}}_4^+ $ and DIN at the sea surface at Stations 1–20 from March 2015 to October 2017. The graphs with lines indicated significant correlation between the two variables, and the correlation coefficients were given in the graphs.
Figure 11. Water budgets for the Zhujiang River Estuary. The water flux is in 106 m3/d. VQ, VP, VE, VR and VX are the river discharge, precipitation, evaporation, the residual flow and the mixing flow between the system of interest and the adjacent system, respectively. The words a and b refer to May and August 2015 cruises, respectively.
Table 1. Discharge, temperature and salinity at Stations 1−20 during March 2015−October 2017. The average values were shown in parentheses
Month Discharge/(m3·s−1) Temperature/°C Salinity March 3669–9904 (7296) 18.7–21.6 (20.4) 0.13–16.7 (5.66) May 10383–19700 (15480) 23.6–27.9 (26.2) 0.08–1.99 (0.30) August 12866–16938 (14968) 28.6–32.1 (30.6) 0.08–7.41 (0.79) October 11855–15034 (13367) 25.8–28.6 (26.8) 0.11–21.4 (5.90) 
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Table 2. Estimation of nutrient fluxes (Mean±SD) via four outlets in four seasons
Discharge/(m3·s−1) $F_{{\rm{NO}}_3^- } $/(106 mol·d−1) FDIN/(106 mol·d−1) FDIP/(106 mol·d−1) FDSi/(106 mol·d−1) FDIN:FDIP FDSi:FDIN FDSi:FDIP Humen March 947 15.0 ± 3.01 25.0 ± 9.90 0.17 ± 0.05 24.7 ± 12.1 148 1.00 150 May 3 302 36.2 ± 15.8 63.7 ± 16.7 0.13 ± 0.03 96.8 ± 37.3 490 1.52 744 August 3 146 35.6 ± 12.5 49.6 ± 11.9 0.19 ± 0.10 69.7 ± 30.0 255 1.41 359 October 2 229 36.7 ± 5.72 42.4 ± 4.37 0.38 ± 0.24 46.3 ± 27.1 113 1.09 124 Jiaomen March 893 10.7 ± 4.54 12.3 ± 4.54 0.11 ± 0.05 20.5 ± 8.41 109 1.67 183 May 3 114 30.0 ± 8.47 33.0 ± 7.60 0.26 ± 0.04 76.5 ± 25.8 124 2.32 289 August 2 966 23.0 ± 12.4 24.2 ± 12.7 0.38 ± 0.16 58.5 ± 23.3 63.0 2.42 152 October 2 101 19.8 ± 2.10 22.9 ± 1.29 0.33 ± 0.22 41.8 ± 21.0 69.7 1.82 127 Hongqimen March 325 3.75 ± 1.65 4.51 ± 1.62 0.05 ± 0.03 7.05 ± 2.84 90.3 1.56 141 May 1 132 10.5 ± 4.02 11.0 ± 3.81 0.11 ± 0.04 26.0 ± 9.51 114 2.17 248 August 1 078 8.40 ± 4.80 8.80 ± 4.87 0.12 ± 0.04 21.2 ± 7.96 75.8 2.41 183 October 764 7.49 ± 0.70 8.93 ± 0.44 0.11 ± 0.09 14.9 ± 7.06 83.6 1.67 140 Hengmena March 541 3.80 5.48 0.04 N/A 138 N/A N/A May 1 887 16.3 19.8 0.13 N/A 150 N/A N/A August 1 797 16.6 20.5 0.16 N/A 130 N/A N/A October 1 274 10.9 12.7 0.11 N/A 111 N/A N/A Note: N/A means no data; a, data from Liu (2006).
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Table 3. Summary of the yearly average nutrient flux (103 t/a)
$F_{{\rm{NO}}_3^-} $ FDIN FDIP FDSi Humen 157 230 1.11 303 Jiaomen 106 118 1.39 251 Hongqimen 38.4 43.6 0.48 88.2 Hengmen 60.7 74.6 0.57 N/A Riverine flux 362 466 3.55 N/A Export flux 491 585 10.8 609 Note: N/A means no data.
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Table 4. Estimation of riverine fluxes to Zhujiang River Estuary and export nutrient fluxes (106 mol/d) of the Zhujiang River Estuary during May and August, 2015
May 2015 August 2015 $F_{{\rm{NO}}^-_3}$ FDIN FDIP FDSi $F_{{\rm{NO}}^-_3}$ FDIN FDIP FDSi Riverine flux (VC) 71.5 108 0.75 145 75.4 91.1 0.78 170 Residual flow (VRCR) –63.3 –77.6 –0.50 –31.1 –44.5 –51.1 –0.49 –47.7 Mixing exchange (VXCX) –57.5 –71.7 –0.55 –28.6 –27.3 –29.3 –0.37 –12.3 Export flux (VRCR + VXCX) 121 149 1.05 59.7 71.8 80.4 0.86 60.0 Δ = (export flux – riverine flux) –49.3 –46.7 –0.30 85.3 3.60 10.7 –0.08 110 Note: VRCR denoted residual nutrient transport out of the Zhujiang River Estuary; VXCX: mixing exchange flux of nutrients. Nutrient fluxes from the Zhujiang River Estuary to the South China Sea (seaward export flux) were estimated as the sum of the VRCR and VXCX. Positive and negative values of ∆ indicate that other processes may magnify or reduce the riverine fluxes.
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