Li Zhang, Yumin Yang, Weihong He, Jie Xu, Ruihuan Li. Fluxes of riverine nutrient to the Pearl River Estuary and its potential eutrophication effect[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1919-7
Citation:
Li Zhang, Yumin Yang, Weihong He, Jie Xu, Ruihuan Li. Fluxes of riverine nutrient to the Pearl River Estuary and its potential eutrophication effect[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1919-7
Li Zhang, Yumin Yang, Weihong He, Jie Xu, Ruihuan Li. Fluxes of riverine nutrient to the Pearl River Estuary and its potential eutrophication effect[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1919-7
Citation:
Li Zhang, Yumin Yang, Weihong He, Jie Xu, Ruihuan Li. Fluxes of riverine nutrient to the Pearl River Estuary and its potential eutrophication effect[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1919-7
Marine Environmental Testing Center, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
2.
State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
3.
Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), 511458, China
4.
Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China
5.
Guangdong Environmental Monitoring Center, Guangzhou 510308, China
6.
College of Marine Science, University of Chinese Academy of Sciences, Qingdao 100049, China
Funds:
Special Project for Marine Economic Development (Six Major Marine Industries) of Guangdong Province under contract No. GDNRC[2020]064; Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) under contract Nos GML2019ZD0303, GML2019ZD0305 and GML2019ZD0402; Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences under contract Nos ISEE2019ZR02 and ISEE2019ZR03; National Natural Science Foundation of China under contract Nos 41676075 and 41706085; Department of Science and Technology of Guangdong Province under contract No. 2018B030320005.
The Pearl River Estuary is becoming eutrophic due to the impact of anthropogenic activities in the past decades. To understand nutrient dynamics and fluxes to Lingdingyang via four outlets (Humen, Jiaomen, Hongqimen and Hengmen), we investigated the spatial distribution and seasonal variation of dissolved nutrients in the Pearl 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 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 Lingdingyang most resulted through Humen Outlet. The estuarine export fluxes of DIN out of the Pearl 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 Pearl River Estuary, eutrophication effect was not as severe as expected in the Pearl River Estuary, since the light limitation restricted the utilization of nutrients by phytoplankton.
Figure 1. Location of sampling stations in the Pearl River Estuary during March 2015–October 2017. The solid circles denoted the sampling stations (1–20) during March 2015–October 2017 and triangles represented sampling stations (21–34) in May and August 2015. PRE, HM, JM, HQM and HeM represented the Pearl River Estuary, Humen, Jiaomen, Hongqimen and Hengmen, respectively. Dashed line A was the boundary of riverine flux to Lingdingyang 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 Pearl 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 Pearl River Estuary during March 2015−October 2017. Vertical bars denoted standard deviation errors.
Figure 4. Spatial and temporal variations in the surface ${\rm{NO}}_3^- $, ${\rm{NO}}_2^- $, ${\rm{NH}}_4^+ $/, DIN, DIP and DSi in the Pearl 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 molar ratios of DIN:DIP, DSi:DIN and DSi:DIN at the 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 Pearl River Estuary at the surface and the 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) at the surface during March 2015–October 2017. The solid line in (b) represented the value of DO = 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 Lingdingyang via four outlets and the river discharge at the surface.
Figure 9. The relationship of riverine nutrient fluxes at the 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 surface at Stations 1–20 during March 2015–2017. 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 Pearl River Estuary. The water flux is in 106 m/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.
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