A numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the Pearl River Estuary

Abstract. As an important biogeochemical indicator of aquatic ecosystem, dissolved oxygen (DO) is affected by the boundary conditions and biogeochemical processes. Biogeochemical processes can affect DO concentrations by directly consuming or generating oxygen locally, or through changing the DO fluxes from the ambient water bodies. However, the latter mechanism is still unclear. In this study, a novel method named physical modulation of biogeochemical terms is therefore proposed and coupled to a physical-biogeochemical model to investigate their contributions to the hypoxia during the summer of the Pearl River Estuary (PRE). According to the result of modulation method, re-aeration and sediment oxygen demand are the most important biogeochemical processes, and determine the distribution, the spatial extent, and the duration of hypoxia in the PRE. A DO balance analysis is conducted and reveals that although the re-aeration occurs on the air-sea interface, the reoxygenation leads to a strong DO gradient form between the surface and lower layers. As a result, the majority (89 %) of oxygen entering the surface layer from the atmosphere will be transported to the lower layer through the vertical diffusion, and 28 % eventually reach the bottom layer. Similarly, after consuming the bottom DO, sediment oxygen demand facilitates the downward DO flux of vertical diffusion and decreases the upward DO flux of vertical advection. Under the modulation of physical processes, sediment oxygen demand causes a most significant decrease in DO concentration by 4.31 mg L−1 in the bottom of the HFZ (a high frequency zone of hypoxia located off the Modaomen sub-estuary) and the west of lower estuary. However, the re-aeration supplements an average of 4.84 mg L−1 DO on the west of lower estuary, which leads to hypoxia only occur in HFZ. Numerical experiments show that turning off the re-aeration leads to an expansion of hypoxic area from 237 km2 to 2203 km2 and results in a shift of hypoxic center to the west of lower estuary. Moreover, a persistent hypoxia (hypoxic frequency > 80 %) is observed in the west of lower estuary. When compared with re-aeration and sediment oxygen demand, photosynthesis and water column respiration have fewer effects on DO conditions. In the bottom of the HFZ, photosynthesis exceeds the water column respiration and eventually supplements DO concentration by 0.98 mg L−1, causing an increase of hypoxic area to 591 km2.

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Effects of Physical Forcing on Summertime Hypoxia and Oxygen Dynamics in the Pearl River Estuary

Water ◽

10.3390/w11102080 ◽

2019 ◽

Vol 11 (10) ◽

pp. 2080 ◽

Cited By ~ 1

Author(s):

Huang ◽

Hu ◽

Li ◽

Wang ◽

Xu ◽

...

Keyword(s):

River Discharge ◽

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

Vertical Diffusion ◽

Physical Forcing ◽

The Pearl River Estuary ◽

Hypoxic Area ◽

Water Stratification ◽

The Pearl River

A validated hydrodynamic-biogeochemical model was applied to investigate the effects of physical forcing (i.e., river discharge, winds, and tides) on the summertime dissolved oxygen (DO) dynamics and hypoxia (DO < 3 mg L−1) in the Pearl River estuary (PRE), based on a suite of model sensitivity experiments. Compared with the base model run in 2006 (a wet year), the simulated hypoxic area in the moderate year (with 75% of river discharge of the base run) and the dry year scenario (with 50% of river discharge of the base run) was reduced by ~30% and ~60%, respectively. This is because under the lower river discharge levels, less particulate organic matter was delivered to the estuary that subsequently alleviated the oxygen demand at the water–sediment interface, and in the meantime, the water stratification strength was decreased, which facilitated the vertical diffusion of DO. Regarding the effect of winds, the highly varying and intermittent strong winds had a significant impact on the replenishment of bottom DO by disrupting water stratification and thus inhibiting the development of hypoxia. Sensitivity experiments showed that the hypoxic area and volume were both remarkably increased in the low wind scenario (with a bottom hypoxic zone extending from the Modaomen sub-estuary to the western shoal in Lingdingyang Bay), whereas hypoxia was almost absent in the strong wind scenario. The DO budget indicated that winds altered the bottom DO mostly by affecting the DO flux due to vertical diffusion and horizontal advection, and had a limited influence on the DO consumption processes. Moreover, the DO concentration exhibited remarkable fluctuations over the spring-neap tidal cycles due to the significant differences in vertical diffusion. The results of a tide-sensitivity experiment indicated that without tide forcing, most of the shallow areas (average water depth < 5 m) in the PRE experienced severe and persistent hypoxia. The tides mainly enhanced mixing in the shallow areas, which led to higher vertical diffusion and enhanced replenishment of bottom DO.

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Impacts of anthropogenic inputs on the hypoxia and oxygen dynamics in the Pearl River Estuary

10.5194/bg-2018-134 ◽

2018 ◽

Author(s):

Bin Wang ◽

Jiatang Hu ◽

Shiyu Li ◽

Liuqian Yu ◽

Jia Huang

Keyword(s):

Organic Carbon ◽

Particulate Organic Carbon ◽

River Estuary ◽

Oxygen Demand ◽

Pearl River Estuary ◽

Sediment Oxygen Demand ◽

Pearl River ◽

The Pearl River Estuary ◽

Anthropogenic Inputs ◽

The Pearl River

Abstract. In summer, the Pearl River Estuary experiences hypoxia, largely driven by the high input of freshwater with low dissolved oxygen (DO) and abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated coupled physical-biogeochemical model to study the response of hypoxia and oxygen dynamics to variations of anthropogenic inputs (i.e. DO, nutrients, and particulate organic carbon). Model results showed that hypoxia in the Pearl River Estuary was confined to the shelf off the Modaomen sub-estuary with a hypoxic area of ~ 200 km2 mainly due to the combined effect of re-aeration and sediment oxygen demand. Numerical experiments suggested that hypoxia in the Pearl River Estuary was most sensitive to riverine inputs of particulate organic carbon , followed by DO concentrations and nutrients. Specifically, a 50 % decrease (increase) in riverine input of particulate organic carbon led to a 47 % decrease (64 % increase) in hypoxic area, with the sediment oxygen demand and water column production being the two most important processes contributing to the changes in DO concentration and hypoxic extent. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration, i.e. the re-aeration compensated the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The Pearl River Estuary features shallow waters (with averaged depth of 10 m) where oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion that largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in determining the hypoxic extent and the buffering effects of re-aeration in reducing hypoxia variability in shallow estuary.

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A numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the Pearl River estuary

Biogeosciences ◽

10.5194/bg-14-2979-2017 ◽

2017 ◽

Vol 14 (12) ◽

pp. 2979-2999 ◽

Cited By ~ 12

Author(s):

Bin Wang ◽

Jiatang Hu ◽

Shiyu Li ◽

Dehong Liu

Keyword(s):

River Estuary ◽

Pearl River Estuary ◽

Modulation Method ◽

Pearl River ◽

The West ◽

The Pearl River Estuary ◽

Hypoxic Area ◽

The Pearl River ◽

Bottom Hypoxia ◽

Source And Sink

Abstract. A three-dimensional (3-D) physical–biogeochemical coupled model was applied to explore the mechanisms controlling the dissolved oxygen (DO) dynamics and bottom hypoxia during summer in the Pearl River estuary (PRE). By using the numerical oxygen tracers, we proposed a new method (namely the physical modulation method) to quantify the contributions of boundary conditions and each source and sink process occurring in local and adjacent waters to the DO conditions. A mass balance analysis of DO based on the physical modulation method indicated that the DO conditions at the bottom layer were mainly controlled by the source and sink processes, among which the sediment oxygen demand (SOD) at the water–sediment interface and the re-aeration at the air–sea interface were the two primary processes determining the spatial extent and duration of bottom hypoxia in the PRE. The SOD could cause a significant decrease in the bottom DO concentrations (averaged over July–August 2006) by over 4 mg L−1 on the shelf off the Modaomen sub-estuary, leading to the formation of a high-frequency zone of hypoxia (HFZ). However, the hypoxia that occurred in the HFZ was intermittent and distributed in a small area due to the combined effects of re-aeration and photosynthesis, which behaved as sources for DO and offset a portion of the DO consumed by SOD. The bottom DO concentrations to the west of the lower Lingdingyang Bay (i.e. the western shoal near Qi'ao Island) were also largely affected by high SOD, but there was no hypoxia occurring there because of the influence of re-aeration. Specifically, re-aeration could lead to an increase in the bottom DO concentrations by ∼ 4.8 mg L−1 to the west of the lower Lingdingyang Bay. The re-aeration led to a strong vertical DO gradient between the surface and the lower layers. As a result, the majority (∼ 89 %) of DO supplemented by re-aeration was transported to the lower layers through vertical diffusion and ∼ 28 % reached the bottom eventually. Additional numerical experiments showed that turning off re-aeration could lead to an expansion of the hypoxic area from 237 to 2203 km2 and result in persistent hypoxia (hypoxic frequency >  80 %) to the west of the lower Lingdingyang Bay. Compared to re-aeration and SOD, photosynthesis and water column respiration had relatively small impacts on the DO conditions; turning off these two processes increased the hypoxic area to 591 km2. In summary, our study explicitly elucidated the interactive impacts of physical and biogeochemical processes on the DO dynamics in the PRE, which is critical to understanding hypoxia in this shallow and river-dominated estuarine system.

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Distribution of Coomassie Blue Stainable Particles in the Pearl River Estuary, China, Insight Into the Nitrogen Cycling in Estuarine System

Frontiers in Marine Science ◽

10.3389/fmars.2021.733240 ◽

2022 ◽

Vol 8 ◽

Author(s):

Cui-Ci Sun ◽

Wei-Zhong Yue ◽

You-Shao Wang ◽

Wei-Hong He ◽

Yi-Guo Hong ◽

...

Keyword(s):

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

Phytoplankton Production ◽

Estuarine System ◽

Lower Estuary ◽

The Pearl River Estuary ◽

Coomassie Blue ◽

The Pearl River ◽

Insight Into

Distributions of Coomassie Blue stainable particles (CSP), the sources and transports, as well as their implications for nitrogen biogeochemical cycles in the Pearl River estuary (PRE) were investigated during two cruises in August 2016 and January 2017. CSPcolor concentrations (CSP concentration determined spectrophotometrically) were 73.7–685.3 μg BSA eq L–1 [μg Bovine serum albumin (BSA) equivalent liter–1] in August 2016 and 100.6–396.4 μg BSA eq L–1 in January 2017, respectively. CSP concentrations were high in low-salinity waters (<5), and declined from the river to the middle estuary by 80% in the wet season and 55.6% in the dry season, respectively, then increased again in the lower estuary due to high primary production. CSP concentrations were mainly associated with chlorophyll a (Chl a) concentration except for the turbid mixing zone, suggesting that autochthonous phytoplankton production served as the primary source of CSP in the PRE. The concentrations of nitrogen (N) as CSP in the PRE were comparable to the nitrogen content of particulate hydrolysable amino acids (PHAA). Pictures of CSP taken by microscopy and the correlation between composition of PHAA and the ratio of Chl a/CSPcolor showed that CSP were relatively degraded due to delivery of old terrestrial protein to river section and extensive microbial degradation during mixing at the upper and middle parts of the estuary, whereas CSP in lower estuary appeared to be more labile due to higher fresh algal production. The contribution of CSP nitrogen (CSP-N) to the particulate nitrogen (PN) pool was 34.98% in summer and 30.8% in winter. The conservative estimate of CSP-N input flux in the Pearl River Delta was about 6 × 106 mol N d–1. These results suggested that CSP was a significant pool of organic nitrogen in the PRE. The study of CSP composition in terms of nitrogen provides new insight into the roles of CSP on nitrogen biogeochemical processes in the turbid and productive estuarine system.

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Control survey for a 6.7 km immersed tunnel in Chinese Lingding ocean

Journal of Applied Geodesy ◽

10.1515/jag-2018-0023 ◽

2019 ◽

Vol 13 (3) ◽

pp. 257-265 ◽

Cited By ~ 2

Author(s):

Guanqing Li ◽

Shengxiang Huang

Keyword(s):

Hong Kong ◽

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

The West ◽

Control Networks ◽

Surface Control ◽

Immersed Tunnel ◽

Control Survey ◽

The Pearl River

AbstractThe immersed tunnel which is composed of elements has drawn more attention nowadays because of new advancements and developments. The elements are prefabricated somewhere else and floated to the tunnel site to be sunk into the prepared trench. Each element must line up exactly for the watertight gaskets to seal properly. The HZM immersed tunnel, a key part of the Hong Kong-Zhuhai-Macao Bridge (HZMB) that crosses the Pearl River Estuary and links Hong Kong to the east, and Zhuhai and Macao to the west, is 6.7 km long, one of the longest immersed tunnel ever realized in the world. For the construction of such a mega immersed tunnel, particular care should be taken in the perspective of geodetic control. This paper described the design and implementation of the geodetic basis and hierarchical surface control networks. And to decrease the influence of lateral refraction and to improve the configuration strength and increase the number of redundant observation compared with traverse, the design of underground surveying network named duo-linear joint chain and the results of gyro checks are presented. In the end HZM immersed tunnel surveying data is analyzed and the performance of the control networks is demonstrated.

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Assessment of Water Quality Evolution in the Pearl River Estuary (South Guangzhou) from 2008 to 2017

Water ◽

10.3390/w12010059 ◽

2019 ◽

Vol 12 (1) ◽

pp. 59 ◽

Cited By ~ 1

Author(s):

Yanping Zhao ◽

Yumei Song ◽

Jinli Cui ◽

Shuchai Gan ◽

Xi Yang ◽

...

Keyword(s):

Water Quality ◽

Monitoring Program ◽

River Estuary ◽

Oxygen Demand ◽

Pearl River Estuary ◽

Control Measures ◽

Pearl River ◽

Estuarine Water ◽

The Pearl River Estuary ◽

The Pearl River

To control the water pollution in the Pearl River Estuary (PRE), a series of measures have been enacted in recent years. The efficacy of these measures on water quality improvement is, however, currently unknown. To evaluate the variation of water quality in response to the pollution control measures in the PRE during the last decade (2008–2017), our study conducted a long-term monitoring program of estuarine water in the representative city Guangzhou that targeted fecal coliform (F. Coli), biochemical oxygen demand (BOD5), chemical oxygen demand (CODCr), potassium permanganate index (CODMn), petroleum, total nitrogen (TN), ammonia nitrogen (NH3–N) and total phosphorus (TP). In the last decade, F. Coli, BOD5, CODCr and CODMn, petroleum and NH3–N have shown a significant reduction by 78.8%, 50.9%, 37.5%, 18.9%, 75.0% and 25.0%, respectively. In contrast, TN and TP remained stable. Water quality index calculations indicated that the water quality was elevated from the marginal–fair level to the good level, particularly after 2012. The biochemical pollutants and nutrients in the estuarine water most likely originated from the upper river due to the wastewater discharge, fecal pollution and agricultural input. The success of pollutant reduction could thus be attributed to industrial upgrading and relocation, as well as the improvement of the sewage treatment system in Guangzhou. However, efficient approaches to reduce TN pollution should be implemented in the future.

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Characteristics of Absorption Spectra of Chromophoric Dissolved Organic Matter in the Pearl River Estuary in Spring

Remote Sensing ◽

10.3390/rs11131533 ◽

2019 ◽

Vol 11 (13) ◽

pp. 1533 ◽

Cited By ~ 3

Author(s):

Xia Lei ◽

Jiayi Pan ◽

Adam Devlin

Keyword(s):

Organic Matter ◽

Organic Compounds ◽

River Estuary ◽

Molecular Size ◽

Pearl River Estuary ◽

Chromophoric Dissolved Organic Matter ◽

Pearl River ◽

Lower Estuary ◽

The Pearl River ◽

Hong Kong Waters

In this study, absorption variation of chromophoric dissolved organic matter (CDOM) was investigated based on spectroscopic measurements of the water surface and bottom during a cruise survey on 2–12 May 2014 in the Pearl River Estuary (PRE). Multiple spectral signatures were utilized, including the absorption ratios E2/E3 (a(250)/a(365)) and E2/E4 (a(254)/a(436))) as well as the spectral slopes over multiple wavelength ranges. The horizontal variations of a(300), E2/E3, spectral slope (S) of Ultraviolet C (SUVC, 250–280 nm), Ultraviolet B (SUVB, 280–315 nm), and S275–295 (275–295 nm) were highly correlated, revealing that CDOM of terrigenous origin in the upper estuary contained chromophores of larger molecular size and weight, while the marine CDOM in the lower estuary comprised organic compounds of smaller molecular size and weight; the molecular size of surface CDOM was generally larger than that at the bottom. Results of Gaussian decomposition methods showed that CDOM in the middle estuary of terrigenous origin produced more Gaussian components per spectrum than those of marine origin in the lower estuary and the adjacent Hong Kong waters. The surface CDOM composition was more diverse than at the bottom, inferred by the finding that the average number of Gaussian components yielded per surface sample (5.44) was more than that of the bottom sample (4.8). A majority of components was centered below 350 nm, indicating that organic compounds with relatively simple structures are ubiquitous in the estuary. Components centered above 350 nm only showed high peaks at the head of the estuary, suggesting that terrigenous CDOM with chromophores in complex structures rapidly lose visible light absorptivity during its transport in the PRE. The relatively low and homogenous peak heights of the components in Hong Kong waters imply higher light stability and composition consistency of the marine CDOM compared with the terrigenous CDOM.

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Two social communities in the Pearl River Estuary population of Indo-Pacific humpback dolphins (Sousa chinensis)

Canadian Journal of Zoology ◽

10.1139/z2012-071 ◽

2012 ◽

Vol 90 (8) ◽

pp. 1031-1043 ◽

Cited By ~ 21

Author(s):

S.Z. Dungan ◽

S.K. Hung ◽

J.Y. Wang ◽

B.N. White

Keyword(s):

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

The West ◽

Social Communities ◽

The North ◽

Animal Populations ◽

Sousa Chinensis ◽

The Pearl River ◽

Northern Community

The way human activities impact animal populations can depend on social structure, which is important to understand in social species such as cetaceans. We investigated association patterns in Indo-Pacific humpback dolphins ( Sousa chinensis (Osbeck, 1765)) inhabiting the Pearl River Estuary near Lantau Island, Hong Kong, using a 10-year data set for 88 individuals. Our analyses revealed two social communities. Each had its own region of core use, to the north and to the west of the island, but their overall ranges partially overlapped northwest of Lantau. The northern community had a fission–fusion structure characterized by short-term associations, while the western community had more long-term associations. Mixed-community groups included calves more often than exclusive groups, so between-community associations may arise from common habitat usage, by females especially, in the overlap area. Recent range extensions by the northern community into the west are likely a response to habitat destruction north of Lantau. This suggests ease of movement between the north and the west is necessary for northern-community dolphins to access suitable habitat, and gives new concern to construction projects planned for the region. We emphasize our study as an example of how sociobiological information can be important in understanding human impacts on animal populations.

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A Real-Time Mathematical Model for Three-Dimensional Tidal Flow and Water Quality

Water Science & Technology ◽

10.2166/wst.1991.0154 ◽

1991 ◽

Vol 24 (6) ◽

pp. 171-177 ◽

Cited By ~ 1

Author(s):

Zeng Fantang ◽

Xu Zhencheng ◽

Chen Xiancheng

Keyword(s):

Mathematical Model ◽

Water Quality ◽

Real Time ◽

Control Volume ◽

Tidal Flow ◽

Three Dimensional ◽

River Estuary ◽

Pearl River Estuary ◽

Practical Application ◽

The Pearl River

A real-time mathematical model for three-dimensional tidal flow and water quality is presented in this paper. A control-volume-based difference method and a “power interpolation distribution” advocated by Patankar (1984) have been employed, and a concept of “separating the top-layer water” has been developed to solve the movable boundary problem. The model is unconditionally stable and convergent. Practical application of the model is illustrated by an example for the Pearl River Estuary.

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