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

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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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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Seasonal Variation of Dissolved Oxygen in the Southeast of the Pearl River Estuary

Water ◽

10.3390/w12092475 ◽

2020 ◽

Vol 12 (9) ◽

pp. 2475

Author(s):

Guangping Liu ◽

Weihong He ◽

Shuqun Cai

Keyword(s):

Dissolved Oxygen ◽

River Estuary ◽

Pearl River Estuary ◽

Empirical Orthogonal Functions ◽

Pearl River ◽

The Pearl River Estuary ◽

Main Period ◽

Do Concentration ◽

The Pearl River ◽

Bottom Hypoxia

Dissolved oxygen (DO) concentration in estuaries is highly variable at different spatial and temporal scales, which is affected by physical, chemical and biological processes. This study analyzed the spatial–temporal distributions of dissolved oxygen concentration and bottom hypoxia in the southeast of the Pearl River Estuary (PRE) using monthly water quality monitoring and hydrographic data covering the period 2000–2017. The seasonal spatial–temporal variation of DO concentration was studied using various methods, such as rotated empirical orthogonal functions, harmonic analysis, and correlation analysis. The results showed that DO stratification was significant in summer, but it was not distinct in winter, during which DO concentration peaked. DO stratification exhibited a significantly positive correlation with water stratification. In the south and west of Hong Kong (SHK and WHK, respectively), DO concentration fields exhibited distinct seasonal changes in the recent 18 years. In SHK, the main periods of the surface DO variation were 24, 12, and 6 months, whereas the main period was 12 months in WHK. The main period of the bottom DO variation was 12 months in both SHK and WHK. In SHK, the spatial–temporal variations in surface and bottom DO were highly related to the variations of salinity, dissolved inorganic nitrogen (DIN), and active phosphorus, and the variation of surface DO was also connected to the variation of temperature and chlorophyll a. In WHK, the variations in surface and bottom DO were highly related to the variations of salinity and temperature, and the variation of surface DO was also connected to the variation of DIN. The river discharge and wind had a different important influence on the temporal variability of DO in WHK and SHK. These findings suggested that the variations of DO may be controlled by coupled physical and biochemical processes in the southeast of PRE. From 2000 to 2017, bottom hypoxia in the southeast of PRE occurred in the summers of 7 years. SHK appeared to be more vulnerable to hypoxia than WHK.

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

Biogeosciences ◽

10.5194/bg-15-6105-2018 ◽

2018 ◽

Vol 15 (20) ◽

pp. 6105-6125 ◽

Cited By ~ 6

Author(s):

Bin Wang ◽

Jiatang Hu ◽

Shiyu Li ◽

Liuqian Yu ◽

Jia Huang

Keyword(s):

Organic Carbon ◽

Particulate Organic Carbon ◽

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

The Pearl River Estuary ◽

Concentration Changes ◽

The Pearl River ◽

Oxygen Dynamics ◽

Source And Sink

Abstract. In summer, the Pearl River estuary (PRE) experiences hypoxia, largely driven by the high input of fresh water with low dissolved oxygen (DO), abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated physical–biogeochemical model together with a DO species-tracing method to study the responses of hypoxia and oxygen dynamics to the anthropogenic perturbations of riverine inputs (i.e. DO, nutrients, and particulate organic carbon) in July–August 2006. Model results showed that hypoxia in the PRE 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 changes in DO concentration. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration (DO fluxes across the air–sea interface); i.e. the re-aeration responded to the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The PRE features shallow waters (with averaged depth of 10 m) in which oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion and largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in reducing hypoxia variability in shallow estuaries.

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Effects of climate change and fishing on the Pearl River Estuary ecosystem and fisheries

Reviews in Fish Biology and Fisheries ◽

10.1007/s11160-019-09574-y ◽

2019 ◽

Vol 29 (4) ◽

pp. 861-875

Author(s):

Zeyu Zeng ◽

William W. L. Cheung ◽

Shiyu Li ◽

Jiatang Hu ◽

Ying Wang

Keyword(s):

Climate Change ◽

River Estuary ◽

Pearl River Estuary ◽

Pearl River ◽

The Pearl River Estuary ◽

The Pearl River

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The Role of Environmental Factors on the Fishery Catch of the Squid Uroteuthis chinensis in the Pearl River Estuary, China

Journal of Marine Science and Engineering ◽

10.3390/jmse9020131 ◽

2021 ◽

Vol 9 (2) ◽

pp. 131

Author(s):

Dongliang Wang ◽

Lijun Yao ◽

Jing Yu ◽

Pimao Chen

Keyword(s):

River Estuary ◽

Pearl River Estuary ◽

Sea Surface ◽

Spatiotemporal Variability ◽

Pearl River ◽

Coastal Current ◽

Catch Per Unit Effort ◽

Chl A ◽

The Pearl River Estuary ◽

The Pearl River

The Pearl River Estuary (PRE) is one of the major fishing grounds for the squid Uroteuthis chinensis. Taking that into consideration, this study analyzes the environmental effects on the spatiotemporal variability of U. chinensis in the PRE, on the basis of the Generalized Additive Model (GAM) and Clustering Fishing Tactics (CFT), using satellite and in situ observations. Results show that 63.1% of the total variation in U. chinensis Catch Per Unit Effort (CPUE) in the PRE could be explained by looking into outside factors. The most important one was the interaction of sea surface temperature (SST) and month, with a contribution of 26.7%, followed by the interaction effect of depth and month, fishermen’s fishing tactics, sea surface salinity (SSS), chlorophyll a concentration (Chl a), and year, with contributions of 12.8%, 8.5%, 7.7%, 4.0%, and 3.1%, respectively. In summary, U. chinensis in the PRE was mainly distributed over areas with an SST of 22–29 °C, SSS of 32.5–34‰, Chl a of 0–0.3 mg × m−3, and water depth of 40–140 m. The distribution of U. chinensis in the PRE was affected by the western Guangdong coastal current, distribution of marine primary productivity, and variation of habitat conditions. Lower stock of U. chinensis in the PRE was connected with La Niña in 2008.

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