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

Huang Huang,Hu Hu,Liang Liang,Liu Liu,Xu Xu,Wang Wang,Li Li

doi:10.3390/w11102080

Abstract

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.

Highlights

As a result of intensified human activities, many coastal and estuarine waters worldwide have received a large amount of nutrients and pollutants [1,2]
We used a hydrodynamic-biogeochemical model to examine the impacts of river discharge, winds and tides on hypoxia in the Pearl River estuary (PRE) and identify the key processes controlling the dissolved oxygen (DO) dynamics based on model sensitivity experiments
The results showed that summertime hypoxia was greatly affected by river discharge

Summary

Introduction

As a result of intensified human activities, many coastal and estuarine waters worldwide have received a large amount of nutrients and pollutants [1,2]. Persistent hypoxia has detrimental effects on the aquatic organisms and leads to major losses in biodiversity [8] This has become a serious threat to aquatic ecosystems globally in coastal and estuarine waters, where oxygen depletion has been increasing rapidly over the past several decades [9]. Biochemical processes such as the photosynthesis, nitrification, and degradation of organic matter, directly contribute to the production and consumption of DO [12] Physical factors such as river discharge, winds, and tides can strengthen or weaken water column stratification, which affect the oxygen exchange between upper and lower waters and the formation and disruption of hypoxia [13,14]. Hydrodynamic–biogeochemical models are widely used to investigate the mechanisms controlling hypoxia [15,16,17]

Methods

Results

Discussion

Conclusion