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

Guangping Liu,Shuqun Cai,Weihong He

doi:10.3390/w12092475

Abstract

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.

Highlights

Oxygen (O2 ) is fundamental to life and a critical constraint on marine ecosystems [1,2].Dissolved oxygen (DO) concentration affects marine biogeochemical processes as well as the survival and distribution of marine organisms [3,4]
Surface DO concentration was higher in south of Hong Kong (SHK) than in WHK (Figure 5a)
Considering the correlation of rotated empirical orthogonal functions (REOF) and RPCs between DO and other environmental factors, in SHK, the spatial–temporal variations in surface and bottom DO were highly related to the variations of salinity, dissolved inorganic nitrogen (DIN), and PO4, and the variation of surface DO was connected to the variation of temperature and chlorophyll a (Chl a)

Summary

Introduction

Oxygen (O2 ) is fundamental to life and a critical constraint on marine ecosystems [1,2]. Dissolved oxygen (DO) concentration affects marine biogeochemical processes as well as the survival and distribution of marine organisms [3,4]. Under the combined actions of physical, chemical, and biological factors, the distribution of DO concentration shows obvious spatial–temporal differences in coastal and estuarine areas [5,6]. Water 2020, 12, 2475 the contents of DO are decreasing sharply, with increasing hypoxia in coastal and estuarine areas [2,4,7]. Hypoxia generally refers to a DO concentration below 2 mg/L [8,9,10], which is extremely hazardous to marine ecosystems [2,7,11]. The variations of DO and hypoxia have been documented in many coastal and estuarine systems worldwide such as the Gulf of Mexico, Chesapeake Bay and the Changjiang (Yangtze River) Estuary [2,12,13]

Objectives

Methods

Results

Conclusion