Abstract
Dissolved oxygen (DO) is one of the most important factors for maintaining a healthy marine ecosystem. The information of DO in large estuaries or bays with large entrances has been widely studied, while it is relatively limited for a bay with a narrow entrance which is vulnerable to human activities. The Zhanjiang Bay, located in the northwestern South China Sea, has a very narrow entrance and suffers from strong anthropogenic activities and obvious seasonal variations in environmental parameters. In this study, we analyzed the spatial and monthly variations of DO, apparent oxygen utilization (AOU), percent oxygen saturation (DO-saturation), and related environmental parameters in the surface water of Zhanjiang Bay to find out the factors controlling the dynamics of DO. Different from many other coastal ecosystems, DO concentrations in the Zhanjiang Bay reached minimum values in late spring and early autumn. The phytoplankton bloom in summer months, which was related to the high concentrations of nutrients brought by rainfall-induced terrestrial inputs, contributed to that phenomenon. Though high chlorophyll a (Chl a) concentrations were observed in both the summer months and December, the DO-saturation values were relatively low and AOU values were relatively high in summer months. Rainfall-induced terrestrial discharge in summer months, which had high concentrations of chemical oxygen demand, contributed much to that phenomenon. The average DO concentrations and DO-saturation values in a hydrological year decreased seaward, and AOU values increased seaward, indicating the anthropogenic influence from terrestrial input. The highest annual average Chl a concentration, relatively high annual average DO-saturation value and relatively low annual AOU value were observed near the Donghai Dam. This indicated that the construction of Donghai Dam has significant influences on the environment of Zhanjiang Bay.
Highlights
Dissolved oxygen (DO) is essential for the survival of almost all aquatic organisms
In the surface water of Zhanjiang Bay (ZJB), there was a gradual increase of annual average DO and DO-saturation, and a gradual decrease in annual average NH4-N, NO3-N, NO2-N, PO4-P, SiO3-Si, chemical oxygen demand (COD) and apparent oxygen utilization (AOU) from the inner bay to the bay mouth
Heavy rainfall can lead to increased land runoff, which was responsible for the low DO-saturation values in the outer bay in July and September
Summary
Dissolved oxygen (DO) is essential for the survival of almost all aquatic organisms It is one of the most important factors for maintaining a healthy marine ecosystem [1,2]. Besides the influencing factors of DO for the open sea, river discharge, anthropogenic activities, geomorphologic features and environment changes make the variations of DO in coastal waters more complex [3,4,6]. In the Deep Bay, Hong Kong, low DO concentrations were observed in summer when the water temperature was high [7]. The DO concentrations for the water of Sanya Bay, Hainan showed a different seasonal variation pattern from that of the Deep Bay [6]. Xu et al [6] found that the lowest DO concentration in the Sanya Bay appeared in autumn, which was related to the upwelling in this area
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