Modeling the impact of river discharge and wind on the hypoxia off Yangtze Estuary

Jingjing Zheng,Xueming Zhu,Guimei Liu,Shan Gao,Hui Wang

doi:10.5194/nhess-16-2559-2016

Jingjing Zheng, Xueming Zhu + Show 3 more

Open Access

https://doi.org/10.5194/nhess-16-2559-2016

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Abstract

Abstract. The phenomenon of low dissolved oxygen (known as hypoxia) in a coastal ocean system is closely related to a combination of anthropogenic and natural factors. Marine hypoxia occurs in the Yangtze Estuary, China, with high frequency and long persistence. It is related primarily to organic and nutrient enrichment influenced by river discharges and physical factors, such as water mixing. In this paper, a three-dimensional hydrodynamic model was coupled to a biological model to simulate and analyze the ecological system of the East China Sea. By comparing with the observation data, the model results can reasonably capture the physical and biochemical dynamics of the Yangtze Estuary. In addition, the sensitive experiments were also used to examine the role of physical forcing (river discharge, wind speed, wind direction) in controlling hypoxia in waters adjacent to the Yangtze Estuary. The results showed that the wind field and river discharge have significant impact on the hypoxia off the Yangtze Estuary. The seasonal cycle of hypoxia was relatively insensitive to synoptic variability in the river discharge, but integrated hypoxic areas were sensitive to the whole magnitude of river discharge. Increasing the river discharge was shown to increase hypoxic areas, while decreasing the river discharge tended to decrease hypoxic areas. The variations of wind speed and direction had a great impact on the integrated hypoxic areas.

Highlights

In recent decades, the eutrophication of waterbodies driven by excess nutrient loads from lands due to human activities has increased year by year, which has led to an enhancement of hypoxic zone (Murphy et al, 2011; Rabouille et al, 2008)
The model results and some observational data in the East China Sea were available to validate the model. These include monthly climatological values for sea surface temperature (SST) and sea surface salinity (SSS) with 1/4◦ spatial resolution from Generalized Digital Environment Model (GDEM), sea surface chlorophyll and SST with a spatial resolution of 4 km from MODIS in August 2011, and in situ biochemical data obtained from a summer cruise in August 2011
A three-dimensional coupled physical– biological model was used to analyze the hypoxia off the Yangtze Estuary

Summary

Introduction

The eutrophication of waterbodies driven by excess nutrient loads from lands due to human activities has increased year by year, which has led to an enhancement of hypoxic zone (Murphy et al, 2011; Rabouille et al, 2008). When DO is less than 2.0 mg L−1, the majority of marine aquatic organisms will die, especially benthic animals (Karlson et al, 2002). Hypoxia is one of the most severe environmental issues affecting estuarine and coastal marine ecosystems around the world. Hypoxia can reduce the diversity of marine species, change the community structure of marine organisms, reduce the richness of fish and benthic animals, and affect the fishery production and bring about direct or indirect economic loss (Yin et al, 2004). The hypoxia off Yangtze Estuary was first found in 1959 (Gu, 1980). With the increase of global warming and pollutant emissions, the hypoxic area off Yangtze Estuary expanded fast and became one of world’s largest coastal hypoxia (Vaquer-Sunyer and Duarte, 2008). In the 1950s, the occurrence probability of hypoxia off Yangtze Estuary in summer was 60 %, while after the 1990s hypoxia occurrence probability reached 90 %, and hypoxic areas which were greater than 5000 km basically occurred after the end of 1990s (Wang, 2009)

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