Impact of urban effluents on summer hypoxia in the highly turbid Gironde Estuary, applying a 3D model coupling hydrodynamics, sediment transport and biogeochemical processes

Abstract

Estuaries are increasingly degraded due to coastal urban development and are prone to hypoxia problems. The macro-tidal Gironde Estuary is characterized by a highly concentrated turbidity maximum zone (TMZ). Field observations show that hypoxia occurs in summer in the TMZ at low river flow and a few days after the spring tide peak. In situ data highlight lower dissolved oxygen (DO) concentrations around the city of Bordeaux, located in the upper estuary. Interactions between multiple factors limit the understanding of the processes controlling the dynamics of hypoxia. Highlights ► A biogeochemical model coupled with a 3D hydrodynamics and sediment transport model was implemented in the Gironde Estuary, accounting for realistic watershed and urban inputs of POC. ► The simulated DO concentrations showed good agreement with field observations and satisfactorily reproduced the seasonal and neap-spring time scale variations around the city of Bordeaux, including summer hypoxia in the upper estuary ► A reduction of POC from sewage overflows would increase the DO minimum in the vicinity of Bordeaux by 3% of saturation, and omitting all wastewater discharge DO would improve by 10% of saturation and mitigate hypoxic events. Abbreviations  DO, dissolved oxygen;  DOC, dissolved organic carbon;  DOM, dissolved organic matter;  NT, neap tide;  POC, particulate organic carbon;  POM, particulate organic matter;  SSC, suspended sediment concentration;  SO, sewage overflow;  ST, spring tide;  TMZ, turbidity maximum zone;  TOC, total organic carbon;

Major fertilization sources and mechanisms for Mediterranean Sea coastal ecosystems

“…We are also missing potentially important pollution sources in the form of big coastal cities delivering waste and rain water directly to the sea. Such point sources of pollution have been shown to greatly modify production seasonality and nutrients ratios (e.g., Romero et al ) and to contribute to bottom hypoxic conditions nearby the sewage areas (e.g., Lajaunie‐Salla et al ). We also lack the direct input of OM by the river waters, which, in reaching the bottom, could contribute to oxygen depletion.…”

Modeling Mud and Sand Transfers Between a Macrotidal Estuary and the Continental Shelf: Influence of the Sediment Transport Parameterization

“…To account for the influence of salinity on flocculation, a dependency between the mud settling velocity and salinity ( S ) was computed (Migniot, 1968; Mikes et al, 2004; Thill et al, 2001). This effect was previously accounted for by Lajaunie‐Salla et al (2017) in the Gironde estuarine system and largely contributed to simulate high SSC levels and maintain the ETM in the tidal rivers. A critical salinity S cr of 5 psu was chosen, below which settling velocity decreases with salinity, as follows: …”

Modeling Summer Hypoxia Spatial Distribution and Fish Habitat Volume in Artificial Estuarine Waterway

“…They showed that the area exhibiting hypoxia was greatly affected by wind speed and direction, but was insensitive to river discharge. Lajaunie-Salla et al [13] also applied a modeling method to assess the impact of urban wastewater treatment plants (WWTPs) and sewage overflows (SOs) on hypoxia in the Gironde Estuary, France. The modeling results revealed that particulate organic carbon (POC) from urban waste was resuspended during the spring tide, thereby expanding the hypoxia area.…”

“…Those researchers also found that hypoxic events could be mitigated by blocking the POC from the WWTPs and SOs. Overall, the findings to date can be summarized as follows: DO depletion of an estuarine area is mainly caused by excessive nutrients and organic matter, and the formation of a hypoxia zone is closely related to deteriorated circulation due to stratification and upstream freshwater inflow [10][11][12][13][14][15][16][17][18]. In addition, the spatial distribution of hypoxia in a small estuarine area is affected by river inflow [10,11,13], while that in a large estuarine area is influenced by wind [12,[14][15][16][17][18].…”

“…Overall, the findings to date can be summarized as follows: DO depletion of an estuarine area is mainly caused by excessive nutrients and organic matter, and the formation of a hypoxia zone is closely related to deteriorated circulation due to stratification and upstream freshwater inflow [10][11][12][13][14][15][16][17][18]. In addition, the spatial distribution of hypoxia in a small estuarine area is affected by river inflow [10,11,13], while that in a large estuarine area is influenced by wind [12,[14][15][16][17][18]. However, although diverse modeling studies have considered hypoxia in estuaries [10][11][12][13][14][15][16][17][18], and the impact of hypoxia on fish has been investigated in various ways [19][20][21][22][23][24], few studies have attempted to assess the spatial distribution of hypoxia in connection with fish habitats.…”