hypoxia in the highly turbid Gironde estuary, applying a 3D model coupling hydrodynamics, sediment transport and biogeochemical processes

Estuaries are increasingly degraded due to coastal urban development, and prone to hypoxia problems. The macro-tidal Gironde estuary is characterized by the presence of a highly concentrated Turbidity Maximum Zone (TMZ). The water quality monitoring data show hypoxia occurring in the TMZ, under conditions of low river flow, increasing water temperature and during the transition from spring to the neap tide. In-situ data also highlights that summer hypoxic is particularly pronounced around the city of Bordeaux, located in the upper estuary. Interactions between these multiple factors limit the understanding of processes controlling the dynamics of dissolved oxygen (DO).

In this study we developed a 3D biogeochemical model coupling hydrodynamics, sediment transport and biogeochemical processes, to assess the contribution of the TMZ and the impact of urban effluents through wastewater treatment plant (WWTP) and sewage overflow (SO) on the hypoxia. Our model describes the transport of solutes and suspended material, the major biogeochemical mechanisms impacting oxygen: primary production, degradation of natural and anthropogenic organic matter, nitrification, and surface exchange. The composition and the degradation rates of each variable have been characterized by in-situ measurements and from experimental data performed in the study area. The DO model was calibrated and validated against observations, and simulate DO dynamics at two times scales (seasonal and neap-spring time scale) in Bordeaux city.

The simulated time series of DO concentrations shows a good agreement with field observations and reproduces satisfactorily the seasonal and neap-spring time scale variations around the city of Bordeaux. Simulations show a strong spatial and temporal correlation between the formation of summer hypoxia and the location of the TMZ, with minimum DO centered in the vicinity of Bordeaux. To understand the contribution of anthropogenic forcing, we compare different simulations with the presence or absence of urban effluents. Our results show that a reduction of POC loads by eliminating SO would increase DO concentration minimum in the vicinity of Bordeaux by 3 % of saturation in summer; omitting both SO and WWTP discharge would improve DO concentrations in summer by 10 % of saturation and mitigate hypoxic events.

Hypoxia is a consequence of an increase in eutrophic areas induced by high concentrations of nutrients and organic substances delivered by rivers and human activities (Rabalais et al., 2010; Verity et al., 2006). This supply of material perturbs the biogeochemical cycle in aquatic systems. When oxygen consumption by community respiration is greater than the replenishment of DO by the atmosphere, vertical mixing or photosynthesis, DO depletion occurs (Conley et al., 2009). In temperate eutrophic estuaries, the input of nutrients and organic material intensifies primary production in spring and summer, producing organic matter in surface waters. Moreover, anthropogenic nutrient and organic matter enrichment intensifies hypoxic events by the rapid decay of labile organic matter during warmer months. Depending on the physical structure of estuarine waters, contrasting mechanisms drive the occurrence of hypoxia. In stratified estuaries, the settling of particulate organic matter and associated heterotrophic processes cause summertime DO depletion in bottom waters, as in Chesapeake Bay (Hagy et al., 2004) and the plume of the Yangtze River (Li et al., 2002). In contrast, in well-mixed estuaries, hypoxia is less likely to occur due to wind and tide mixing driving DO replenishment from the atmosphere. In turbid estuaries, photosynthesis is low and DO depletion can be caused by the degradation of organic matter, which is associated with the suspended sediment (Lanoux et al., 2013; Talke et al., 2009; Thouvenin et al., 1994).

The analysis of 7 years of DO data from the water quality monitoring of the Gironde Estuary (Etcheber et al., 2011) showed that summer hypoxia events occur in the TMZ coincident with lower river discharge and higher water temperature. Moreover, DO minima occur a few days after the spring tide (ST) peak, with lower concentrations near the city of Bordeaux in the upper estuary (Lanoux et al., 2013). Various factors explain the hypoxia events in the Gironde Estuary, including temperature, river flow, turbidity and urban effluents, but we do not know the relative contribution of each factor. Increasing turbidity, water temperature and urban discharge, together with decreased flushing due to low river flow, could lead to severe summer hypoxia in the coming years. Therefore, it is essential to find solutions to mitigate these hypoxia events, for example, by reducing organic matter and nutrient inputs or improving wastewater management (Kemp et al., 2009). In the Thames and Scheldt Estuaries, the installation of wastewater treatment plants (WWTPs) notably increased oxygen concentrations and eliminated hypoxic zones (Amann et al., 2012; Soetaert et al., 2006; Tinsley, 1998).