Katixa Lajaunie-Salla

Aldo Sottolichio

Sabine Schmidt

Xavier Litrico

Guillaume Binet

Gwenaël Abril

Future intensification of summer hypoxia in the Tidal Garonne River (SW France) simulated by a coupled 1 hydro sedimentary-biogeochemical model 2

Keywords: future changes, hypoxia, modeling, Tidal Garonne River, wastewater, water quality saturation, e.g., Rabalais

Projections for the next 50 years predict a widespread distribution of hypoxic zones in the open and coastal ocean due 31 to environmental and global changes. Previously, the Tidal Garonne River (SW France) has experienced few episodic 32 hypoxic events. However, predicted future climate and demographic changes suggest hypoxia could become more 33 severe near the city of Bordeaux in the next few decades. A 3D model, which couples hydrodynamic, sediment 34 transport and biogeochemical processes, is applied to assess the impacts of global and regional changes on 35 oxygenation in the turbidity maximum zone (TMZ) of the Tidal Garonne River during low-discharge periods. The 36 model predicts an intensification of summer hypoxia with an increase in temperature, a decrease in river flow or an 37 increase in the local population but not with sea level rise, which has a negligible impact on dissolved oxygen. We 38 tested different scenarios by combining these different factors according to the regional projections for 2050 and 39 2100. All simulations predict a spatial and temporal extension of summer hypoxia that needs to be considered by local 40 water authorities to impose management strategies to protect the ecosystem. 41

1 Introduction

43

Climate models predict a general decrease in dissolved oxygen (DO) and a widespread distribution of hypoxia in the 44 ocean [START_REF] Cocco | Oxygen and indicators of stress for marine life in multi-model global 371 warming projections[END_REF]. Hypoxic waters are defined by a concentration of DO lower than 2 mg.L -1 (or < 30%

The objective of this work is to evaluate the potential impacts of global and local changes on the oxygenation of the 92 Tidal Garonne River (SW France). Continuous monitoring data [START_REF] Lanoux | Factors contributing to hypoxia in a highly turbid, macrotidal estuary 419 (the Gironde, France)[END_REF]Schmidt et al. 2017) have 93 revealed that the Tidal Garonne River (TGR) had previously experienced few hypoxic events. However, continuing 94 changes (temperature increase, increase in TMZ duration and concentration, development of the Bordeaux 95 Metropolis) suggest that summer hypoxia near the city of Bordeaux could be more pronounced in the next decades. In 96 this work, we use a recently developed 3D numerical model of dissolved oxygen [START_REF] Lajaunie-Salla | Impact of urban effluents on summer hypoxia in the 416 highly turbid Gironde Estuary , applying a 3D model coupling hydrodynamics , sediment transport and 417 biogeochemical processes[END_REF] to 97 estimate the influence of the main identified factors likely to evolve in the next 50-100 years (fluvial discharge, 98 temperature, urban development, and sea level rise) that may affect DO. The aim of the study is to simulate the 99 probable future summer oxygenation according to the influence of these pressures, considered alone or combined, and 100 to discuss the future risk of hypoxia in the TGR.

103

The Gironde Estuary, located in southwestern France on the Atlantic coast, is formed by the confluence of the 104

Garonne and the Dordogne Rivers. This macrotidal fluvio-estuarine system is characterized by the presence of a TMZ 105 with suspended sediment concentration (SSC) > 1 g.L -1 [START_REF] Allen | Étude des processus sédimentaires dans l'estuaire de la Gironde[END_REF]). The TMZ is formed by the so-called tidal 106 pumping effect [START_REF] Uncles | Observed fluxes of water and suspended sediment in a partly mixed 466 estuary[END_REF], consisting of mud trapping due to tidal asymmetry, i.e., the flood phase is 107 shorter but with more intense currents than the ebb phase [START_REF] Allen | Effects of tides on mixing and suspended sediment transport in macrotidal 366 estuaries[END_REF]. The TMZ moves seasonally along the 108 estuary axis according to fluvial discharges. In summer, when river flows decrease, the TMZ moves toward the tidal 109 Dordogne and Garonne rivers, i.e., the portions of the rivers where the tidal waves propagate. A TMZ is present in the 110 Tidal Garonne River between 3 and 8 months per year (Jalón-Rojas et al. 2015).

111

The Garonne River is the main tributary of the Gironde Estuary: it drains a watershed of 55 000 km 2 ; the mean annual 112 discharge is 600 m 3 .s -1 , ranging between extreme daily values of 54 and 4720 m 3 .s -1 . The Bordeaux Metropolis is

117

DO levels in the TGR are strongly influenced by water temperature, river flow, TMZ occurrence and urban effluents 118 [START_REF] Lajaunie-Salla | Impact of urban effluents on summer hypoxia in the 416 highly turbid Gironde Estuary , applying a 3D model coupling hydrodynamics , sediment transport and 417 biogeochemical processes[END_REF][START_REF] Lanoux | Factors contributing to hypoxia in a highly turbid, macrotidal estuary 419 (the Gironde, France)[END_REF]Schmidt et al., 2017). Hypoxia events preferentially occur during 119 summer, when water temperature is increased to 24°C, a few days after a spring tide [START_REF] Lanoux | Factors contributing to hypoxia in a highly turbid, macrotidal estuary 419 (the Gironde, France)[END_REF], as the 120 result of an increased oxygen demand followed by reduced reaeration due to high turbidity (Lajaunie-Salla et al [START_REF] Brenon | Modelling the Turbidity Maximum in the Seine Estuary ( France ): Identification of 369 Formation[END_REF] and [START_REF] Cugier | Development of a 3D Hydrodynamic Model for Coastal Ecosystem Modelling. Application 380 to the Plume of the Seine River (France)[END_REF] for the Seine Estuary, were implemented in the 131 Gironde Estuary by [START_REF] Sottolichio | Modeling mechanisms for the stability of the turbidity maximum in the 454 Gironde estuary, France[END_REF]. operator of the sewer system, both regarding rejects of WWTP and SO (Fig. 1).

150

The computational domain of the model extends from the 200 m isobath on the continental shelf to the upstream 151 limits of the tidal propagation, at 170 km from the mouth (La Réole and Pessac-sur-Dordogne; Fig. 1) (Sottolichio et 152 al. 2000). The 3D model is implemented for the Gironde Estuary on an irregular Cartesian grid with 2421 wet cells in 153 the horizontal plane. In the vertical plane, the water column is split into 12 layers. The grid cell lengths progressively 154 increase in the upstream direction from 500 m to 2 km. In the TGR, the width is described by three cells in the 155 downstream section and by one cell in the upstream section. However, the average width and depth are rigorously 156 well represented at all the cross sections along the tidal rivers, ensuring a good calculation of the oscillating volumes 157 of water and tidal currents.

158

The originality of this biogeochemical model is to rely on extensive local data, particularly in the Tidal Garonne 159

River. Except for nitrification, the degradation kinetics of different organic matter were measured for mixtures of 160 estuarine turbid waters, riverine waters rich in phytoplankton or terrestrial litter, and domestic waters from SOs and 161 WWTPs [START_REF] Veyssy | Transferts de matière organiques das bassins versants aux estuaires de la Gironde et de l'Adour 475[END_REF][START_REF] Lemaire | Effet de la turbidité sur la dégradation des pigments 427 phytoplanctoniques dans l'estuaire de la Gironde[END_REF][START_REF] Etcheber | Particulate organic carbon in the estuarine turbidity maxima of the 392 Gironde, Loire and Seine estuaries: origin and lability[END_REF][START_REF] Lanoux | Factors contributing to hypoxia in a highly turbid, macrotidal estuary 419 (the Gironde, France)[END_REF].

162

In addition to temperature, the MAGEST network has recorded turbidity, salinity and DO every 10 minutes at 168

Reference year

169

The simulations were compared to a reference simulation, which corresponds to the year 2006 (Fig. 2). This year was 170 chosen as a relatively critical reference simulation from the point of view of river discharge, temperature and hypoxia.

171

The summer water temperature was on average 24.6°C, with a maximum of 29.4°C during a 21-day heat wave event.

172

The mean summer Garonne River flow was 145 m 3 .s -1 (minimum of 54 m 3 .s -1 ) with 60 continuous days of river flow

195

three simulations to test the impact of sea level rise by +10, +20 and +40 cm.

196

one simulation to test the impact of the increase in population assuming an associated increase in wastewater 197 discharge by 50% and in a theoretical case of uncontrolled increase of urban discharges.

198

the four forcing factors were combined to produce academic scenarios close to the expected scenarios for the 199 years 2050 and 2100 (Table 1).

200

Each of the 14 simulations were run over 10 months, from January 1 to October 31. To form a TMZ, a stock of

211

The reference simulation highlights the occurrence of the TMZ at Bordeaux from June to September, a summer DO 212 decrease and two events of low DO, which were close to hypoxia (end of July and mid-September) (Fig. 2b). In fact,

213

in summer, when the river flow decreases, the TMZ arrival in the tidal Garonne is always accompanied by a 214 simultaneous decline in DO in Bordeaux waters [START_REF] Etcheber | Monitoring water quality in estuarine environments: lessons from 389 the MAGEST monitoring program in the Gironde fluvial-estuarine system[END_REF]Schmidt et al, 2017). The lowest DOs occur at 215 the end of July during a period characterized by a persistent low river flow, a heat wave and a sewage overflow 216 discharge (Fig. 2). Moreover, as detailed in Lajaunie-Salla et al. ( 2017), the reference simulation shows that lower 217 DO levels occur a few days after the spring tide peak and the deoxygenated zone moves between PK-10 and PK-20.

218

In fact, at spring tide, the highest tidal currents promote resuspension and advection of suspended sediment, and the

226

At Bordeaux, the hypoxia threshold is not reached (DO > 2 mg.L -1 ); however, at Portets, the hypoxia event lasted 3 227 days during this reference year (Tab. 2 & Fig. 4c).

228

Impact of increasing temperatures

The impact of an increase in temperature is negligible during winter and spring. From June, modeled scenarios show 230 a reduction in DOmin at Bordeaux and Portets when the temperature is higher than 20°C (Fig. 3; Tab. 2). For a 231 temperature increase of 5°C, at Bordeaux station, DOmin decreases from 28.9 to 14.3 %sat, and 26 days of hypoxia are 232 simulated, whereas at Portets, DOmin decreases from 22.4 to 15.8 %sat, and 11 days of hypoxia are simulated (Tab. 2).

233

At Bordeaux station, the deoxygenation observed in 2006 is amplified to hypoxia at the end of July when the 234 temperature increases by +2°C. An increase of +4°C is required to observe hypoxia in mid-September (Tab. 2 and 235 Fig. 3a).

236

In the area near Bordeaux, for the scenario of +2°C, DO consumption rates by nitrification and the mineralization of 237 urban OM and watershed OM increase by 12%, 7% and 4%, respectively, whereas with an increase of 5°C, the

258

This is explained by the TMZ development and its high SSC, which favors DO consumption by heterotrophic 259 processes. Indeed, the strongest deoxygenation occurs when the river flow is the lowest and SSC is the highest (Fig.

4o

). In the case of a discharge of 40 m 3 .s -1 , a DOmin of 13.5 %sat and 8 %sat and 13 and 52 days of hypoxia are 261 simulated at Bordeaux and Portets, respectively (Tab. 2). However, the comparison of low river flow scenarios 262 suggests that heterotrophic processes are not the only controlling factor of DO distribution (Fig. 4). First, in the 263 reference simulation, a short event of increasing river flow occurred at the end of July, which resulted in a temporary 264 downstream relocation of a low-DO core near Bordeaux (Fig. 4c). For 100 m 3 .s -1 , the hypoxia is also located close to 265 Bordeaux, whereas the waters of the upper tidal river (PK -10 to -20) are still reoxygenated by fluvial inputs (Fig. 4f).

266

In contrast, for the lowest simulated river flow (40 m 3 .s -1 ), the dilution of the upper Tidal Garonne River waters by 267 oxygenated freshwaters becomes negligible, and then, several successive hypoxia events spread from PK-10 to PK-25 268 (Fig. 4o). This indicates that the upper section could be reoxygenated by the input of oxygenated advected freshwater.

269

At Bordeaux, hypoxia is reached for the four simulated scenarios, whereas at Portets, it occurs only when the river 270 discharge is lower than 60 m 3 .s -1 (Tab. 2).

271

We calculated the input of oxygenated freshwater (with a typical DO concentration of approximately 90 %sat)

272

introduced by the Garonne River into the tidal river during a semidiurnal tidal cycle: the volume is 4.3 10 6 m 3 for a 273 discharge of 100 m 3 .s -1 , and 1.7 10 6 m 3 for 40 m 3 .s -1 . This represents 3.8 and 1.5% of the average water volume 274 between La Réole (the river end-member) and Bordeaux; and 22% and 9.3% of the averaged volume between La cycle by +2.3 %sat for a river flow of 100 m 3 .s -1 , but only by +0.9 %sat for a river flow of 40 m 3 .s -1 (Portets, +13.2 278 %sat for 100 m 3 .s -1 and +5.6 %sat for 40 m 3 .s -1 ). Then, the fluvial DO renewal effect is reduced when the river flow 279 decreases.

280

The last consequence of a lower fluvial discharge is an increase in DO consumption by nitrification and 281 mineralization of OM from wastewater, whereas there is a decrease in the mineralization of OM from watersheds near 282 Bordeaux and Portets (Tab. 3). This is a consequence of i/ a lower downstream dispersion of urban inputs, which tend 283 to remain concentrated near Bordeaux and ii/ an upstream displacement of the TMZ to the upper section (PK-25, Fig. Finally, we tested the potential impact of sea level rise (SLR) on DO. The simulations do not show a clear trend 289 among the 3 scenarios (+10 cm; +20 cm; +40 cm) (Fig. 5). In the simulation SLR+10 cm, the TMZ is intensified and 290

DOmin decreases from 28.9%sat to 25.6%sat at Bordeaux, but never reach the hypoxia threshold (Tab.2). At Portets 291 the impact seems more critical with a DOmin decline from 22.4 %sat to 15.0 %sat, reaching 5 days of hypoxia 292 (instead of 3 days for the reference year, Tab.2). But, surprisingly, in the simulation SLR+20 cm, the trend is 293 reversed: the TMZ is attenuated and DOmin remains similar to the reference year values, with only two days of 294 hypoxia at Portets (Tab.2). For the scenario SLR+40 cm, there is an amplification of deoxygenation at Portets: DOmin 295 decreases at 14.7% and hypoxia occurs 6 days (Tab.2). The intensification of hypoxia for a sea level rise of 10 cm and 296 40 cm is explained by an increase of the tidal range and asymmetry in the Tidal Garonne River due to an overall 297 higher depth, as observed in other urbanized estuaries [START_REF] Winterwerp | Man-induced regime shifts in small 481 estuaries-II: A comparison of rivers[END_REF]. These changes in tide are likely to 298 increase the tidal pumping effect and therefore sediment resuspension and upstream transport (Winterwerp et al., 299 2013). For the simulation SLR+20 cm, flood dominance increases also, but simulated SSC decreases, which creates 300 better conditions for oxygenation. As tidal currents are not expected to reduce, we hypothesize this could be attributed 301 to a dilution effect due to a larger water volume in the tidal Garonne section (compared to SLR+10 cm). Indeed, the 302 fate of suspended sediments in the water column of shallow tidal rivers results from a complex interaction between 303 the tide, river flow, sediment supply and morphology (Robins et al., 2016). In particular, the evolution of the flood 304 dominance depends on the relative importance of the local tidal range, the cross-section averaged depth, and the 305 presence of channel and intertidal flats [START_REF] Dronkers | Tidal asymmetry and estuarine morphology[END_REF][START_REF] Friedrichs | Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis[END_REF]. For the TGR, a sea level rise of 306 approximately 20 cm seems to be a threshold value from which the effects of morphology and estuarine volume 307 counteract the effects of increased tidal pumping and flood dominance. Then, at higher SLRs, the amplification of 308 tidal pumping prevails over the dilution effect. Unfortunately, the description of lateral flats and channel bathymetry 309 in the present model is not robust enough to make conclusions for these opposing effects. This implies the 310 implementation of a morphodynamic model that more precisely describes the lateral flats and, eventually, the 311 morphological evolution, which is far beyond the scope of the present study. Indeed, simulated results demonstrate 312 this factor does not induce major changes in oxygenation compared with temperature and fluvial discharge changes 313 (Tab. 2). 314

Scenarios of the combined effects for 2050 and 2100

315

To produce more realistic simulations, we combined the forcing factors to produce expected scenarios for 2050 and 316 2100. When compared to the reference scenario (Fig. 6a), the 2050 scenario shows no upstream TMZ migration nor 317 extension, only a slight increase in SSC and a longer duration until October near Bordeaux (Fig. 6c). Simultaneously, 318 there is a clear DO decrease and an increase in the hypoxia duration in Bordeaux (29 days). Conversely, the hypoxia 319 threshold is never attained at Portets (Tab. 2). This is caused by two processes: the summer river flow is fixed at 80 320 m 3 .s -1 , water in the upper sections is still reoxygenated by dilution with freshwater (Fig. 4i/6d), and there is no TMZ 321 intensification (Fig. 6c).

322

For the 2100 scenario, the TMZ is far more concentrated until October compared with the reference, and it extends up 323 to Portets in September (Fig. 6e). In the meantime, high and continuous DO depletion occurs over the entire section, 324 from PK-15 to PK-25 (Fig. 6f). The extremely low summer river flow (40 m 3 .s -1 ) promotes an increased intensity and 325 duration of the TMZ. In addition, the input of well-oxygenated freshwater is limited, as already discussed in section 326 3.4. The duration of hypoxia increases at Bordeaux and Portets (91 and 78 days, respectively, Tab. 2). DO in the TGR 327 decreases far below the threshold of 2 mg.L -1 (DOmin at Bordeaux and Portets is less than 0.3 mg.L -1 ) and almost 328 reaches anoxia in early August (Tab. 2). Then, the hypoxia area extends over 40 km during the entire summer season.

329

As these low DO values are modeled for surface waters, we assume a "dead zone" [START_REF] Diaz | Spreading dead zones and consequences for marine ecosystems[END_REF] could 330 appear in the Tidal Garonne River near Bordeaux.

331

The average summer rates of biogeochemical processes impacting DO near Bordeaux and Portets show a significant 332 increase (more than twice) in nitrification and degradation of urban OM in the 2050 and 2100 simulations due to the 333 combined effect of increased temperature and population growth (Tab. 3). An increase in DO consumption rates of 334 urban substrates (ammonia and urban matter) occurs for both scenarios (Tab. 3): in Bordeaux close to the urban 335 source and at Portets 25 km upstream. In the case of Bordeaux, DO consumption by wastewater organic matter is 336 higher for the 2050 scenario than for 2100, probably due to a stronger upstream dispersion of urban effluents from 337

Bordeaux to Portets (Tab. 3).

338

The presence of a permanent summer hypoxia may cause serious ecological problems in the TGR, and in particular 339 could perturb benthic and pelagic fauna [START_REF] Diaz | Spreading dead zones and consequences for marine ecosystems[END_REF][START_REF] Vaquer-Sunyer | Thresholds of hypoxia for marine biodiversity[END_REF]. A permanent 340 seasonal hypoxia could have serious repercussions for the downstream migration of juvenile fishes (shad or sturgeon) 341 [START_REF] Lanoux | Effects of hypoxia on the fish and crustacean fauna in the Gironde 421 Estuary[END_REF]. These ecological problems could also impact the economy of the fishing sector. Moreover, the 342 potential formation of a « dead zone » would have an impact on atmospheric chemistry, causing greenhouse gas 343 emissions [START_REF] Naqvi | Marine hypoxia/anoxia as a source of CH4 and N2O[END_REF]Howarth et al. 2011). Howarth R, Chan F, Conley DJ, et al (2011)

  from the confluence. The sewage system of the metropolis drains an urban surface area of 578 km² and 114 serves a population estimated at 749 595 inhabitants in 2015. Two wastewater treatment plants (WWTPs, Clos de 115 Hilde and Louis Fargue) continuously discharge treated wastewater into the TGR (Fig. 1); in addition, nine sewage 116 overflows (SOs) occasionally discharge untreated wastewater into the TGR.

  163different sites since 2005 (Fig.1). The model was compared with data available for the TGR and tested on the basis of 164 three criteria: i/ the ability to reproduce the observed DO variability at a seasonal scale; ii/ the ability to reproduce the 165 spring-neap tidal cycle; and iii/ a statistical evaluation based on the Willmott skill score (WSS,[START_REF] Willmott | Some comments on the evaluation of model performance[END_REF]). In 166 summary, the model performed well (WSS > 0.7) in the lower TGR around Bordeaux but had lower performance 167 levels at the upper section (WSS < 0.5) (for details, see[START_REF] Lajaunie-Salla | Impact of urban effluents on summer hypoxia in the 416 highly turbid Gironde Estuary , applying a 3D model coupling hydrodynamics , sediment transport and 417 biogeochemical processes[END_REF]).

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  3 .s -1 . In 2006, WWTP and DO discharges of urban water to the Garonne River were 51.2 and 9carried out to study the DO sensitivity of the Tidal Garonne River to the evolution of the 177 main forcing factors: water temperature, river flow, sea level and population growth of Bordeaux Metropole. Present-178 day trends were used to extrapolate the expected values for 2050 and 2100: 179 -the water temperature increases following a mean rate of 0.06°C year -1 (Le Treut 2013), suggesting an increase 180 of +2°C in 2050 and +5°C in 2100;181the mean Garonne river flow is expected to be reduced by 30 to 40% in 2050 and by 50 to 60% in 2100 182[START_REF] Etcheber | Impact 387 of climate change in the Aquitaine region[END_REF] Schmidt et al. 2016); -in the Bay of Biscay, the sea level has increased at a mean rate of 2 mm year -1 over the last few decades and 184 should increase by 0.14 to 0.24 m in 2050 and by 0.28 to 0.48 m in 2100[START_REF] Treut | Impact of climate change in the Aquitaine region[END_REF]; 185 Bordeaux Metropolis is taking advantage of its attractiveness by implementing a dynamic policy to develop 186 the city with the objective of reaching 1 million inhabitants by 2030 and then stabilizing the population until 187 the end of the 21 st century. (http://www.bordeaux-metropole.fr).188These expected trends were used to perform 14 simulations, including sensitivity analysis and two combined 189 scenarios (Tab. 1), as follows:190four simulations to test the impact of increasing temperature by adding a constant value (2four simulations to test the impact of a decrease in the Garonne flow, consisting of 78 days of constant river 193 flow (100, 80, 60 and 40 m 3 .s -1 ) during the driest season, from July 15 to September 30. The selected values 194 correspond to a decrease in the reference summer flow by 24%, 36%, 46% and 57%, respectively.

  201 deposited mud was introduced in the estuary at the start of each simulation, which was easily resuspended during the 202 first tidal cycle (see Lajaunie-Salla et al., 2017 for details). The mass of the TMZ was therefore the same for all the 203 simulations, including those with a decreasing summer river flow, which did not significantly change the total mass of 204 mud in the estuary. We used three quantitative indicators to assess the oxygenation state of the TGR waters: (1) the 205 minimum DO value attained (DOmin); (2) the number of hypoxia days, i.e., DO < 2 mg.L -1 ; and (3) the rates of 206 biogeochemical processes consuming DO near Bordeaux and Portets, averaged for the summer months. The grid cells 207 in front of Bordeaux and Portets were chosen because Bordeaux is under the impact of urban effluents and Portets 208 represents the upstream presence of a TMZ.

  219 TMZ moves upstream of Bordeaux. Then, heterotrophic processes are favored and DO depletion is more intense in 220 the upper TGR section than near Bordeaux. At neap tides, suspended matter settles, as well as organic matter coming 221 from the watershed, and consequently reduces the mineralization of organic matter in the water column. Considering 222 that at Bordeaux 32% of the DO consumption is due to wastewater effluents verses 4% at Portets (Lajaunie-Salla et 223 al., 2017), at neap tides, DO depletion is more intense in Bordeaux waters than in the upper section of the TGR. In 224 fact, in the area of Bordeaux, nitrification processes account for 19% of the DO consumption, and the watershed, 225 49%. Whereas in the area of Portets, nitrification accounts for 1% and the watershed for 95% of the DO consumption.

  238consumption rates increase by28%, 11% and 16%, respectively (Tab. 3). The results highlight that an increase in 239 temperature affects biogeochemical processes and accelerates reactions. However, in the area of Portets, although the 240 nitrification processes increase with temperature, DO consumption by urban OM decreases. This is explained by a 241 more intense degradation of the urban OM close to its source, i.e., Bordeaux, leading to a lower amount of OM wastewater discharge with demographic growth also results in lower DO values compared with those 245 of the reference year only in summer (Fig.3b). At Bordeaux, the model simulates 3 days of hypoxia with a DOmin of 246 23.6 %sat, whereas at Portets, urban effluents have less impact because DOmin decreases by only 1 %sat (DOmin = 21.3 247 %sat, Tab. 2). Close to Bordeaux, DO consumption due to urban OM degradation and nitrification increases by 50% 248 and 42%, respectively (Tab. 3). At Portets, the same processes increase by 65% and 7%, respectively (Tab. 3), 249 whereas the impact of the mineralization of OM from the watershed remains constant (Tab. 3). Population growth in 250 Bordeaux Metropole moderately affects the oxygenation status of the TGR, despite the increase in urban labile 251 organic matter inputs and then in DO consumption. shown an influence of the multidecadal discharge decrease on the intensification of the 254 TMZ in the Tidal Garonne River (Jalón-Rojas et al., 2015). The model was thus used to test whether a discharge 255 decrease also influences DO. The model simulates a marked saline intrusion (>4 at Bordeaux) and an obvious TMZ 256 intensification and extension in the upper section of the TGR with a long period of low to very low water discharge 257 (Fig. 4). DOmin decreases and the number of hypoxic days increases with the river flow reduction (Fig. 4; Tab. 2).

  strongest deoxygenation observed for the simulation of a river flow of 40 m 3 .s -1 is thus a consequence of 285 combined factors: reduced wastewater dispersion and dilution with oxygenated freshwaters and an increase in 286 heterotrophic processes in the uppermost section where large amounts of fine sediments are trapped during summer.

  3D biogeochemical model of the Tidal Garonne River coupling hydrodynamic and sediment 346 transport models was particularly useful for understanding the future evolution of DO under global and local changes.347 An increase in temperature accelerates oxygen consumption by biogeochemical reactions. Population growth supplies 348 additional labile organic matter in the Tidal Garonne River. A reduction in the river flow during summer droughts 349 reduces the dilution of estuarine water with oxygenated freshwater and the downstream dispersion of the TMZ. Sea 350 level rise can intensify mud trapping and oxygen consumption, but the competition between dilution and tidal 351 pumping remain unclear and need specific investigations with high-resolution models. 352 The simulations of the combined effects of temperature, river flow, sea level and urban effluent changes predict a 353 probable critical to very critical hypoxic state of the Tidal Garonne River for 2050 and 2100. A severe and permanent 354 summer hypoxia is likely to develop between Bordeaux and Portets, reaching DO levels close to anoxia and 355 potentially causing substantial repercussions on the ecosystem. These model results indicate the need for management 356 actions to maintain the water quality of the Tidal Garonne River in the future. Possible actions could be to provide 357 low river flow support, while still minimizing the impact of population growth through even more stringent 358 wastewater treatment objectives. Moreover, the DO response to fluvial discharge highlights the need to maintain river 359 discharge at a minimal threshold during summer, for example, by improving the release of water stored in dams or by 360 limiting irrigation in the watershed during drought periods.

Figure 1 :

 1 Figure 1:The Tidal Garonne River in southwestern France. "PK" denotes the distances in km from the city center of Bordeaux; the control grid cell at Bordeaux is at PK-4 and Portets is at PK-20. In insert A, the purple triangles represent the position of sewage overflows (SOs) and the green squares represent the position of wastewater treatment plants (WWTPs). The area in orange represents the area of Bordeaux where the biogeochemical fluxes are calculated. In inset B, the purple line represents the 200 m isobath.

Figure 2 :

 2 Figure 2: Influencing factors of the reference simulation (year 2006): time series of river flow (a, m 3 .s -1 ), water temperature (b, °C), discharge of sewage overflow (SO) and of wastewater treatment plants (WWTPs) (c, m 3 .s -1 ), simulated DO saturation (d, %), and SSC (d, g.L -1 ) and tidal range (d, m) from January 1 to October 31. In figure b, the blue line and the green area show the median and the envelope of measured temperature for the period 2005-2015, respectively.

Figure 3 :

 3 Figure 3: Temporal evolution of DOmin (over tidal cycle in %sat) at Bordeaux, from January to October, according to the scenarios of temperature increases (from 2°C to 5°C, a) or population growth (b). The blue line represents the 2006 reference simulation.

Figure 4 :

 4 Figure4: Spatiotemporal evolution of simulated surface daily average parameters along the Tidal Garonne River section: salinity (left), SSC (middle, in g.L -1 ) and DO (right, in %sat). The simulations correspond to the reference with a summer mean Garonne discharge of 145 m 3 .s -1 (a, b and c) and to the following four scenarios: 100 m 3 .s -1 (d, e and f), 80 m 3 .s -1 (g, h and i), 60 m 3 .s -1 (j, k and l) and 40 m 3 .s -1 (m, n and o). The y-axis represents the kilometric points, and the white lines represent Bordeaux and Portets.

Figure 5 :

 5 Figure5: Spatiotemporal evolution of simulated surface daily average parameters along the Tidal Garonne River section: salinity (left), SSC (middle, in g.L -1 ) and DO (right, in %sat). The simulations correspond to the three scenarios of SLR: +10 cm (a, b and c), +20 cm (d, e and f) and +40 cm (g, h and i). The y-axis represents the kilometric points, and the white lines represent Bordeaux and Portets.

Figure 6 :

 6 Figure 6: Spatiotemporal evolution of daily average surface SSC (left, in g.L -1 ) and DO (saturation, right in %) along the Tidal Garonne River section for the reference year 2006 (a and b) and the scenarios of 2050 (c and d) and 2100 (e and f). The y-axis represents the kilometric points, and the white lines represent Bordeaux and Portets.

Figure 2 :

 2 Figure 2:

  We recently implemented the biogeochemical model for the Gironde Estuary to simulate biogeochemical processes 139 that produce or consume oxygen in the water column, taking into account different types of dissolved and particulate 140 organic matter[START_REF] Lajaunie-Salla | Impact of urban effluents on summer hypoxia in the 416 highly turbid Gironde Estuary , applying a 3D model coupling hydrodynamics , sediment transport and 417 biogeochemical processes[END_REF]. These biogeochemical processes include the degradation of organic 141 matter (mineralization of organic carbon and ammonification using the C/N ratio), nitrification, photosynthesis, 142 respiration and mortality of phytoplankton, and DO gas exchange with the atmosphere. The model includes 11 state

	143
	144	variables: dissolved oxygen (DO), ammonia (NH4 + ), nitrate (NO3 -) and particulate and dissolved organic carbon (POC
	145	and DOC) from the watershed (POC from litter, DOC from rivers) and wastewater (from wastewater treatment plants
	146	and from sewage overflows), and POC from phytoplankton and detritus. The biogeochemical model requires inputs of
	147	the seasonal water temperature (data from MAGEST, a high-frequency water quality monitoring network,
	148	http://www.magest.u-bordeaux1.fr), wind and incident light intensity (from French meteorological office, Météo
	149	France). Urban wastewater discharge points are included in the model based on measurements provided by the

The hydrodynamic model is based on the Navier-Stokes equations under 132 the Boussinesq approximation and hydrostatic assumptions. The transport model solves the advection/dispersion 133 equations for dissolved and particulate variables, i.e., suspended sediment, salinity and biogeochemical variables. For 134 suspended sediment, the main cohesive sediment processes are taken into account to appropriately reproduce the 135 dynamics of the TMZ (Van Maanen and Sottolichio, in revision). The hydrodynamic model is forced by tidal 136 elevation at the shelf and by daily river flow of the Garonne and Dordogne Rivers at the upstream boundary (from 137 French national hydrological data, www.hydro.eaufrance.fr).

138

Table 1 :

 1 Coupled biogeochemical cycles: eutrophication and hypoxia in temperate Forcing of the different scenarios simulated with the model.

	403
	404	estuaries and coastal marine ecosystems. Front Ecol Environ 9:18-26. doi: 10.1890/100008
	405	Howarth RW, Swaney DP, Butler TJ, Marino R (2000) Rapid Communication: Climatic Control on Eutrophication of
	406	the Hudson River Estuary. Ecosystems 3:210-215. doi: 10.1007/s100210000020
	407	IPCC (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth
		Assessment Report of the Intergovernmental Panel on Climate Change. 1535

Table 2 :

 2 Minimum simulated DO (in % of saturation and in mg.L -1 ), the corresponding temperature and the number of hypoxia days in Bordeaux and Portets for each scenario.

				Bordeaux			Portets
	Scenarios		T (°C)	DOmin (%)	DOmin (mg.L -1 )	Days of hypoxia	T (°C)	DOmin (%)	DOmin (mg.L -1 )	Days of hypoxia
	Reference	Ref	27.2	28.9	2.3	0	24	22.4	1.8	3
		+2°C	29.2	22.8	1.7	5	26	18.9	1.5	7
		+3°C	30.2	19.8	1.5	9	27	18.5	1.4	8
	Effect of temperature
		+4°C	31.2	17.0	1.2	15	28	16.5	1.3	13
		+5°C	32.2	14.3	1.0	26	29	15.8	1.2	14
	Effect of demographic growth	+POP	27.2	23.6	1.9	3	24	21.3	1.7	4
		100 m 3 .s -1	26.9	23.7	1.9	4	24	33.5	2.8	0
		80 m 3 .s -1	26.8	19.5	1.5	8	23.7	32.2	2.7	0
	Effect of river flow
		60 m 3 .s -1	26.8	15.6	1.2	11	24.4	19.5	1.6	12
		40 m 3 .s -1	27.4	13.5	1.0	13	24.4	8	0.7	52
		+10 cm	27.2	25.6	2.0	0	23.1	15	1.2	5
	Effect of sea level	+20 cm	26.4	33	2.6	0	24.6	22.5	1.9	2
		+40 cm	23.7	29.7	2.3	0	22.6	14.7	1.2	6
		2050	28.9	14	1.1	29	26.7	29.1	2.4	0
	Combined impact
		2100	32.4	2.8	0.2	91	29.7	3.4	0.3	78

Table 3 :

 3 Differences (in %) of biogeochemical process rates impacting DO between the scenarios and reference simulations during summer at Bordeaux.

									Bordeaux			Portets
	Scenarios							nitrification	mineralization TOC WS	mineralization TOC WW	nitrification	mineralization TOC WS	mineralization TOC WW
		+2°C			+12%	+7%	+4%	+15%	-3%	-2%
		+3°C			+17%	+11%	+6%	+22%	-2%	-7%
	Effect of temperature
		+4°C			+23%	+13%	+9%	+30%	-3%	-11%
		+5°C			+28%	+16%	+11%	+37%	-2%	-15%
	Effect of demographic growth	+POP		+42%	-1%	+50%	+7%	-3%	+65%
		100 m	3 .s	-1	+8%	-1%	+4%	-4%	-8%	+20%
		80 m	3	.s	-1	+25%	0	+3%	+15%	-9%	+29%
	Effect of river flow	60 m	3	.s	-1	+45%	-4%	+6%	+67%	-14%	+41%
		40 m	3	.s	-1	+66%	-14%	+6%	+159%	-18%	+52%
		+10 cm	+3%	+3%	-7%	+4%	-4%	+14%
	Effect of sea level	+20 cm	0	0	0	-26%	-19%	-5%
		+40 cm	+3%	-13%	-12%	+11%	+3%	+18%
		2050			+86%	-8%	+59%	+44%	-5%	+65%
	Combined effect
		2100			+144%	-28%	+52%	+241%	-15%	+131%

Réole and Portets (Fig.1). The time to renew half of the water volume is shorter at Portets (between 1.1 and 2.7 days)

than at Bordeaux (from 6.7 to 16.3 days). At Bordeaux, a water initially at 30 %sat would increase after one tidal

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hypoxia. Biogeosciences 7:585-619. doi: 10.5194/bgd-6-9359-2009

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potential projections. Estuar Coast Shelf Sci 169:119-135. doi: 10.1016/j.ecss.2015.12.016

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a case study in the tidal Garonne River (South-West France). Environ Sci Pollut Res. doi: 10.1007/s11356-016-

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Schmidt S, Etcheber H, Sottolichio A, Castaing P (2016) Le réseau MAGEST: bilan de 10 ans de suivi haute-

fréquence de la qualité des eaux de l'estuaire de la Gironde. In: Schmitt FG, Lefevre A (eds) Mesures haute

résolution dans l'environnement marin côtier. Presses du CNRS

This study was funded by the Aquitaine Region and LyRE (SUEZ research center) who provided a PhD grant to K.

13

Lajaunie-Salla. This research was also supported by the Cluster of Excellence COTE at the Université de Bordeaux 14 (ANR-10-LABX-45). The authors are also grateful to the MAGEST network for the availability of data and to the 15 SGAC and Bordeaux Metropole for providing urban effluent data and fruitful discussions.