Carbon Dynamics Along the Seine River Network: Insight from a Coupled Estuarine/River Modeling Approach

Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach Goulven Laruelle, Audrey Marescaux, Romain Le Gendre, Josette Garnier, Christophe Rabouille, Vincent Thieu To cite this version: Goulven Laruelle, Audrey Marescaux, Romain Le Gendre, Josette Garnier, Christophe Rabouille, et al.. Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuar- ine/River Modeling Approach. Frontiers in Marine Science, Frontiers Media, 2019, 6, pp.216. 10.3389/fmars.2019.00216. hal-02139408 HAL Id: hal-02139408 https://hal.sorbonne-universite.fr/hal-02139408 Submitted on 24 May 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. fmars-06-00216 April 26, 2019 Time: 16:29 # 1 ORIGINAL RESEARCH published: 26 April 2019 doi: 10.3389/fmars.2019.00216 Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach Goulven G. Laruelle1,2*, Audrey Marescaux1,2, Romain Le Gendre3, Josette Garnier1,4, Christophe Rabouille5 and Vincent Thieu1 1 UMR 7619 Metis, CNRS, EPHE, Institute Pierre Simon Laplace (IPSL), Sorbonne Université, Paris, France, 2 FR636 Institute Pierre Simon Laplace (IPSL), CNRS, Sorbonne Université, Paris, France, 3 IFREMER LEAD NC, Noumea, New Caledonia, 4 FR3020 Fédération-Ile-de-France de Recherche sur l’Environnement (FIRE), CNRS, Sorbonne Université, Paris, France, 5 Laboratoire des Sciences du Climat et de l’Environnement, UMR CEA-CNRS-UVSQ et IPSL, Gif-sur-Yvette, France The Seine river discharges over 700 Gg of carbon (C) every year into the sea mostly under the form of dissolved inorganic carbon (DIC) and emits 445 Gg under the form of Edited by: carbon dioxide (CO2) to the atmosphere over its entire river network. The watershed, Il-Nam Kim, which drains 76,000 km2, is heavily populated with 18 106 inhabitants and is thus Incheon National University, South Korea submitted to large anthropic pressure. The offline coupling of two Reactive Transport Reviewed by: Models is used to understand the complex spatial and temporal dynamics of carbon, Carlos Noriega, oxygen and nutrients and quantify the CO2 exchange at the air-water interface along Federal University of Pernambuco, Brazil the main axis of the river. The estuarine section of the Seine is simulated by the generic John Christopher Lehrter, estuarine model C-GEM (for Carbon Generic Estuarine Model), while the upstream University of South Alabama, part of the network, devoid of tidal influence is simulated by the pyNuts-Riverstrahler United States modeling platform which also includes an explicit representation of the drainage network *Correspondence: Goulven G. Laruelle ecological functioning. Our simulations provide a process-based representation of [email protected]; nutrients, oxygen, total organic carbon (TOC) and the carbonate system (DIC and [email protected] alkalinity) over the entire year 2010. Our coupled modeling chain allows quantifying the Specialty section: respective contributions of the estuarine and freshwater sections of the system in the This article was submitted to removal of carbon as well as following the fate of TOC and DIC along the river network. Marine Biogeochemistry, a section of the journal Our results also allow calculating an integrated carbon budget of the Seine river network Frontiers in Marine Science for year 2010. Received: 29 December 2018 Keywords: dissolved inorganic and organic carbon, carbon dioxide, reactive-transport model, Seine river and Accepted: 05 April 2019 estuary, biogeochemical budget Published: 26 April 2019 Citation: Laruelle GG, Marescaux A, INTRODUCTION Le Gendre R, Garnier J, Rabouille C and Thieu V (2019) Carbon Dynamics At the interface between the oceanic and terrestrial realms, rivers and estuaries transport ∼0.9 Pg·C Along the Seine River Network: Insight From a Coupled Estuarine/River (organic and inorganic) every year into coastal seas (Regnier et al., 2013a; Resplandy et al., 2018). Modeling Approach. Generally oversaturated in carbon dioxide (CO2) with respect to the atmosphere, these systems Front. Mar. Sci. 6:216. collectively further emit between 0.8 and 2.0 Pg·C every year into the air under the form of CO2 doi: 10.3389/fmars.2019.00216 (Cai, 2011; Laruelle et al., 2013; Raymond et al., 2013; Regnier et al., 2013a; Lauerwald et al., 2015). Frontiers in Marine Science| www.frontiersin.org 1 April 2019| Volume 6| Article 216 fmars-06-00216 April 26, 2019 Time: 16:29 # 2 Laruelle et al. Carbon Dynamics Along the Seine Because of this significant role as carbon filters, rivers and (OC) and inorganic carbon (IC) cycles allows simulating and estuaries are critical components of the Land-Ocean Aquatic quantifying the different processes controlling the OC/IC ratio Continuum (LOAC) and have been under growing scrutiny over and the emission of CO2 into the atmosphere along the entire the past decade (Cole et al., 2007; Battin et al., 2009; Bauer river network. Earlier modeling work in tidal estuaries using et al., 2013; Regnier et al., 2013a). However, large uncertainties C-GEM alone evidenced the intense carbon processing taking are still currently associated to the quantification of global place in the widest, marine part of estuaries (Regnier et al., CO2 fluxes at the air-water interface in these systems, partly 2013b; Volta et al., 2014, 2016a). The first integrated study due to the scarcity of data available to constrain such global presented here will allow further testing the hypothesis that budgets and also due to the difficulty to derive reliable annual the most downstream section of the river/estuarine system estimates of the CO2 exchange with the atmosphere at the disproportionately contributes to the retention of carbon and system scale (Regnier et al., 2013b). For instance, the global CO2 outgassing at the scale of an entire river network. The outgassing of CO2 taking place in rivers have been estimated first part of the results section thus consists of a validation of at 0.6 and 1.8 Pg·C·yr−1 only 2 years apart by Raymond et al. the hydrology of C-GEM in the estuarine section of the river (2013) and Lauerwald et al.(2015), respectively, and the global domain. Then, the ability of both models to capture carbon, outgassing of CO2 from estuaries has been successfully revised oxygen and nutrients dynamics is assessed over the year 2010. downward from 0.4 to 0.6 Pg·C·yr−1 (Abril and Borges, 2004; These simulations allow better understanding the fate of organic Borges, 2005; Borges et al., 2005; Chen and Borges, 2009) to and inorganic carbon throughout the river network and the 0.1–0.15 Pg·C·yr−1 over the past decade (Chen et al., 2013; respective contributions of the different sections of this river Laruelle et al., 2013). Even at the scale of a single system, network to the conversion of organic carbon into DIC and the short scale spatial and temporal gradients (of the order of outgassing of CO2 into the atmosphere. A comprehensive carbon kilometers and days) cannot be captured by a realistic monitoring budget is then assembled which allows quantifying the organic network. Moreover, the origin of the carbon outgassed [i.e., to inorganic carbon ratio in the different section of the Seine oversaturated waters in dissolved inorganic carbon (DIC) versus river network (i.e., the estuary, between Poses and Paris and degradation of organic carbon] is also largely unknown in most upstream of Paris). systems. Spatially and temporally resolved numerical models are necessary to capture the CO2 dynamics at the air-water interface, both in time and space (Regnier et al., 2013b). In MATERIALS AND METHODS this context, reactive transport models (RTM) allow reproducing the changes in chemical species concentrations due to both Study Area transport and biogeochemical transformations, thus providing a The Seine river is the largest river flowing into the English mechanistic insight into the fate of chemical species of interest Channel both in terms of nutrients, carbon loads and fresh water in riverine and estuarine environments (Volta et al., 2014, 2016a; discharge. The river’s central part has a large urban area (the Marescaux, 2018; Romero et al., 2018). Paris conurbation: 12.4 M inhab), a very dense industrial network Here, the application to the Seine estuary-river system of (40% of French industries) and the biggest wastewater treatment the generic transport reactive model C-GEM (for Carbon plant in Europe (The Seine Aval WWTP: 1.5 Mm3·d−1). Further Generic Estuarine Model, Volta et al., 2014) combined with downstream, the Rouen conurbation (0.4 M inhab) appears pyNuts-Riverstrahler (Thieu et al., 2015; Raimonet et al., nowadays as a new pole of economic development, thus forming 2018) allows understanding the carbonate system dynamics the “Grand Paris” (Attali, 2010) which steer urban sprawling in a well monitored and intensively studied system. Fully along the Seine axis from Paris to the sea shore at Le Havre transient simulations over the entire year 2010 are performed (Figure 1). The Seine watershed also supports intensive cropping to quantify the annually integrated CO2 exchange at the air- activities with 57% of land devoted to intensive agriculture. water interface and better understand the evolution over the The Seine estuary is characterized by tidal amplitudes year and along the estuarine-river continuum of the CO2 comprised between 3 and 8 m at its mouth in Le Havre and exchange with the atmosphere and organic carbon degradation a tidal penetration of 168 km up to a dam located at Poses as well as the longitudinal variations of inorganic to organic (upstream area: 64,860 km2 which corresponds to 85% of the carbon ratio in response to anthropic pressures.

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