28 Jun 2021

28 Jun 2021

Review status: this preprint is currently under review for the journal BG.

Nutrient transport and transformation in macrotidal estuaries of the French Atlantic coast: a modelling approach using C-GEM

Xi Wei1, Josette Garnier1, Vincent Thieu1, Paul Passy2, Romain Le Gendre3, Gilles Billen1, Maia Akopian4, and Goulven Gildas Laruelle5 Xi Wei et al.
  • 1UMR Metis 7619, Sorbonne Université, CNRS EPHE, 4 Place Jussieu, Paris, 75005, France
  • 2UMR 8586 PRODIG, Université de Paris, 8 rue Albert Einstein, 75013 Paris, France
  • 3Ecosystèmes et Aquaculture Durable, Unité de Recherche Lagons, IFREMER, Noumea, 98897, New-Caledonia
  • 4Department of Research and Scientific Support, French Biodiversity Agency (OFB), 5 square Félix Nadar, 94300 Vincennes, France
  • 5Department of Geosciences, Environment and Society, Université Libre de Bruxelles, Brussels, 1050, Belgium

Abstract. Estuaries are key reactive ecosystems along the land–ocean aquatic continuum, with significant ecological and economic value. However, they have been facing strong morphological management changes as well as increased nutrient and contaminant inputs, possibly leading to ecological problems such as coastal eutrophication. Therefore, it is necessary to quantify the ingoing and outgoing fluxes of the estuaries, their retention capacity, and estuarine eutrophication potential. A 1-D Carbon–Generic Estuary Model (C-GEM) was used to simulate the transient hydrodynamics, transport, and biogeochemistry for estuaries with different sizes and morphologies along the French Atlantic coast during the period 2014–2016 using readily available geometric, hydraulic, and biogeochemical data. These simulations allowed us to evaluate the budgets of the main nutrients (phosphorus [P], nitrogen [N], silica [Si]) and total organic carbon (TOC), and their imbalance with respect to estuarine eutrophication potential. Cumulated average annual fluxes to the Atlantic coast from the seven estuaries studied were 9.6 kt P yr−1, 259 kt N yr−1, 304 kt Si yr−1, and 145 kt C yr−1. Retention rates varied depending on the estuarine residence times, ranging from 0–27 %, 0–34 %, 2–39 %, and 8–96 % for TP, TN, DSi, and TOC, respectively. Large-scale estuaries had higher retention rates than medium and small estuaries, which we interpreted in terms of estuarine residence times. As shown by the indicator of eutrophication potential (ICEP), there might be a risk of coastal eutrophication, i.e., the development of nonsiliceous algae that is potentially harmful to the systems studied due to the excess TN over DSi.

Xi Wei et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-164', Pierre ANSCHUTZ, 12 Aug 2021
    • AC1: 'Reply on RC1', Xi WEI, 22 Sep 2021
  • RC2: 'Comment on bg-2021-164', Anonymous Referee #2, 17 Aug 2021
    • AC2: 'Reply on RC2', Xi WEI, 22 Sep 2021
  • RC3: 'Comment on bg-2021-164', Anonymous Referee #3, 18 Aug 2021
    • AC3: 'Reply on RC3', Xi WEI, 22 Sep 2021

Xi Wei et al.


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Short summary
Estuaries are key reactive ecosystems along the land–ocean aquatic continuum and are often strongly impacted by anthropogenic activities. We calculated nutrient in and out fluxes by using a 1-D transient model for seven estuaries along the French Atlantic coast. Among these, large estuaries with high residence times showed higher retention rates than medium and small ones. All reveal coastal eutrophication due to the excess of diffused nitrogen from intensive agricultural river basins.