07 Sep 2023
 | 07 Sep 2023
Status: this preprint is currently under review for the journal BG.

Hypoxia also occurs in small highly turbid estuaries: the example of the Charente (Bay of Biscay) 

Sabine Schmidt and Ibrahima I. Diallo

Abstract. The French coast facing the Bay of Biscay (north-east Atlantic) is characterised by the presence of small macrotidal and turbid estuaries, including the Charente, geographically located between the two large estuaries of the Gironde and the Loire (south-west France). Multi-year, multi-site, high-frequency water quality surveys have shown that the Loire, and to a lesser extent the Gironde, suffer from summer hypoxia. These observations raised the question of the possible occurrence of hypoxia, particularly in one of these small estuaries, the Charente, which flows into the Bay of Marennes-Oléron, the first oyster-producing area in France. Unlike its two large neighbours, the Charente estuary is not continuously monitored, although it is subject to similar climatic changes and anthropogenic pressures, making it impossible to assess potential risks to the ecosystem. Here we present a first investigation of dissolved oxygen in the Charente estuary, based on a combination of longitudinal studies along the estuary axis and instrumented sites to determine the intensity and spatial extent of deoxygenation. Oxygen and salinity sensors were deployed at several sites during the summers of 2018, 2019 and 2020 to record temperature, salinity and dissolved oxygen every 15 minutes. This high-frequency dataset will be compared with a long-term low-frequency dataset (1975–2022) to determine whether or not there is a degradation of oxygenation in the Charente estuary. The high-frequency dataset shows a high variability of dissolved oxygen (DO), with a clear influence of tidal cycles. During summer, DO concentrations are often below 5 mg L-1, and sometimes even below 2 mg L-1, indicating the presence of a summer oxygen minimum zone with an extension of about 20–25 km. Temperature is the DO main controlling factor, limiting preventive management strategies and, in the context of global warming, raising questions about the long-term suitability of the estuarine conditions for community needs, in particular migratory fishes.

Sabine Schmidt and Ibrahima I. Diallo

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-2023-150', Anonymous Referee #1, 13 Sep 2023
    • AC1: 'Reply on RC1', Sabine Schmidt, 17 Sep 2023
  • RC2: 'Comment on bg-2023-150', Anonymous Referee #2, 26 Nov 2023
    • AC2: 'Reply on RC2', Sabine Schmidt, 03 Dec 2023
Sabine Schmidt and Ibrahima I. Diallo
Sabine Schmidt and Ibrahima I. Diallo


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Short summary
Along the French coast facing the Bay of Biscay, the two main estuaries, the Gironde and the Loire, experience hypoxia, motivating this study of the small Charente estuary between them. This work highlights a minimum oxygen zone in the Charente estuary extending for about 25 km, temperature being the main factor controlling the hypoxia. This calls for the monitoring of small highly turbid macrotidal estuaries vulnerable to hypoxia, a risk that will increase with global warming.