Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi,Pearl, and Rhône rivers |
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| Authors: | C Rabouille DJ Conley MH Dai W-J Cai CTA Chen B Lansard R Green K Yin PJ Harrison M Dagg B McKee |
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| Institution: | 1. Laboratoire des Sciences du Climat et de l’Environnement, UMR CEA-CNRS-UVSQ and IPSL, Av. de la Terrasse, 91198 Gif sur Yvette, France;2. GeoBiosphere Science Centre, Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden;3. State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China;4. Department of Marine Sciences, The University of Georgia, Athens, Georgia 30602, USA;5. Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan;6. Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USA;g Atmospheric, Marine and Coastal Environment Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China;h Department of Marine Sciences, University of North Carolina, Chapel Hill, NC 25599, USA;i Australian Rivers Institute, Griffith University (Nathan Campus), Brisbane, Qld. 4111, Australia |
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| Abstract: | We examined the occurrence of seasonal hypoxia (O2<2 mg l−1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m−2 d−1), and lower off the Rhône River (<1 g C m−2 d−1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia. |
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| Keywords: | Oxygen Hypoxia Carbon cycle Coastal oceanography Estuaries River plumes Nutrients Continental shelves |
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