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Acidification of subsurface coastal waters enhanced by eutrophication


Human inputs of nutrients to coastal waters can lead to the excessive production of algae, a process known as eutrophication. Microbial consumption of this organic matter lowers oxygen levels in the water1,2,3. In addition, the carbon dioxide produced during microbial respiration increases acidity. The dissolution of atmospheric carbon dioxide in ocean waters also raises acidity, a process known as ocean acidification. Here, we assess the combined impact of eutrophication and ocean acidification on acidity in the coastal ocean, using data collected in the northern Gulf of Mexico and the East China Sea—two regions heavily influenced by nutrient–laden rivers. We show that eutrophication in these waters is associated with the development of hypoxia and the acidification of subsurface waters, as expected. Model simulations, using data collected from the northern Gulf of Mexico, however, suggest that the drop in pH since pre-industrial times is greater than that expected from eutrophication and ocean acidification alone. We attribute the additional drop in pH—of 0.05 units—to a reduction in the ability of these carbon dioxide-rich waters to buffer changes in pH. We suggest that eutrophication could increase the susceptibility of coastal waters to ocean acidification.

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Figure 1: Spatial distributions of near-bottom water properties in May 2007 and August 2007 from the nGOM.
Figure 2: Relationships between subsurface water pH and O2 concentration from the nGOM and ECS shelves and model simulations.
Figure 3: A conceptual model for a large river plume eutrophication and subsurface water hypoxia and acidification.
Figure 4: Seawater buffering capacity as a function of CO2 addition.

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The Mississippi River plume/nGOM work was funded by US EPA, NSF, and NASA. The Changjiang/ECS work was funded by the National Natural Science Foundation of China through an international collaboration grant, the National Basic Research Program of China, and the National Science Council of the Republic of China. We thank N. Zheng, F. Chen and Z. Gao for help with sample collection. R. Wanninkhof is acknowledged for helpful comments.

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W-J.C., X.H., W-J.H., X.G., and Y.W. are responsible for CO2 and pH data collection in the nGOM and W-C.C., W.Z., and M.D. for those in the ECS. J.C.L., M.C.M., S.E.L. and K.G. are responsible for O2 and nutrient data collection in the nGOM and W.Z. and G-C.G. for those in the ECS. All authors discussed the results and commented on the manuscript.

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Correspondence to Wei-Jun Cai.

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The authors declare no competing financial interests.

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Cai, WJ., Hu, X., Huang, WJ. et al. Acidification of subsurface coastal waters enhanced by eutrophication. Nature Geosci 4, 766–770 (2011).

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