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Acidification of subsurface coastal waters enhanced by eutrophication
Citation:
CAI, W., X. HU, W. HUANG, M. C. MURRELL, J. C. LEHRTER, S. E. LOHRENZ, W. CHOU, W. ZHAI, Y. WANG, X. GUO, K. GUNDERSEN, J. T. HOLLIBAUGH, M. DAI, AND G. GONG. Acidification of subsurface coastal waters enhanced by eutrophication. Nature Geoscience. Nature Publishing Group, London, Uk, 4(11):766-770, (2011).
Impact/Purpose:
This paper describes the potential synergistic effect of atmospheric CO2 increase and hypoxia on coastal water quality.
Description:
Uptake of fossil-fuel carbon dioxide (CO2) from the atmosphere has acidified the surface ocean by ~0.1 pH units and driven down the carbonate saturation state. Ocean acidification is a threat to marine ecosystems and may alter key biogeochemical cycles. Coastal oceans have also been affected by eutrophication, excessive production of organic matter (OM) in surface waters due to river inputs of agricultural fertilizer, and the related incidence of bottom water hypoxia due to oxygen consumption by OM respiration. Accompanying this hypoxia, low pH is expected as CO2 is produced during respiration and accumulates in bottom waters. While the chemistry of ocean acidification is well known, the interaction between fossil-fuel-derived CO2 and respiration-derived CO2 is not. We report here that these two sources of CO2 have acted in concert to lower pH greatly, as large as 0.45 units between today and preindustrial and pre-hypoxic time on subsurface waters of continental shelves impacted by two nutrient-laden larger rivers, the Mississippi (USA) and the Changjiang (China). The combined pH decrease is greater than the sum of the components due to a synergism between these distinct sources of CO2. We call this effect respiration-enhanced ocean acidification. The chemical nature of this synergism can be viewed as the progressive weakening of seawater buffering capacity with further CO2 addition. We show that these two processes, when combined, will lead to carbonate under-saturation in subsurface waters within this century. The large decreases in pH and carbonate saturation in coastal hypoxic areas provide unique opportunities to study how future ocean acidification may alter important ocean biogeochemical processes.