Impacts of a weather event on shelf circulation and CO2 and O2 dynamics on the Louisiana shelf during summer 2009

Other literature type English OPEN
Huang, W.-J. ; Cai, W.-J. ; Wang, Y. ; Hopkinson, C. S. (2013)

While much is known about the physics of coastal currents, much less is known about the biogeochemical effects of surface currents on shelf carbon dioxide (CO<sub>2</sub>) and oxygen distribution and dynamics. The Mississippi and Atchafalaya River plume is usually observed along the Louisiana shelf with easterly winds. Such a typical pattern was observed in August 2007, i.e. a plume of low salinity and low partial pressure of CO<sub>2</sub> (<i>p</i>CO<sub>2</sub>), indicating high biological production on the inner shelf; and higher salinity and <i>p</i>CO<sub>2</sub> on the outer shelf. This high biological production induced by riverine nitrogen flux thus provided major organic matter sources for the shelf-wide hypoxia (dissolved oxygen [DO] < 2 mg L<sup>−1</sup>) accompanied by high dissolved inorganic carbon (DIC) concentrations in the bottom water. The slope of the DO and DIC relationship also demonstrated Redfield-type respiration in this shelf-wide hypoxia. In contrast, summer 2009 was an abnormal season characterized by a cool temperature in the central North America. Our observation and satellite chlorophyll <i>a</i> patterns both displayed a greatly distinct situation, i.e., the river plume was relocated to the eastern part of the Louisiana shelf; and high salinity and high <i>p</i>CO<sub>2</sub> values occurred in surface waters of the western inner shelf. This plume relocation shifted the Louisiana shelf from a normally weak CO<sub>2</sub> sink (as in 2007) to a strong CO<sub>2</sub> source for the atmosphere. Although riverine nitrogen flux was enough to support a shelf-wide hypoxia in 2009, the plume relocation changed the location of high biological production and resulted in a limited hypoxic area. Furthermore, DIC concentration in bottom waters was higher than those predicted by the Redfield ratio, most likely because of much rapid O<sub>2</sub> compensation than CO<sub>2</sub> loss during air–sea exchange. Numerical models indicate such relocation of plume was mostly affected by the shelf circulation dominated by southerly and southwesterly winds. Consequently, we conclude that wind-forcing and shelf circulation are critical factors that influence the plume trajectories and the associated biogeochemical properties in coastal waters.
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