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

Abstract. 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 (CO₂) 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 CO₂ (pCO₂), indicating high biological production on the inner shelf; and higher salinity and pCO₂ 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⁻¹) 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 a 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 pCO₂ values occurred in surface waters of the western inner shelf. This plume relocation shifted the Louisiana shelf from a normally weak CO₂ sink (as in 2007) to a strong CO₂ 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 O₂ compensation than CO₂ 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.