Inorganic carbon distribution and dynamics in the Mississippi River plume on the northern Gulf of Mexico
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The continental shelf, especially the river-dominated shelf, plays an important role in the global carbon cycle. Thirteen cruises were conducted to measure sea surface salinity, temperature, and partial pressure of carbon dioxide (pCO2) on the Louisiana (LA) shelf, a shelf great influenced by the Mississippi and Atchafalaya River System (MARS) from 2004 to 2010. The result showed that the LA shelf was a sink of atmospheric CO2 , particularly strong on the eastern shelf, with a seasonal variation: strong in spring, weak during summer; shifting to a weak source in fall and back to near equilibrium in winter. In terms of salinity sub-divisions, the small low salinity region (S = 0 to 17) acted as a strong CO2 source to the atmosphere, the wide middle salinity region (17<S<33) acted as a CO2 sink, and middle-to-high salinity region (33<S<35) was near neutral to the atmosphere. The MARS plume was subjected to local wind forcing, thus the pCO2 spatial variation on the LA shelf was mostly dominated by this varied plume trajectory. The plume associated processes, including mixing, autotrophic and heterotrophic activities, and gas exchanges, further controlled the CO2 dynamics in the plume. A widespread plume in March 2010 induced by northerly wind forcing provided a clear example illustrating that the MARS plume was affected by wind forcing and transition from autotrophy to heterotrophy along the plume trajectory. The altered shelf circulation in July 2009 further demonstrated that not only surface water but also the bottom water conditions were affected, such as the extent of hypoxia (dissolved oxygen less than 2 mg L-1). This study also revealed that the Mississippi riverine nitrate plus nitrite fluxes were positively correlated with the magnitude of the CO2 sink in the Louisiana Bight, providing the first quantitative estimation of the relationship between the anthropogenic export nitrogen and air-sea CO2 fluxes. To sum up, this dissertation describes the CO2 dynamics under river-to-sea interaction and also air-to-sea interaction, i.e. the local wind forcing dominated the river plume trajectory while the transition from autotrophic to heterotrophic activities and the compensatory gas exchanges along the trajectory were largely responsible the CO2 variation in the plume.