Summer-time Co2 Fluxes and Carbonate System Behavior in the Mississippi River and Orinoco River Plumes

High-resolution underway measurements of surface water pH, CO2 fugacity (fCO2), and total dissolved inorganic carbon (DIC), along with discrete bottle sample analyses from depths, were obtained in July 2005 and September 2006 in the Mississippi River Plume (MRP) and the Orinoco River Plume (MRP). The resulting data were analyzed to compare and contrast the CO2 fluxes and carbonate system behavior of the two river plumes. The surface water within the MRP shows a strong atmospheric CO2 sink, while oligotrophic waters of the Gulf of Mexico were a CO2 source. The CO2 sink of the ORP is much less significant, and the adjacent surface water of the Caribbean Sea was approximately in equilibrium with the atmosphere. The carbonate system inside the MRP is strongly influenced by net biological uptake. This is less significant for the ORP. The strength of the carbon sinks in the two plume systems differs significantly due to both natural and anthropogenic influences. 1. BACKGROUND Coastal oceans are a critical component of the global carbon cycle, and their capacity to absorb atmospheric CO2 plays an important role in the global CO2 budget [1, 2]. Coastal oceans are highly dynamic and heterogeneous due to complex physical, chemical, and biological forcings. Thus the factors that influence air-sea CO2 flux are often complex and highly variable in coastal waters, making modeling and prediction difficult. A recent synthesis of CO2 measurements and flux calculations by Chen and Borges [2] indicates open continental shelf waters experience a net influx of atmospheric CO2 of 0.33 – 0.36 Pg C yr. However, these estimates still bear large uncertainty due to limited data coverage in coastal oceans around the globe, and many shelf regions lack any pCO2 measurements. Arguably the greatest uncertainty in our understanding of the global carbon cycle surrounds biogeochemical processes that occur in coastal oceans. As a community we should move forward and broaden our research scope in coastal CO2 studies beyond air-sea CO2 flux to the investigation of biogeochemical mechanisms that control the coastal CO2 source/sink, since these processes may have important implications for the long-term responses of coastal systems to anthropogenic impacts. The Mississippi and Orinoco Rivers are the seventh and eighth largest rivers in terms of discharge in the world, respectively. Their impact on the chemistry, geology, physics, and biology of the ocean basins they enter extends thousands of kilometers from their river mouths. At times, the Mississippi River Plume (MRP) can reach the Florida Straits, and be traced as far north as Georgia [3]. The Orinoco River Plume (ORP) influences the entire southern Caribbean [4], and often combines with the Amazon plume to cause surface salinity anomalies over 2000 km away from the river mouths [5]. Nutrients from the Mississippi River fuel net biological uptake of inorganic carbon in its river plume water that generally produces areas under-saturated in CO2, creating CO2 sinks on the continental shelf [6,7]. Large river plumes may even make significant contribution to the CO2 flux on a ocean-basin scale as implied by the CO2 studies in the Amazon River Plume [8,9]. This study is one part of the project to study ecology and genomics of CO2 fixation in the Mississippi and Orinoco River Plumes. The two overarching goals of this project are: (1) to investigate the CO2 sink and carbonate system in the two large river plumes; (2) to link gene expression analysis and the carbonate system measurements in order to identify phytoplankton species/groups that are mainly responsible for carbon uptake inside vs. outside the plumes, and to study its possible causes. In this study, we will present the high-resolution data of the CO2 system (pH, DIC, and pCO2) measured during the two summer cruises in the Mississippi and Orinoco River Plumes. The main research objectives for this coastal carbon cycle study are: 1) Examine if the two major river plumes consist of significant carbon sink under summer conditions and estimate air-sea CO2 fluxes in surveyed area; 2) Estimate net biological production of the two river plumes in summer; 3) Compare and contrast the carbonate system behaviors between the MRP and ORP and examine their respective controlling biogeochemical processes.