Seasonal variability in air-sea fluxes of CO2 in a river-influenced coastal margin

Recent studies in the northern Gulf of Mexico and elsewhere have demonstrated that enhanced biological production in large river plumes may contribute to a net surface influx of atmospheric CO2. However, large rivers also deliver significant amounts of terrestrial carbon into continental margin waters; hence, the potential for large and variable signals in carbon flux exist in these regions. Here, we used a combination of satellite and ship-based observations to examine variability in surface pCO(2) and air-sea flux of carbon dioxide in relation to variations in river discharge and seasonal environmental conditions. Underway surface pCO(2) showed large seasonal differences based on observations acquired during cruises in August 2004, October 2005, and April 2006. Strong cross-shelf gradients in pCO(2) were observed during August 2004 and April 2006, influenced by river outflow. Uniformly high values observed during October 2005 likely reflected the disturbed nature of the system after two major storm events (hurricanes Katrina and Rita). Satellite-derived assessments of pCO(2) were used in conjunction with estimates of wind fields to produce regional maps of surface water pCO(2) and air-sea fluxes. The region was a net sink for atmospheric CO2 in August 2004 (-0.96 to -1.2 mmol C m(-2) d(-1)) and net source during October 2005 and April 2006 (1.0 to 5.4 mmol C m(-2) d(-1)). Uncertainties in flux estimates, particularly for low salinity waters in April 2006, highlighted the need for more extensive in situ observations. Our results illustrate the utility of satellite approaches for providing regional assessments of coastal carbon budgets.