Air-water CO2 Fluxes and Inorganic Carbon Dynamics in a Microtidal, Eutrophic Estuary

Though the role of coastal margins in the global carbon cycle is on a slow rise up the scientific agenda, carbon dynamics in microtidal estuaries have been largely ignored. High spatial and temporal variability of wind and water conditions in these systems make integration of air-water CO2 fluxes particularly difficult. Given the high degree of monitoring effort required to accurately determine air-water CO2 fluxes, does the short term variability and significance in the coastal carbon cycle justifies the means? In an effort to address this question we examined air-water CO2 fluxes in the microtidal, eutrophic Neuse River Estuary, NC (NRE) from June 2009 to July 2010. High-resolution, continuous-flow surveys of CO2 partial pressure, pCO2, were conducted biweekly spanning the longitudinal axis of the estuary from the tidal freshwater region to the polyhaline border with the Pamlico Sound. Lateral transects were conducted in each of three hydrologically distinct sections. Discrete surface and bottom water dissolved inorganic carbon (DIC) samples were input into a conservative tracer box model to determine estuarine carbon cycle parameters and exchange coefficients. High temporal variability of air-water CO2 fluxes was closely associated with climatological events affecting estuarine stratification and riverine discharge. The greatest flux rate increase over a 10 day period (-3.01 to 54.76 t-C d-1) was observed during the steepest rise (>500%) in river discharge. Smoother trends were observed on seasonal scales with maximum and minimum whole estuary air-water CO2 flux rates of 54.76 and -20.84 t-C d-1 in mid-fall and late-spring respectively. Air-water CO2 fluxes in the NRE were found to account for up to 5% of total estuarine C import during undersaturated conditions and 14% of C export during oversaturated conditions. Though accurate characterization of air-water CO2 fluxes requires intensive monitoring resources, the magnitude and variability of these exchanges warrant the effort to integrate CO2 fluxes into the coastal carbon budget.