Seasonality of Carbonate Chemistry and CO2 Flux in a Northwestern Gulf of Mexico estuary

Estuaries are important CO₂ source to the atmosphere and exhibit significant spatial and temporal variability. Currently, relatively little is known regarding the role of subtropical semiarid estuaries in the carbon cycle and their carbonate chemistry. In this study we examined seasonality of carbonate system and CO₂ flux in the Mission-Aransas estuary, a shallow subtropical semiarid estuary in the Northwestern Gulf of Mexico, during a one-year period (05/2014-04/2015). This estuary includes three interconnected coastal bays (Aransas, Copano, and Mesquite) that have little direct freshwater input from rivers. Average pH (total scale) was 8.017±0.096 and varied between 7.515 and 8.317. Annual mean total dissolved inorganic carbon (DIC) and total alkalinity (TA) were 2183.2±180.4 µmol kg^-1 and 2467.2±206.7 µmol·kg^-1, respectively. Both DIC and TA decreased from June to October, 2014 with increasing salinity, then started to increase when salinity decreased after heavy precipitation evens in November, 2014. Contrary to DIC and TA patterns, the highest carbonate saturation state (4.89) with respect to aragonite (Ω_aragonite) was observed in August 2014, and the lowest (0.20) in March 2015. Overall, high Ω_aragonite (>4.0) occurred in hypersaline seawater (salinity>35). Calculated annual average pCO₂ was 487±138 µatm, with the annual high occurring in early summer (May to June, 2014, 544±76 µatm) and annual low at 352±33 µatm in winter (January to February, 2015). During the flooding period from January to April, 2015, DIC and TA decreased dramatically while pCO₂ first decreased to below the atmosphere level and then increased with maximum level reaching nearly 1700 µatm, indicating a trophic state transition during the development and relaxation periods of the flood. Average annual CO₂ flux in this estuary was estimated to be 7.0±2.0 10⁹g-C·yr^-1. The highest CO₂ efflux (20.6±10.9 mmol·m^-2·d^-1) occurred in August, 2014, and this estuary turned to a CO₂ sink (-1.9±0.6 mmol·m^-2·d^-1) briefly in February, 2015.