Responses of carbonate system and CO2 flux to extended drought and intense flooding in a semiarid subtropical estuary

Globally, estuaries are considered important CO2 sources to the atmosphere. However, estuarine water carbonate chemistry and CO2 flux studies have focused on temperate and high latitude regions, leaving a significant data gap in subtropical estuaries. In this study, we examined water column carbonate system and air–water CO2 flux in the Mission-Aransas Estuary, a subtropical semiarid estuary in the northwestern Gulf of Mexico, by collecting samples at five System Wide Monitoring Program stations from 05/2014 to 04/2015. The carbonate system parameters (total alkalinity [TA], dissolved inorganic carbon [DIC], pH, CO2 partial pressure [pCO2], and carbonate saturation state with respect to aragonite [ΩAr]) and air–water CO2 flux all displayed substantial seasonal and spatial variations. Based on freshwater inflow conditions, a drought period occurred between 05/2014 and 02/2015, while a flooding period occurred from 03/2015 to 04/2015. Average DIC was 2194.7 ± 156.8 μmol kg−1 and 2132.5 ± 256.8 μmol kg−1, TA was 2497.6 ± 172.1 μmol·kg−1 and 2333.4 ± 283.1 μmol kg−1, pCO2 was 477 ± 94 μatm and 529 ± 251 μatm, and CO2 flux was 28.3 ± 18.0 mmol C·m−2·d−1 and 51.6 ± 83.9 mmol·C·m−2·d−1 in the drought and flooding period, respectively. Integrated annual air–water CO2 flux during our studied period was estimated to be 12.4 ± 3.3 mol·C·m−2·yr−1, indicating that this estuary was a net CO2 source. High wind speed, warm climate, riverine input, and estuarine biogeochemical processes all contributed to the high CO2 efflux despite the modest pCO2 levels year round.