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