Globally, estuaries are considered important CO 2 sources to the atmosphere. However, estuarine water carbonate chemistry and CO 2 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 airwater CO 2 flux in the Mission-Aransas Estuary, a subtropical semiarid estuary in the 21 and 2132.5 6 256.8 lmol kg 21 , TA was 2497.6 6 172.1 lmolÁkg 21 and 2333.4 6 283.1 lmol kg 21 , pCO 2 was 477 6 94 latm and 529 6 251 latm, and CO 2 flux was 28.3 6 18.0 mmol CÁm 22 Ád 21 and 51.6 6 83.9 mmolÁCÁm 22 Ád 21 in the drought and flooding period, respectively. Integrated annual air-water CO 2 flux during our studied period was estimated to be 12.4 6 3.3 molÁCÁm 22 Áyr 21 , indicating that this estuary was a net CO 2 source. High wind speed, warm climate, riverine input, and estuarine biogeochemical processes all contributed to the high CO 2 efflux despite the modest pCO 2 levels year round.
Estuaries are generally considered a source of CO2 to the atmosphere, although with significant uncertainties in magnitude and controlling factors between and within estuaries. We studied four northwestern Gulf of Mexico estuaries that experience extreme hydrologic conditions between April 2014 and February 2017 to determine the role of dry/wet cycle on estuarine CO2 system. Annual air–water CO2 flux ranged from 2.7 to 35.9 mol·C·m−2·yr−1; CO2 flux declined by approximately an order of magnitude along with declining river discharge. Episodic flooding made CO2 flux differ between dry (−0.7 to 20.9 mmol·C·m−2·d−1) and wet (11.6–170.0 mmol·C·m–2·d–1) conditions. During wet condition, increases in dissolved inorganic carbon (DIC) and total alkalinity (TA) significantly elevated CO2 degassing. Furthermore, ventilation of river‐borne CO2 strengthened degassing when estuaries became overwhelmingly river‐dominated. During flood relaxation, all estuaries experienced heightened productivity, evidenced by DIC and TA consumption in the mid‐salinity range (10–30). When prolonged drought led to hypersalinity (>36.5), biogeochemical and evaporative effects enhanced DIC and TA consumption and CO2 degassing. Due to flooding and high wind speeds, these estuaries were a strong CO2 source during spring and summer. Then they transitioned to a weak CO2 source or sink during the fall. Low temperatures further depressed CO2 efflux during winter except when a pulse of freshwater input occurred. This study demonstrates that changes in the hydrologic condition of estuaries, such as dry/wet cycle and river discharge gradient, will greatly alter air–water CO2 flux and estuarine contribution to the global carbon budget.
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