High CO2 evasion during floods in an Australian subtropical estuary downstream from a modified acidic floodplain wetland

Dissolved CO2 dynamics associated with flooding and postflood recovery were investigated in the lower estuary of the Richmond River and a tributary draining an acidic wetland. Heavy rains (up to 133 mm d21) resulted in large discharge volumes (up to 250 3 106 m3 d21) flushing the estuary in less than a day. Maximum values of the partial pressure of CO2 (PCO2 , 2006 Pa or 19,801 latm) occurred during the postflood recovery period. High PCO2 values were related to the transport of floodplain metabolic products via surface runoff and groundwater as well as the low pH (4.22) of waters draining a modified wetland with coastal acid sulphate soils. Aerobic respiration alone could not explain the high concentrations of CO2 in the estuary and an area of low buffering developed in the 0–20 salinity range. PCO2 was correlated with freshwater discharge in the Richmond River main channel (R2 5 0.82; p \u3c 0.05; n 5 9) but not in the acidified tributary (R2 5 0.17; p \u3e 0.05; n 5 7). Air-water fluxes of CO2 were at the high end of the range for aquatic systems (maximum 1413 mmol C m22 d21, mean 252 mmol C m22 d21). This study highlights the large spatiotemporal variability of PCO2 , the importance of episodic flooding events, the role of adjacent modified acidic wetlands, and the role of groundwater discharge as a source of CO2 to estuaries. Global estimates of CO2 air-water fluxes in estuaries may be under estimated due to the lack of high-resolution data to capture short-term episodic events (e.g., flooding), in tropical and subtropical areas