Seasonal Drivers of Carbon Dioxide Dynamics in a Hydrologically Modified Subtropical Tidal River and Estuary (Caboolture River, Australia)

Estuaries are dynamic hot spots for carbon cycling and atmospheric evasion. Here we assess the dynamics and drivers of CO 2 in a hydrologically modified subtropical Australian estuary. Over 1 year, 10 high‐resolution spatial surveys of p CO 2 , radon, chromophoric dissolved organic matter, chlorophyll a , and physicochemical parameters were conducted from the estuary mouth to a weir located 25 km upstream. The riverine respiratory quotient revealed that processes besides water column respiration were driving high CO 2 within the tidal river (salinity < 2) but not in the estuary (salinity > 2). Conservative mixing plots for p CO 2 and 222 Rn implied that groundwater may be a contributing source of CO 2 during most surveys, but not during dry conditions. A multiple linear regression model explained 88% of the annual p CO 2 variability, indicating that mixing, metabolism, temperature, and groundwater inputs were key drivers of CO 2 . Inputs from an upstream wastewater outfall potentially fuel observed seasonal algal blooms, resulting in the lowest daytime CO 2 emissions periods. Postbloom surveys had the highest daytime CO 2 emissions. The average tidal river CO 2 atmospheric flux rate was 379 ± 53 mmol m −2 day −1 . The average estuarine CO 2 flux was 78 ± 17 mmol m −2 day −1 , equating to 28 ± 6 mol m −2 yr −1 . Although the tidal river surface area was ~10 times smaller than the estuary, about one third (35%) of the CO 2 emissions were derived from the tidal river. Our results suggest that CO 2 emissions along the tidal river‐estuary continuum are dynamic over small temporal and spatial scales and that a combination of hydrological and biological processes is a controlling factor of this variability.