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

Abstract

Estuaries are dynamic hot spots for carbon cycling and atmospheric evasion. Here we assess the dynamics and drivers of CO2 in a hydrologically modified subtropical Australian estuary. Over 1 year, 10 high‐resolution spatial surveys of pCO2, 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 CO2 within the tidal river (salinity < 2) but not in the estuary (salinity > 2). Conservative mixing plots for pCO2 and 222Rn implied that groundwater may be a contributing source of CO2 during most surveys, but not during dry conditions. A multiple linear regression model explained 88% of the annual pCO2 variability, indicating that mixing, metabolism, temperature, and groundwater inputs were key drivers of CO2. Inputs from an upstream wastewater outfall potentially fuel observed seasonal algal blooms, resulting in the lowest daytime CO2 emissions periods. Postbloom surveys had the highest daytime CO2 emissions. The average tidal river CO2 atmospheric flux rate was 379 ± 53 mmol m−2 day−1. The average estuarine CO2 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 CO2 emissions were derived from the tidal river. Our results suggest that CO2 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.

Carbon outwelling and outgassing vs. burial in an estuarine tidal creek surrounded by mangrove and saltmarsh wetlands

“…Even though high sulphate concentrations in coastal pore waters can inhibit methanogenesis, methanogenesis can still occur in intertidal sediments (Kristensen et al ). Significant methane emissions have been observed in mangrove‐dominated systems driven by pore water exchange (Call et al ; Jeffrey et al ; Rosentreter et al ). It remains unclear to what extent methane emissions offset the carbon sequestration capacity of mangroves and saltmarshes as previously observed in freshwater wetlands (Whiting and Chanton ).…”

Controls on Carbon, Nutrient, and Sediment Cycling in a Large, Semiarid Estuarine System; Princess Charlotte Bay, Australia

“…Groundwater input to estuaries has been previously reported to increase CO 2 efflux to the atmosphere (Jeffrey et al, ; Maher et al, ; Sadat‐Noori et al, ). This study is the first time an opposite effect on estuarine CO 2 flux has been quantified for which we are aware.…”

The mangrove CO2 pump: Tidally driven pore‐water exchange

“…To build carbon budgets and understand mangrove's capacity to ultimately sequester blue carbon, there has been growing interest in soil carbon sequestration (Alongi 2014) and water‐atmosphere CO 2 exchange (e.g., Borges et al 2003; Rosentreter et al 2017; Jeffrey et al 2018 b ) in mangroves. Previous investigations conceptually linked CO 2 to pore‐water exchange (Santos et al 2012 b ; Call et al 2015, 2019 a ; Rosentreter et al 2018) or quantified groundwater/pore water‐derived dissolved inorganic carbon (Maher et al 2013; Faber et al 2014; Chen et al 2018; Taillardat et al 2018 b ; Santos et al 2019).…”

“…We estimate that the CO 2 influx by pore‐water exchange exceeded the CO 2 emissions to the atmosphere by more than 100% in the Coffs Creek estuary. The excess amount of the CO 2 from pore water is likely exported to the ocean (Faber et al 2014; Jeffrey et al 2016, 2018 b ; Maher et al 2018). Therefore, we suggest that the pore water‐derived dissolved CO 2 export is an important component of mangrove carbon budgets and that accounting for pore‐water exchange will help to reduce uncertainties in mangrove carbon budgets.…”

“…We estimate that the CO 2 influx by pore-water exchange exceeded the CO 2 emissions to the atmosphere by more than 100% in the Coffs Creek estuary. The excess amount of the CO 2 from pore water is likely exported to the ocean (Faber et al 2014;Jeffrey et al 2016Jeffrey et al , 2018bMaher et al 2018). Therefore, we suggest that the pore waterderived dissolved CO 2 export is an important component of mangrove carbon budgets and that accounting for pore-water (Rosentreter et al 2018), carbon burial in sediments (Bouillon et al 2008;Mcleod et al 2011;Breithaupt et al 2012), ecosystem respiration (e.g., tree, soil, and water respiration) (Alongi 2009), and net primary production (Alongi 2009).…”