Estuarine canal estate waters: hotspots of CO2 outgassing driven by enhanced groundwater discharge?

Increased water-to-air carbon dioxide fluxes are a potentially important, but as yet unquantified, consequence of canal estate developments in estuaries surrounded by coastal wetlands. We used detailed pCO₂ and radon (²²²Rn, a submarine groundwater discharge tracer) surveys to investigate whether water-to-air CO₂ fluxes were enhanced in residential canal systems, and whether groundwater exchange may drive pCO₂ distribution. Observations were performed along 300 km of canals, rivers, estuaries, and coastal embayments from the Gold Coast (Queensland, Australia), one of the largest estuarine canal systems globally. Overall, residential canal estate waters were supersaturated in CO₂ withpCO₂ ranging from 372 to 3639 μatm and 434 to 3080 μatm in the dry and wet season surveys, respectively. pCO₂ usually increased in areas of reduced connectivity (i.e., poorly flushed dead end canals). A stronger correlation between ²²²Rn and pCO₂ than between dissolved oxygen and pCO₂ implied that groundwater seepage (not pelagic respiration) was the major driver pCO₂ supersaturation within the canal system. Average area-weighted water-to-air CO₂ fluxes within canals were 34 and 67 mmol C m^{− 2} d^{− 1} during the dry and wet seasons respectively. When upscaled to the entire Gold Coast estuarine system, residential canal contributed 46% and 56% of the total flux of CO₂ to the atmosphere during the dry and wet seasons, respectively. These results imply that areas that were previous atmospheric carbon sinks (i.e. coastal wetlands) have become sources of CO₂ to the atmosphere since the development of residential canal estates.