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

• Published in: Marine chemistry 2014-12, Vol.167, p.82-92
• Main Authors: Macklin, Paul A., Maher, Damien T., Santos, Isaac R.
• Format: Article
• Language: English
• Subjects: Brackish; Canals; Carbon dioxide; Coastal; Coastal hydrology; Drying; Earth sciences; Earth, ocean, space; Estuaries; Exact sciences and technology; External geophysics; Floodplain; Fluxes; Geochemistry; Greenhouse gases; Groundwater; Mangrove; Marine; Mineralogy; Moreton Bay; Permeable sediments; Physical and chemical properties of sea water; Physics of the oceans; Residential; Silicates; Water geochemistry
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/j.marchem.2014.08.002

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 300km 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 with pCO2 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 67mmol 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. •pCO2 and 222Rn were measured in residential canals and natural estuarine waterways.•Residential canals had the highest contribution to estuary CO2 emissions (~50%).•Coupling between pCO2 and 222Rn indicated groundwater seepage was a driver of pCO2.