Submarine groundwater discharge estimated from isotopic tracers : implications to carbon cycling in three coastal systems

• Main Author: Escobar Correa, Rogger
• Format: Text
• Language: unknown
• Published: Southern Cross University 2021
• Subjects: Radium; Radon; Estuaries; Blue Carbon; Ocean acidification
• Online Access: https://dx.doi.org/10.25918/thesis.154; https://researchportal.scu.edu.au/esploro/outputs/doctoral/991012951900502368

Submarine groundwater discharge (SGD) is defined as any input of water to the ocean coming from continental margins and sediments, regardless of its source or composition. This thesis aimed to gain a better understanding of carbon transported by SGD to three different coastal environments. To achieve these aims, I estimated SGD fluxes and associated carbon fluxes using isotopic mass balance models. The first environment was an urbanized estuary close to a megacity (Sydney Harbour, Australia), the second was a pristine coral reef in an active volcanic area (Mabini, Philippines) and the third was a salt marsh belonging to a national estuarine reserve (North Inlet, SC, USA). I assessed the relative contribution of SGD to water and carbon fluxes. At the Sydney Harbour estuary, the estimated SGD rates (2.2±1.5 cm d-1) were comparable to the global average of radium-derived-SGD in other urban estuaries (3.1±2.4 cm d-1). SGD can deliver between 124 to 203 x103 mol d-1 of dissolved organic carbon, a flux that exceeds local river inputs by over 100%. Nutrients also exceeded riverine exports to the estuary, meaning that SGD plays a major role sustaining primary productivity. At a coral reef receiving hydrothermal inputs in the Philippines, SGD exports 254 ± 35 mmol m2 d-1 of CO2 sustaining seawater super-saturation and driving localized ocean acidification. Finally, in a salt marsh in North Inlet, USA, SGD exported 57± 43 mmol m2 d-1 of CO2, 602± 460 mmol m2 d-1of DIC and 19± 14 mmol m2 d-1of DOC. These carbon fluxes exceeded carbon burial in the salt marsh soils. Hence, groundwater flushes soil carbon and reduces the potential of the marsh soils to sequester carbon over long time scales. Overall, this work provides insight and perspective on the implications of SGD in three contrasting coastal environments. SGD is often an overlooked component in the carbon cycling of diverse coastal ecosystems, and should be accounted for in coastal carbon budgets.