Influence of submarine groundwater discharge in the blue carbon budget of typical mangrove: A case study from the Zhenzhu Bay, Guangxi

As one of the forms of land-ocean interactions, submarine groundwater discharge (SGD) can release solutes into the coastal sea and has a significant impact on the nutrients budget in coastal seawater. Here, using 222Rn tracer, the SGD and the associated dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) inputted to the Zhenzhu Bay, a typical mangrove-dominated bay, were quantified. The results show that the average concentrations of²²²Rn, DIC and DOC in groundwater are relatively higher than those in river water and surface sea water. A 222Rn mass balance implies that SGD rate is (0.36±0.36) m/d during January 2019. And SGD-derived DIC and DOC fluxes are estimated to be (2.41±2.63)×10⁷ mol/d and (1.96±2.20)×10⁶ mol/d. It confirmed that SGD-derived carbon is the most important carbon source in this bay, with 91% DIC and 89% DOC of the total input fluxes by SGD, respectively. Our results highlight the importance of groundwater-derived carbon fluxes in the Zhenzhu Bay, especially in the blue carbon assessments and biogeochemical process in tidal zones such as mangrove ecosystems.