Abstract:
Mangroves, one of the Earth ’s most carbon-rich ecosystems, serve as important blue carbon sinks. Submarine Groundwater Discharge (SGD), an important land-sea interaction process, plays a crucial role in the lateral transport of carbon within mangrove ecosystems. In this study, we conducted a 24-hour time series observation in the tidal creek of Zhangjiang Mangrove in Yunxiao County, Fujian Province, in April 2017. We collected samples from river, pore water and domestic sewage. Measurements of radium (Ra) isotopes, dissolved inorganic carbon (DIC), total alkalinity (TA), pH, and flow velocity were taken to quantify the flux of dissolved carbon in mangroves, encompassing both pore water and surface water transport. The results show that DIC concentration in surface water is positively correlated with Ra activity (SGD tracer), and pore water serves as a significant source of DIC in tidal creek, which is higher than river (Zhangjiang) input and sewage discharge. Based on the Ra mass balance model and the tidal dynamic model, we estimated the pore water exchange rate to be approximately (0.2±0.1) m/d. The fluxes of DIC, TA, dissolved organic carbon (DOC), and free CO
2 per unit mangrove area were measured at 1013, 1008, 332, and 52 mmol/(m
2·d), respectively. Under the influence of tides, the fluxes of DIC, TA, DOC, and free CO
2 per unit area of tidal channel water transported to the estuary were recorded at 1 136, 1 349, 131, and 170 mmol/(m
2·d), respectively. The primary export form of dissolved carbon was DIC, constituting 82% of the total. The dissolved carbon flux, totaling 3.9×10
4 mol/d, transported by the Zhangjiang Mangrove to the tidal creek contributed 89% of the lateral DIC flux conveyed by the surface water of the tidal creek. Additionally, it accounted for 23% of the carbon sequestration of mangrove vegetation, a proportion comparable to the carbon buried in the sediment of the Zhangjiang mangrove forest (3.2 ×10
4 mol/d). This study represents a significant component of the coastal blue carbon budget.