Tidal dynamics of dissolved carbon flux in ground water of a mangrove tidal creek in Qinglan Bay, Wenchang, Hainan

Submarine groundwater discharge (SGD) is an important pathway for transporting dissolved carbon from land to the ocean. A tidal creek of the Qinglan Bay (Hainan, China) was selected for study in the regard. The field work was conducted in Oct.-Dec., 2019. Results show that the proportion of freshwater SGD in was 2.5%～22.7%, in average of 13.0%, and the net SGD flow rate was 0.86×10⁵ m³/d. Based on the dissolved carbon concentration in groundwater, we estimated that the fluxes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) output from the tidal creek were 5.78×10⁵ mol/d and 0.10×10⁵ mol/d, respectively. Among them, the DIC was the main form of dissolved carbon transported in the tidal creek, accounting for 98.3% of the total dissolved carbon. At low tide, 49% of DOC came from mangrove plant debris, and 51% comes from SGD. In DIC, 78% came from SGD and 22% from organic matter in mangrove sediment. At high tide, 44% of DOC was from contaminated seawater and 56% from mangrove plant debris. In DIC, 65% came from seawater and 35% from pore water exchange. The SGD of mangrove tidal channels transported a large amount of dissolved carbon to the Qinglan Bay, and its impact on the carbonate balance system of nearshore water needs to be clarified in the future.