Mangrove-derived dissolved organic carbon and allochthonous refractory subsidies via submarine groundwater discharge at regional and global scales

Submarine groundwater discharge (SGD) is the primary pathway for the lateral transport of dissolved organic carbon (DOC) in mangroves. However, mangrove-derived DOC flux and its fluorescent components via SGD remains poorly constrained. Based on ²²²Rn- and Ra-derived SGD rates, we calculate mangrove-derived DOC export to the northern Beibu Gulf via SGD to be 1.39 × 10⁶ mol d⁻¹. This flux is comparable to 7.4 % of the net primary productivity (NPP) of mangroves around the northern Beibu Gulf. Through Monte Carlo simulation (n = 1.5 × 10⁸ iterations), we estimated the global flux of mangrove-derived DOC to be 1.79 × 10¹² mol yr⁻¹, representing 11.7 % of the NPP of global mangroves and 10.5 % of global riverine outputs. This indicates that the export of DOC is a very important carbon sink within mangrove ecosystems. Combine with three-dimensional excitation-emission matrix spectroscopy (3D-EEMs), this study indicates that mangrove-derived DOC may have relatively high bioavailability. Based on regional observations from the Beibu Gulf in China, this study provides a preliminary estimation of the global export of refractory fluorescent components from mangroves. The results conservatively estimates that their contribution may be approximately equivalent to 10 % of riverine exports and is significantly higher than that from fresh submarine groundwater discharge (FSGD). These findings establish mangroves as a dual-function source, supplying both labile carbon for coastal productivity while being a significant sink of refractory carbon, highlighting their critical role in regulating coastal carbon cycling and blue carbon storage.