Intertidal Mangroves as Hotspots for Groundwater Transport of Aromatic Dissolved Organic Matter to Coastal Oceans

Submarine groundwater discharge (SGD) from mangroves is an important yet understudied pathway for the transport of aromatic dissolved organic matter (DOM) to the ocean. Here, we investigated the spatial variability and fluxes of two major aromatic DOM components, dissolved black carbon (DBC) and dissolved lignin phenols (DLP), in groundwater from a subtropical intertidal mangrove. Results showed significantly higher DBC and DLP concentrations in shallow groundwater than in deeper layers. The monomer ratios of DBC and DLP revealed distinct transformation patterns: more condensed DBC accumulated at depth, while DLP showed higher degradation in the recharge zone compared with the discharge zone. Hydrological factors such as hydroperiod and infiltration capacity dominated over geochemical factors (e.g., pH, dissolved oxygen, and salinity) in shaping DBC and DLP characteristics. Area-normalized fluxes of DBC (2.38 mg m^-2 d^-1, equivalent to 869.7 kg km^-2 yr^-1) and DLP (0.13 mg m^-2 d^-1, equivalent to 46.0 kg km^-2 yr^-1) via mangrove SGD significantly exceeded those from major global river systems, identifying intertidal mangroves as previously underappreciated sources of aromatic DOM to coastal waters. These findings expand our understanding of mangrove carbon cycling and highlight the need to incorporate SGD-derived aromatic DOM into the refinement of global carbon budgets.