Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat

The distributions of dissolved organic carbon (DOC), total dissolved hydrolysable amino acids (THAA), and colored dissolved organic matter (CDOM) were studied in pore-water/groundwater samples (including seeping water) from a subterranean estuary (STE) of a large tidal flat in Hampyeong Bay, Korea, in July 2011. The relatively low alanine D/L ratios and high THAA concentrations in the pore-water closest to the sediment surface (0–10 cm) indicate the active production of dissolved organic matter (DOM) from benthic algae, and the relatively low THAA concentrations and high D/L ratios in the subsurface pore-water (10–35 cm) indicate a relatively large presence of bio-degraded DOM. In the deep pore-water (35–75 cm), relatively low D/L ratios, high DOC concentrations, and intense humic-like fluorescence were observed, suggesting a net accumulation of less-reactive DOC in this layer. Overall, this STE appears to have net DOM sources because the concentrations of DOC (60–1700 μM) in the pore-water decreased toward the land, the surface, and the low-salinity waters. The concentrations of DOC in the seeping water (185 ± 52 μM) were higher than those in the overlying seawater (144 ± 9 μM), resulting in net DOC fluxes of 2–5 × 109 g·C·yr− 1 through submarine groundwater discharge (SGD) into Hampyeong Bay. The organic matter compositions in the seeping water indicated that SGD introduced DOM from both the surface and subsurface layers. Our results highlight that tidal flats are important sources for DOM, implying that SGD-driven DOM plays an important role in coastal carbon cycles and biogeochemistry.