Spatiotemporal variations of dissolved organic matter in coastal groundwater and controlling biogeochemical factors in tidal flats of Incheon, on the west coast of Korea

초록

To investigate the spatiotemporal variability of dissolved organic matter (DOM) in coastal groundwater as influenced by aquifer type, we analyzed dissolved organic carbon (DOC) along with the optical properties of colored DOM (CDOM) and fluorescent DOM (FDOM) in a mud flat (Sohwangsando) and a sand flat (Seonnyeobawi) on the west coast of Korea (Incheon) from 2022 to 2023. DOC and FDOM concentrations in coastal groundwater were consistently higher in the mud flat, with pronounced increases during the summer. Across all seasons, groundwater exhibited higher concentrations of DOC, humic-like FDOM (FDOMH), and specific ultraviolet absorbance at 254 nm (SUVA254) than coastal seawater, indicating a significant groundwater contribution of humic substances to the coastal ocean. SUVA254 values were particularly elevated in the mud flat, suggesting greater DOM aromaticity, whereas the biological index (BIX) was higher in the sand flat, reflecting fresher and more bioavailable DOM. These results demonstrate distinct DOM signatures governed by aquifer type, with preferential accumulation of refractory DOM in muddy systems and enhanced contributions of labile, marine-derived DOM in permeable sandy environments. This study provides the first comparative insight into DOM dynamics across contrasting tidal aquifers in Korea, highlighting the overlooked role of submarine groundwater discharge (SGD) in regulating coastal carbon fluxes under monsoonal influence. Our results advance the understanding of how aquifer properties regulate the sources, composition, and fate of DOM within the coastal carbon cycle.

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