Isotopic evidence for enhanced tidal flat productivity driven by nutrient‐rich submarine groundwater discharge

Submarine groundwater discharge (SGD) is well recognized as a significant nutrient source in coastal oceans. However, direct evidence linking SGD‐derived nutrients to biological production across trophic levels remains limited. Here, we investigated the influence of SGD on a tidal flat ecosystem on the west coast of Korea, where high microphytobenthos (MPB) productivity persists despite oligotrophic seawater and the absence of riverine input. A significant negative correlation between nitrate (NO 3 − ) concentrations (fresh groundwater: ~ 360 μ M; coastal seawater: ~ 6 μ M) and salinity indicates that SGD is the predominant NO 3 − source. The stable isotope compositions of δ 15 N–NO 3 − and δ 18 O–NO 3 − suggest that multiple processes, including NO 3 − assimilation and potentially remineralization and nitrification, influence NO 3 − dynamics in the coastal aquifer. However, the estimated isotope enrichment factors ( 15 ε = 2.03‰; 18 ε = 2.30‰) fall within the range typical of assimilation, indicating that SGD‐derived NO 3 − is primarily removed through this biological process. This finding is consistent with in situ observations of elevated MPB production. Stable isotope compositions of δ 13 C and δ 15 N for two dominant deposit feeders ( Batillaria cumingii and Macrophthalmus japonicus ) and their potential food sources (MPB and organic matter) indicate that MPB contributes significantly to their diets, accounting for approximately 72% in B. cumingii and 84% in M. japonicus . These results highlight the critical role of SGD as a key nutrient source that stimulates MPB production, thereby enhancing food availability for MPB‐dependent consumers, such as snails and crabs, in tidal flat ecosystems.