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 (NO3−) concentrations (fresh groundwater: ~ 360 μM; coastal seawater: ~ 6 μM) and salinity indicates that SGD is the predominant NO3− source. The stable isotope compositions of δ15N–NO3− and δ18O–NO3− suggest that multiple processes, including NO3− assimilation and potentially remineralization and nitrification, influence NO3− dynamics in the coastal aquifer. However, the estimated isotope enrichment factors (15ε = 2.03‰; 18ε = 2.30‰) fall within the range typical of assimilation, indicating that SGD‐derived NO3− is primarily removed through this biological process. This finding is consistent with in situ observations of elevated MPB production. Stable isotope compositions of δ13C and δ15N 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.