National scale evaluation of nutrient purification capacity in marine sediments along the coast of South Korea: A mesocosm study based in situ assessment

The ecosystem regulating services from tidal flats, such as removal of organic pollutants, provided by natural tidal flats are being increasingly recognized, yet quantitative evaluation remains limited. Here we evaluated a nationwide capacity of natural purification in tidal flats. Using in situ sediments from five along the Korean coast (Incheon, Gunsan, Sinan, Gwangyang, and Busan), we applied a mesocosm system informed by 18 years of riverine monitoring data from national surveys. In particular, we adopted an integrated approach, analyzing nutrient fluxes, removal rates, and their economic value. The results indicated that in all regions, the water-sediment fluxes of all nutrients showed negative values, indicating the migration of nutrients from the overlying water to the sediment. A significant (p < 0.01) difference among regions of water-sediment fluxes was primarily due to exposed nutrient concentrations. Notably, the presence of greater total nitrogen (TN) and total phosphorus (TP) purification capacities in Incheon and Nakdong sediments reflected in remaining loads of river-estuarine continuum. Nationwide, the nutrient purification capacities of Korean tidal flats (2491 km²) were estimated at 0.29 Tg N yr^-1 and 0.15 Tg P yr^-1, equating to economic values of 8.26 billion USD yr^-1 and 8.21 billion USD yr^-1, respectively. Scaling globally, the economic values for TN and TP removal by tidal flats (124,921 km²) reached 414 billion USD yr^-1 and 412 billion USD yr^-1. Overall, the integrated approach successfully provided the first national-scale evaluation of nutrient removal capacity in tidal flats across Korea, addressing the ecological and economic significance of tidal flats, while underscoring the need for their conservation and management strategies of coastal areas globally.