Biological legacy of large-scale coastal reclamation: Potential oxygen demand from pre-existing benthic biomass

Coastal-enclosure assessments commonly emphasize external organic loading, sediment processes, and hydrological exchange but rarely account for living benthic biomass that may become a transient internal organic load after habitat conversion. Omitting this pool may leave early post-closure oxygen budgets incomplete, particularly where water exchange and oxygen renewal are restricted. We evaluated this biological legacy in Saemangeum, Korea, using a three-step analysis: (1) estimating the pre-closure benthic carbon inventory and its complete-decomposition oxygen-demand upper bound; (2) using reanalyzed laboratory data to assess the temporal plausibility of rapid biomass loss and field-observed intertidal macrofaunal deficits to constrain potential demand; and (3) comparing the field-constrained estimates with standing dissolved-oxygen inventories across three effective receiving-water volumes. The compiled inventory contained 9,384 t C and yielded an all-component upper bound of 25,023 t O 2 ; excluding the transferred bacterial estimate reduced this value to 14,461 t O 2 . At 120 days after closure, the net biomass-deficit and taxon-resolved minimum-loss estimators yielded potential demands of 3,547 and 4,663 t O 2 , respectively, equivalent to 54.0% and 70.9% of the intertidal macrofaunal upper bound. The corresponding point-estimate demand-to-inventory ratios exceeded one at an effective water volume of 0.35 km 3 but remained below one at 0.83 and 1.27 km 3 . Among the tested assumptions, the inventory upper bound was most sensitive to the transferred bacterial stock, whereas the demand-to-inventory comparison was strongly influenced by receiving-water conditions. The tested alternative allocation and conversion-factor perturbations had smaller effects on the 120-day estimates. Conditional bootstrap intervals quantified sampling uncertainty in station-level means, while separate sensitivity analyses examined selected structural assumptions. These comparisons represent potential demand rather than realized oxygen consumption or predictions of hypoxia. The proposed tiered framework can identify coastal-enclosure projects requiring targeted monitoring and process-based oxygen-budget assessment.