Threshold-based management of nitrogen speciation for coastal lagoon restoration: Insights from particulate organic matter sourcing and macrophyte physiology

Coastal lagoons are highly vulnerable to nitrogen enrichment; however, simple and integrative indicators for assessing restoration effectiveness remain limited. We evaluated particulate organic matter (POM) sources and stoichiometric responses of the emergent macrophyte Phragmites australis as potential bioindicators of nitrogen status in restored (Sunpo) and natural (Maeho) lagoons along the eastern coast of Korea. Stable isotopes δ 13 C and δ 15 N, C:N ratios, and size-fractionated POM were used to quantify autochthonous production and nutrient dynamics. Compared with the natural lagoon, the restored lagoon exhibited higher chlorophyll- a and increased contributions of phytoplankton-derived autochthonous POM, supported by ammonium concentrations maintained within an optimal range (8–12 μM). Notably, the restored lagoon maintained a higher NH 4 + /DIN ratio than the natural lagoon, suggesting efficient nitrogen cycling without reaching inhibitory levels. In contrast, the natural lagoon suffered from chronic nitrogen over-enrichment, where excessive NH 4 + levels exceeded physiological thresholds, inducing the “ammonium paradox” that hindered phytoplankton growth. The positive correlation between leaf C:N ratios and NH 4 + /DIN ratios across both lagoons indicates that nitrogen speciation, rather than absolute concentration alone, significantly influences macrophyte physiology and stress. These findings suggest that the NH 4 + /DIN ratio is a more sensitive bioindicator for lagoon health and restoration success. Successful coastal management requires a threshold-based approach that balances nitrogen speciation to optimize ecosystem functions and maximize the capacity for nutrient sequestration in temperate lagoons.