Abstract
Under accelerating coastal eutrophication, disentangling the relative roles of light limitation and nitrogen enrichment is essential for diagnosing seagrass decline. We conducted a fully crossed light × ammonium experiment using intact Zostera marina cores from Guzhenkou Bay (4 irradiance levels: 180, 90.6, 17, 0 μmol photons m -2 s -1 ; 4 NH 4 Cl levels: 0, 25, 50, 100 μmol L -1 ; 5 weeks; 16 units; no within-cell replication). Across traits, irradiance—not NH 4 Cl within the tested range—governed responses at the level of dominant main effects. Shading sharply reduced shoot density, above- and below-ground biomass (AGB, BGB) and F v /F m , with near-complete collapse at darkness. Biomass turnover accelerated under shading (higher above-ground shedding, below-ground mass loss), and below-ground carbon content declined, together indicating a shift from carbon accrual to expenditure. By contrast, NH 4 Cl enrichment showed minimal main effects on morphology, pigments, F v /F m , or antioxidant enzymes, although tissue δ 15 N decreased with NH 4 Cl addition, evidencing ammonium assimilation. Antioxidant responses were variable (POD increased with reduced irradiance; SOD, CAT, GSH-Px inconsistent), while MDA did not increase under shading and was occasionally higher under high light, consistent with lower photo-oxidative load at low irradiance. Leaf and rhizome δ 13 C showed no significant treatment effects and only a weak, non-significant tendency toward less negative values under severe shading. Collectively, the trait suite diagnoses light-driven carbon limitation as the proximal stressor, with nitrogen effects contingent on concentration and context. We recommend tiered monitoring that couples rapid photophysiology (F v /F m ) with structural (shoot density, AGB/BGB) and integrative biogeochemical indicators (below-ground C; δ-isotopes), and management focused on water clarity to meet species-specific light requirements.