Revised nitrogen benchmarks identify a critical transition zone in seagrass meadow degradation

Seagrass meadows support biodiversity, coastal protection and blue carbon services but are declining globally, with nutrient enrichment a pervasive driver of eutrophication. Effective management depends on detecting eutrophication before structural collapse occurs, yet widely used tissue nitrogen (N) benchmarks for seagrasses have never been formally validated. We assessed nutrient exposure and ecological condition across nine seagrass meadows in Northern Ireland using tissue carbon, nitrogen and phosphorus content, stoichiometric ratios and stable isotopes (δ¹³C, δ¹⁵N), and tested how tissue nitrogen relates to Zostera marina biomass at Northern Hemisphere scale by integrating regional, UK and global datasets. All Northern Ireland meadows exceeded the global 1.8% tissue nitrogen benchmark, indicating pervasive anthropogenic nitrogen exposure even within Marine Protected Areas. The combined datasets revealed a strong, nonlinear decline in Z. marina biomass with increasing tissue nitrogen that was consistent across sites. Biomass began to decline significantly above 1.8% N, identifying this value as the onset of functional degradation. The rate of biomass loss increased rapidly with further enrichment and peaked at 2.8% N, defining a critical point at which additional nitrogen causes the greatest marginal loss of seagrass biomass. These results provide quantitative support for existing tissue nitrogen benchmarks and refine their ecological interpretation, highlighting 1.8% N as an early-warning threshold and 2.8% N as a high-risk range for rapid biomass loss. Tissue nitrogen therefore represents a integrative, biologically meaningful indicator of eutrophic stress applicable for management and restoration prioritisation in seagrass ecosystems.