Global Patterns in the Nutrient Composition Delivered by Seabirds to Coastal and Island Ecosystems

ABSTRACT Aim Consumer‐mediated nutrient dynamics are crucial for sustaining ecosystems. While most research has focused on the magnitude of seabird‐derived nutrient fluxes, the role of species and diet composition in shaping nutrient quality remains largely unexplored. We aim to test whether the stoichiometric composition of seabird‐derived nutrient subsidies is spatially structured. Location Global. Time Period 1950–2025. Major Taxa Studied Seabirds: Charadriiformes, Pelecaniformes, Phaethontiformes, Procellariiformes, Sphenisciformes and Suliformes. Methods Using dietary data and prey nutrient stoichiometry, we quantified nitrogen and phosphorus excretion for 166 seabird species and analysed variation in excreta nitrogen:phosphorus (N:P) ratios across geographic gradients. Spatial patterns were assessed at three scales: (i) species centroid distributions; (ii) global patterns weighted by breeding abundance at the grid‐cell level (141 species); and (iii) continental patterns weighted by breeding abundance at the colony level (79 species). Results Excreta N:P ratios showed a strong latitudinal gradient, declining from southern high latitudes towards the tropics. Abundance‐weighted analyses confirmed this pattern at both global and continental scales, with increased nitrogen enrichment in Arctic regions, although less pronounced than in the Southern Ocean. Stoichiometry varied strongly across climate zones, with greater stoichiometric variability in polar and subpolar regions than in the tropics. Taxonomic identity and diet composition were the strongest predictors of excretion stoichiometry, but geographical patterns contributed substantially. Main Conclusions Consistent with ecological stoichiometry, species identity and diet are primary drivers of seabird nutrient composition. Spatial patterns identify regions of high stoichiometric variability that are likely sensitive to shifts in prey availability and species redistribution under climate change. N:P ratios of deposited excreta broadly align with marine nutrient limitation patterns but contrast terrestrial plant limitation, delivering relatively P‐rich subsidies in tropical regions and N‐rich inputs at high latitudes. These results demonstrate that seabirds impose a spatial structure on cross‐ecosystem nutrient subsidies, linking marine and terrestrial nutrient regimes. Changes in seabird distributions and abundance are therefore likely to alter both the magnitude and composition of nutrient inputs, with consequences for island ecosystem functioning.