Freshwater eutrophication: spatially explicit fate factors for nitrogen and phosphorus emissions at the global scale

PURPOSE: Spatially explicit freshwater eutrophication indicators in life cycle assessment focus on phosphorus as the sole contributor to such impacts. Nitrogen may also be an ecological limiting factor in freshwater systems, but commonly not modelled. This work aims at filling this gap by consistently developing fate factors (FFs) for both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), using the same underlying model of nutrient export by rivers. METHODS: The present FFs were developed for application to both diffuse emissions from soil and point emissions of nutrients to freshwater. The fate processes modelled include nutrient attenuation from land to stream, in the river network, in artificial reservoirs and lakes and associated with water consumption. FFs were calculated at a river basin resolution with a global coverage and at the country and global scales by means of emission-weighting aggregation and distinguishing agricultural from non-agricultural emissions. RESULTS AND DISCUSSION: River basin-scale FFs range from 7.7 × 10⁻⁸ to 330 days for N emissions and from 3.0 × 10⁻⁸ to 520 days for P emissions. Fate factors are aggregated at country and global scale with applicability at such scales in mind. Global average FFs (in days) are FFₛₒᵢₗ Ngₗₒbₐₗ = 125; FFfw N gₗₒbₐₗ = 257; FFₛₒᵢₗ P gₗₒbₐₗ = 23; FFfw P gₗₒbₐₗ = 247. Comparison of FFs calculated at various scales showed the importance of using FFs at the highest spatial resolution (i.e. river basin). However, the river basin resolution may be too coarse for certain large basins as demonstrated for the Waikato basin in New Zealand, where FFs calculated at the sub-basin scale varied significantly. The characterisation factors represent the potential contribution of N and P to freshwater eutrophication (in Nₑq and Pₑq). The N and P components can be aggregated into a single indicator expressed in “algae-equivalent” for co-limited catchments or when the limitation status is unknown. The present fate model for freshwater eutrophication is consistent with and complements recent advances in marine eutrophication impact assessment. CONCLUSION: Applying these FFs in conjunction with a spatially explicit inventory data of N and P emissions may improve the environmental relevance of freshwater eutrophication impact assessment in LCA. One limitation is the focus on dissolved N and P, as highlighted by a comparison of attenuation factors with a New Zealand-specific hydrological model. The inclusion of particulate and organic N and P from NEWS2 should be part of future freshwater eutrophication FF developments.