Table 5.

Recommendations for the improvement of freshwater and marine eutrophication LCIA methods. Priority of each recommendation is assessed on a scale of 1-3, as follows: an immediate need (1), beneficial in the medium-term (2), or requires validation to justify the effort (3). The level of effort associated with each recommendation is assessed as: easy (E), medium (M), or difficult (D). Easy recommendations represent adoption of the best, currently available methods. Recommendations assessed as medium difficulty represent extensions of existing approaches. Difficult recommendations require novel modeling techniques or applications.

LCIA Method	Environmental Compartment	Priority	Level of Effort	Recommendation
Freshwater and Marine	All Compartments	1	E	Separate freshwater and marine eutrophication LCIA methods.
	Soil & Freshwater	2	D	Use IMAGE-GNM1 to develop sub-watershed-level, spatially differentiated terrestrial and freshwater FFs.
Freshwater Eutrophication	Freshwater	1	E	Adopt freshwater FFs from Helmes et al., providing state or watershed aggregated fate factors for the US.
		2	M	Adapt O2 depletion midpoint indicator approach developed in Cosme et al. for freshwater systems.
		2	M	Validate performance of Helmes et al. retention rates and resulting FFs against other models and monitoring data to assess long-term needs for method improvement.
	Soil	1	E	Apply standard emission fractions to terrestrial nutrient loads, as in the ReCiPe approach (e.g. 10% land-applied P to freshwater).
		1	M	Provide spatially differentiated guidance on emission fractions based on landscape characteristics.
Marine Eutrophication	Freshwater & Marine	1	E	Adopt soil, freshwater, and marine FFs, and marine XFs and EFs, from Cosme et al., reaggregating to the state level for the US.
	Marine	2	D	Refine residence time values that serve as the basis of Cosme et al. marine fate and transport factors1.
		3	D	Use hydrodynamic water quality models to develop marine FFs and XFs, increasing spatial resolution beyond 66 LMEs.
	Air	2	M	Adapt the research of Roy et al. (2012) to develop global marine eutrophication FFs for atmospheric N emissions. Consider updating FFs based on more recent inventory data.
		3	D	Run a series of nested CMAQ or CAMx model runs at a regional scale using GEOS-Chem at a coarser grid resolution to provide boundary conditions. Explore options to apply the SRM approach of Roy et al. or internal source apportionment functions.

¹This possibility was suggested in Cosme et al. (2016).