Spatially-explicit LCIA model for marine eutrophication as a tool for sustainability assessment

The increasing emissions from human activities are overrunning the ecosystems’ natural capacity to absorb them. Nutrient emissions, mostly nitrogen- and phosphorus-forms (N, P) from e.g. agricultural runoff and combustion processes, may lead to social-economic impacts and environmental quality degradation.

Life Cycle Assessment (LCA) is as a tool to comparatively quantify the environmental impacts from product systems throughout their life cycle. Marine eutrophication is one of the LC Impact Assessment (LCIA) categories and it is still lacking an overall model linking nutrients over-enrichment to impacts on marine ecosystems.

Emitted nitrogen reaches marine coastal waters where it promotes the growth of phytoplankton biomass in the surface photic zone from where it eventually sinks to bottom waters. This downward flux of organic matter is respired there by bacteria resulting in the consumption of dissolved oxygen. An excessive depletion of oxygen affects the exposed organisms and loss of species diversity may be expected.

A model framework was built to estimate the potential impacts arising from N-emissions (see figure). It combines the fate of N in rivers and coastal waters, the exposure of receiving ecosystem to the N enrichment, and the effects of oxygen depletion on relevant species. The estimated impacts are quantified by means of substance-specific factors that translate the emission into potential impacts, i.e.

Characterization Factors (CFs). These express the impacts to the ecosystem quality as potentially affected fraction of species (PAF) per mass of N emitted to the environment, volume and time integrated, or (PAF·)[m3·yr·kg-1]. Preliminary results present spatially differentiated CFs for 214 country-to-ecosystem combinations and for 143 countries.

Such CFs can be implemented into impact assessment methods in LCA to help characterizing the eutrophication impact of product systems related to agricultural production or involving combustion processes, and ultimately to assess the environmental sustainability of human activities.