What is eutrophication?
Eutrophication occurs when an ecosystem receives an excessive input of nutrients, particularly nitrogen and phosphorus compounds. This enrichment can stimulate the growth of algae and aquatic plants, alter ecosystem structure and, as organic matter decomposes, reduce oxygen concentrations in water. Under certain conditions, oxygen depletion and mortality of aquatic organisms can occur.
Relevant nutrient sources include agricultural fertilisers, wastewater, organic waste and atmospheric emissions that are subsequently deposited in ecosystems. The relative importance of each source depends on location, emission pathway and the characteristics of the receiving environment.
Why distinguish freshwater, marine and terrestrial environments?
Life Cycle Impact Assessment (LCIA) methods represent eutrophication through distinct impact categories. The European Union's Environmental Footprint method, for example, includes freshwater, marine and terrestrial eutrophication. The relevant substances and environmental transport and fate mechanisms differ between these categories.
In traditional approaches, phosphorus is often treated as the limiting nutrient in freshwater systems and nitrogen in marine systems. This simplification is useful for modelling, but it does not describe every situation: multiple nutrients may jointly limit ecosystem response, depending on the receiving environment.
How to interpret LCA results
An eutrophication result should not be interpreted as a direct measurement of algae, oxygen or ecological status at a specific location. LCA results are modelled indicators of potential impacts, calculated from the emissions inventory and characterisation factors defined by the selected method.
It is therefore important to check which LCIA method was used, which environmental compartment is represented, which substances are included and whether the characterisation factors have appropriate geographical coverage. Freshwater, marine and terrestrial results should remain separate when aggregation would conceal relevant differences.
Eutrophication is only one of the impact categories needed to understand the environmental profile of a product or service. A comprehensive assessment also considers other impact categories and avoids focusing interpretation exclusively on climate change.