Eutrophication is the process through which lakes, streams, or bays become overloaded with nutrient-rich water. When this occurs, large blooms of algae and aquatic plants occur, fed by the excess nitrogen and phosphorus. When the algae die, microorganisms in the water begin feeding on the remains as part of the decomposition process and consequently use up the available oxygen in the water. This leaves little oxygen for fish and other aquatic animals, resulting in the suffocation of aquatic life.
Eutrophication can occur in both freshwater and saltwater systems. Sources of excess nutrients to these systems include agricultural runoff, overuse of synthetic fertilizers, septic tank or sewage leaks, and erosion. Excess phosphorus often causes eutrophication in freshwater, while nitrogen is responsible for the phenomena in saltwater.
As the soils used in green infrastructure can be a source of both of these nutrients, special care should be taken to minimize nutrient loss from green stormwater management systems. Nutrients can be managed by testing soils for adequate ratios of phosphorus to iron and aluminum, and/or appropriate carbon to nitrogen (C:N) ratios. In addition, pairing plants with soils that do not require the use of synthetic fertilizers will reduce the amount of nutrients in runoff.