Eutrophication is a process in which an ecosystem accumulates mineral nutrients. It can occur naturally, but is usually associated with human activity that releases nutrients into the environment.
Anthropogenic eutrophication has caused a widespread loss of biodiversity in many systems. Recent attempts to reverse the process are proving difficult and expensive.
Symptoms of eutrophication are most readily seen in aquatic systems, where the additional nutrients lead to the explosive growth of algal or bacterial populations. The large biomass produced excludes light from the water and can result in the deoxygenation of the water, killing fish and other animals.
In terrestrial systems, additional nutrients boost the productivity of competitive plant species. These then exclude less competitive species by shading them, leading to a decrease in species richness. The humped-back curve describes the relationship between biomass and species richness.
Estuaries are particularly prone to eutrophication, and like other aquatic environments can suffer from algal blooms that eliminate other species. Loss of key species, such as seagrass, results in an entire habitat type and all its dependent species disappearing.
The main agents of eutrophication are compounds containing the elements phosphorus and nitrogen. It is these elements that, under natural conditions, usually limit the primary production in ecosystems. Increasing their supply therefore increases productivity.
Sources of anthropogenic phosphorus entering the environment include sewage discharges, intensive livestock farms and the spreading of artificial fertilizers and animal manures onto agricultural land. The majority of phosphorus comes from point sources.
Sources of anthropogenic nitrogen entering the environment include gaseous emissions from car exhausts and power stations and artificial fertilizers applied to agricultural land. The majority of nitrogen comes from diffuse sources.
Recent European legislation has tried to limit further eutrophication of the environment by measures such as the stripping of phosphorus from waste water and the control of nitrogen fertilizer applications in sensitive zones.
Living organisms can be used as monitors of the trophic status of ecosystems.
Removal of nutrients from an ecosystem in order to reverse the effects of eutrophication is a difficult and expensive undertaking.