Eutrophication involves rising plant productivity as a result of the increased availability of nutrients. Excess nutrient pollution in wastewater, runoff from farmland, and atmospheric deposition can trigger harmful eutrophication processes.
Eutrophication on land and in the water
In terrestrial ecosystems, eutrophication is typically reflected in the accelerated growth of forests. This increased growth is particularly promoted by the deposition of nitrogen compounds from the air, and increasing atmospheric concentrations of carbon dioxide.
In water bodies, eutrophication particularly boosts the growth of planktonic algae. Its effects can be seen in reduced water clarity and the increased growth of filamentous algae and aquatic plants. In the worst cases, eutrophication may result in the increased occurrence of massive blue-green algal blooms, in winter oxygen depletion, and in dramatic changes in fish stocks.
Eutrophication is basically a natural phenomenon. Certain lakes or habitats are naturally poorer in nutrients than others, but over time they may become richer in nutrients through natural processes. Where nutrient pollution is widespread, however, eutrophication often becomes a problem.
Eutrophication may lead to increases in biodiversity – at least locally. Birds are particularly attracted to lakes and wetlands affected by eutrophication, for example. But where eutrophication becomes predominant, overall diversity is likely to decline, since the species typically associated with nutrient-poor habitats will gradually disappear.
Problems and benefits
Eutrophication is widely seen as a negative trend in lakes and the sea, since it benefits animals and plants normally considered undesirable – including smaller cyprinid fishes such as roaches, aquatic plants that can overgrow shores and bays, and toxic blue green algae.
On the land, increases in the productivity of plants are more welcomed, particularly where crops and commercially managed forests are concerned. Terrestrial ecosystems are also normally spared from the more harmful side effects of eutrophication such as oxygen depletion.
The challenge of preventing eutrophication
Once a process of eutrophication has begun, it may be hard to curb. Nutrients accumulate over many years in the water and the soil. Even if the external loads of nutrient pollution entering aquatic ecosystems can be cut, a self-perpetuating process can continue as internal loads of stored nutrients are repeatedly reabsorbed into the water, where they feed the renewed growth of plants. This is particularly a problem in the Baltic Sea.
Badly affected lakes can be restored to some extent by laboriously removing nutrients from the ecosystem through selective fishing or the removal of excess plant growth. The nutrient-rich silt on lake-beds may also be dredged or covered over. During the winter, air may also be pumped into lakes to improve the oxygen content of their deeper waters and slow the release of nutrients from bottom sediments.