Eutrophication happens as water bodies, such as estuaries, slow-moving streams and lakes, receive excess nutrients that trigger excessive plant growth. This increased plant growth, often referred to as an algal bloom, reduces the amount of oxygen dissolved in the water when dead plants decompose and cause other organisms to die.
There are two types of eutrophication: natural eutrophication and anthropogenic eutrophication. Eutrophication is widespread in freshwater ecosystems, and it plays a major role in the normal aging process of many ponds and lakes.
The main cause of anthropogenic eutrophication is human activity. Nutrients from fertilized fields, lawns, and farms are deposited in rivers, streams, lakes and oceans when it rains. The nutrients increase the metabolic activity of algae, causing an algal bloom. The algal bloom then covers the surface of water and prevents light from reaching other plants. The plants lacking light begin to die because they cannot photosynthesize and eventually decompose. The decomposed plants increase bacteria in the water, which further decreases oxygen concentration, and other living organisms begin to die.
Eutrophication can also occur as a result of natural events, such as climate change and geology. However, natural eutrophication is dramatically increased by human activity. The main difference between anthropogenic eutrophication and natural eutrophication is that the natural process is slow.