Causes of Eutrophication

Three main factors:
   - The lake or reservoir basin
   - Thermal stratification
   - Biological processes

The Lake or Reservoir Basin:

The potential of a lake or reservoir to become eutrophic is largely influenced by the nature of the nutrient, organic matter and silt inputs from the surrounding land. This will vary depending on whether the land is used for agricultural, forestry or industrial purposes. The response of the lake or reservoir to the various inputs is directly linked to its size, shape and depth, algae tending to proliferate more in small, shallow lakes.

Thermal Stratification

Another major contributory factor is the annual temperature cycle of the lake. This determines its physical, chemical and biological behaviour. In temperate climates, most lakes stratify thermally in the summer months into a deep, dark, stagnant, cold layer (the epilimnion). This thermal stratification gives rise in turn to the chemical stratification of the water body, with oxygen concentrations declining rapidly with depth and sometimes reaching zero at the level of the sediment.

Biological Processes:

In eutrophic lakes, algae proliferate and then sink to the bottom where they form a layer of sediment. Their decay leads to the release of organic matter and nutrients which consume the dissolved oxygen in the bottom layers. As oxygen is reduced, elements such as phosphorus locked in iron-hydroxy complexes, may be released. A vicious circle is created, since the increased phosphorus concentrations promote, in turn, the production of algae.