Eutrophication means a sudden and dramatic increase in the growth factor for photosynthesis, such as sunlight, carbon dioxide or nutrients. Nutrients, especially nitrogen and phosphorus, exist in low levels in the aquatic ecosystem, therefore, they become the limiting factor for algal and plant growth. The main cause of eutrophication is the leaching of fertilizers from farm fields into the surrounding watershed, as well as sewage from industrial and urban areas. Read more about eutrophication results.

The image graphically explains the phenomenon of eutrophication. The rays of sun, depicted by the yellow arrows, shine on a thick algae layer on the surface of the water basin. Below, the image of a fish is shown, together with few irregularly shaped floating organisms, called decomposers. At the bottom of the basin, the images of plants are presented, together with a brown pile of nutrient material. The stages of eutrophication are numbered. Number 1 - Nutrient load up: excessive nutrients from fertilisers are flushed from the land into rivers or lakes by rainwater. Number 2 - Plants flourish: these pollutants cause aquatic plant growth of algae, duckweed and other plants. Number 3 - Algae blooms, oxygen is depleted: algae blooms, preventing sunlight reaching other plants. The plants die and oxygen in the water is depleted. Number 4 - Decomposition further depletes oxygen: dead plants are broken down by bacteria decomposers, using up even more oxygen in the water. Number 5: Death of the ecosystem: oxygen levels reach a point where no life is possible. Fish and other organisms die.

Figure 1: Eutrophication