8.1.2  Effects of excess nutrients on the environment

Phosphorus and nitrogen are common pollutants generated from residential areas and agricultural run-off. They are usually associated with human and animal wastes and/or fertiliser. Nitrogen and phosphorus are plant nutrients that plants need in order to grow. If there are large quantities of nutrients, they can encourage excess plant growth in the water. This can cause the phenomenon known as an algal bloom, which means a sudden increase in the population of microscopic algae. If a water body has high nutrient levels it is said to be eutrophic; the process is called eutrophication. Eutrophication is a common phenomenon in Ethiopia and has been observed in Lake Alemaya, Lake Boye, Lake Aba Samuel and Lake Koka (Figure 8.2).

Figure 8.2  Eutrophication in Lake Koka, Oromia Region.

The density of microscopic green algae, as shown in Figure 8.2, blocks sunlight from penetrating the water causing larger plants under the surface to die and decompose. The main problem of eutrophication is that the sudden algal bloom can die off equally quickly. The decay of the algae by bacteria can cause deoxygenation of the water.

Water that contains large amounts of nitrates is unpleasant to drink and can be toxic to humans and animals. Also, some species of cyanobacteria (also known as blue-green algae) that flourish under these conditions produce toxins that cause liver, nerve and skin problems in humans and animals. Toxic levels of cyanobacteria have been found in several Ethiopian lakes (Mankiewicz-Boczek et al., 2015; Willén et al., 2011).

Eutrophication also encourages the growth of larger plants, such as the floating and invasive water hyacinth (Eichhornia crassipes) which can cover large areas of lakes (Figure 8.3). When these plants die, they add to the problems of deoxygenation caused by decaying organic material.

Figure 8.3  Water hyacinth: an aquatic weed growing on the surface of Lake Tana.