Effects of water pollution on the environment

This study session begins by describing the effect of water pollution on surface water ecosystems, such as lakes, rivers and streams. Pollution from human activities enters surface waters, either directly when wastes are dumped into lakes and rivers, or indirectly when wastes released on land or into the air are washed into surface waters.

An ecosystem was defined in Study Session 1 as ‘all living organisms and their physical environment and the interactions between them’. Pollution can disturb and unbalance those interactions in a number of ways.

Effects of organic pollution

You will remember from Study Session 7 that organic matter is any material derived from living organisms. Organic pollution is any contamination of water by organic matter. Examples include human and animal wastes and urban run-off.

Many aquatic (water-living) organisms depend on oxygen dissolved in the water to survive. Aquatic animals include fish, amphibians (e.g. frogs, toads) and many invertebrate species such as insect larvae, snails and worms. Their supply of oxygen in the water is maintained from atmospheric oxygen in the air above the water and from oxygen produced by green aquatic plants in the process of photosynthesis. Fast-flowing, turbulent water will be aerated (gain oxygen) more than still water because the boundary between air and water is more active.

If organic pollutants such as human and animal wastes are released into a water body, bacteria will use the waste as food. Bacteria break down the complex organic chemicals (proteins, fats, carbohydrates) into simpler chemicals that are further oxidised into nitrates, sulphates and carbonates. This process, known as biodegradation, provides energy to the bacteria and uses dissolved oxygen from the water.

If the quantity of organic matter is small and there is plenty of dissolved oxygen, then this natural breakdown process will remove the pollution quite quickly. However, if there are high levels of organic pollution, the population of bacteria increases and may use up all the oxygen from the water. This is called deoxygenation. Complete deoxygenation is unlikely in a river where the water is moving, but it can happen in lakes or slow-flowing channels. Anaerobic (without oxygen) conditions are unsightly and cause unpleasant odours. Fish and other aquatic organisms that need oxygen to survive will eventually die if they cannot migrate elsewhere (Figure 8.1). The reduction in fish populations and other aquatic animals not only disrupts the ecosystem but also results in a loss of food for local people and loss of jobs for local fishermen.

Figure 8.1 Dying fish due to oxygen starvation.

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.

Effects of persistent pollutants

Persistent pollutants, such as heavy metals and persistent organic pollutants (POPs), were mentioned in Study Session 7.

These pollutants may be present at very low concentrations in water but, over time, they build up in the tissues of organisms by a process called bioaccumulation. These chemicals have no known biological function in animals and there is no process to expel them from the body. If the chemicals are ingested or otherwise absorbed from the environment, they remain in the body and gradually accumulate. This is a particular problem for fish and shellfish that feed by filtering plankton from very large volumes of water. If the plankton are contaminated, this will pass into the fish. The pollutants become more and more concentrated and can reach toxic levels.

Figure 8.4 shows how levels of heavy metals, in this case mercury, increase through a food chain. A food chain is the sequence of who eats whom, or what. In Figure 8.4 you can see that the concentration of mercury in water is very low, but increases in the bodies of phytoplankton (small aquatic plants), then in zooplankton (small aquatic animals) as they eat the phytoplankton, and then in the bodies of fish that eat the zooplankton and finally in fish-eating birds. The chemicals can also be passed on to eggs and damage reproduction of the birds. This gradual increase in concentrations through the levels of a food chain is a form of bioaccumulation called biomagnification.

Figure 8.4 Biomagnification of a heavy metal (mercury) in a food chain. The concentration of the heavy metal is expressed as parts per million (ppm) in the environment.

The aquatic food chain extends to terrestrial (land-living) animals and humans. The amount of a pollutant such as mercury can reach a level dangerous enough to cause harm if consumed. Mekuyie (2014) analysed the levels of heavy metals in milk from cows that had drunk water from lagoons that contained wastewater from a textiles factory in Hawassa. The milk contained unsafe levels of heavy metals. The study concluded that this could have toxic effects on people who drank the milk, as well as decreasing livestock productivity and causing damage to the aquatic ecosystem.

Pollutants will continue to bioaccumulate in plants and animals as long as the pollution continues. The Hawassa example demonstrates that some pollutants do not go anywhere, but instead become a part of our life system in the environment.