(a) (i) What is pollution? (ii) State four pollutants of water (b) Describe the effects of each of the pollutants named on aquatic organisms.

(a)(i) Meaning of pollution

Pollution is the introduction into the environment (air, water or soil) of harmful or unwanted substances (pollutants) in quantities that are injurious to living organisms, degrade the quality of the environment and upset the balance of nature.

(a)(ii) Four pollutants of water

1. Sewage and other domestic organic wastes.
2. Industrial effluents containing toxic chemicals and heavy metals (for example lead, mercury).
3. Crude oil and petroleum products (oil spillage).
4. Agricultural chemicals: fertilizers and pesticides/herbicides washed into water.

(Also acceptable: hot water/thermal discharge, detergents.)

(b) Effects of each pollutant on aquatic organisms

• Sewage and organic waste: It is decomposed by bacteria which use up the dissolved oxygen. The resulting low oxygen (high biochemical oxygen demand) suffocates fish and other aquatic animals, killing them. It also spreads water-borne disease organisms and encourages eutrophication (excessive algal growth).
• Industrial effluents / heavy metals: Toxic chemicals such as mercury and lead poison aquatic organisms directly, killing fish and invertebrates. The poisons accumulate along the food chain (bioaccumulation) and may kill top consumers and humans that eat the fish.
• Crude oil (oil spillage): The oil forms a film on the water surface that prevents oxygen from dissolving into the water, so aquatic organisms are suffocated. It coats the gills of fish and the feathers/bodies of aquatic birds, and blocks light, killing plankton and reducing photosynthesis.
• Fertilizers/agricultural chemicals: Excess nitrates and phosphates cause eutrophication, leading to rapid algal blooms; when the algae die and decay, oxygen is depleted and aquatic animals die. Pesticides directly poison fish and other aquatic organisms.