Step 1: Nutrient enrichment

Excess nitrate and phosphate fertilisers are washed off farmland into rivers and lakes by rainwater. This process is called leaching. Sewage discharge can also introduce excess nutrients into waterways. The increased mineral ion concentration in the water is the starting point of eutrophication.

Step 2: Algal bloom

The high concentration of nitrates and phosphates stimulates rapid growth of algae and aquatic plants on the water surface. This is called an algal bloom. The dense layer of algae on the surface blocks light from reaching submerged aquatic plants.

Without light, submerged plants cannot photosynthesise. They die, adding to the organic matter in the water. The algae on the surface eventually die too, as nutrients become depleted or the bloom becomes too dense for all algae to get enough light.

Step 3: Decomposition and oxygen depletion

The dead algae and plants provide a large food source for aerobic decomposing bacteria. The bacterial population increases rapidly. These bacteria use dissolved oxygen in the water for aerobic respiration as they decompose the dead organic matter.

The demand for oxygen exceeds the supply. The dissolved oxygen concentration drops dramatically. This is called biochemical oxygen demand (BOD). High BOD means bacteria are using up oxygen faster than it is being replaced.

Step 4: Death of aquatic organisms

Fish and other aerobic aquatic organisms require dissolved oxygen for respiration. As oxygen levels fall, they are unable to survive and die. The water becomes dominated by organisms that can tolerate low oxygen, such as anaerobic bacteria and certain worm species.

The complete sequence is: fertiliser leaching → algal bloom → light blocked → plants die → bacteria decompose dead matter → oxygen used up → fish and other aerobic organisms die. Each step must be included for full marks in an extended answer.