Explain the process of eutrophication

The process begins when excess fertiliser, which has been used for farming, is flushed into rivers or lakes by rainwater. The fertiliser then provides nutrients such as nitrates which allow plant life in the water to grow more favourably, such as algae and duckweed. This subsequent extra growth of aquatic plants forms a layer on the surface of the water, known as an 'algal bloom'. The bloom prevents sunlight from reaching the plants underneath, and so they cannot carry out photosynthesis, resulting in death of the plants and a loss of oxygen being produced by photosynthesis. These dead plants are then broken down by bacteria, which uses up even further oxygen. Thus, the fish in the water now have so little oxygen that they die. Eutrophication, therefore, is a process which harms the ecosystem, and demonstrates one of the many negative impacts of fertilisers.