Describe the stages of the europhication process (6 marks)

Farmers use nitrate containing fertilisers on their crops which are leached into river water due to rains and water run off, causing a build up of nitrates in pools of water. This increases the ion concentration in the water, leading to an algal bloom (1 mark). A layer of algae covers the surface of the pool which reduces the light intensity (1 mark) in the deeper waters. This means plants beneath the surface cannot photosynthesise and die (1 mark). Saprobiotic bacteria (1 mark) decompose the dead organisms but they respire aerobically so their activity causes the deoxygenation of the water (1 mark) and the remaining aerobic respiring organisms, such as fish, die (1 mark). This causes the water to become stagnant.