A wheat farmer uses fertiliser on his crop for several months. During this time the pond at the end of the field becomes covered in a thin green film and starts to smell. What process is responsible for this and how does it occur?

Fertilisers contain nitrates for use by plants for protein synthesis.In eutrophication, when it rains excess soil nitrates are washed in run-off to the pond. Water becomes nitrogen rich so surface algae thrive - an 'algeal bloom'. Algae block sunight reaching pond flood plants, blocking photosynthesis and causing them to die. Plant material is digested by aerobic bacteria, using up water oxygen so that fish in the pond asphyxiate and die. These are then also digested.