How to answer an extended answer question in an exam such as 'Describe the process of eutrophication (6 marks)'

When answering a 6 mark question such as this you need to ensure you are using the correct scientific terminology and checking off all the key marking points in order to score highly. In this example there are usually more than 6 marking points which I will explain below.
First it is important to mention that fertilisers used by farms to increase crop yield wash into water sources.
These contain nitrates and phosphates which cause algal bloom (this is an important term to use).
As a result algae grows on the upper surface of the water and blocks light from reaching the plants at the bottom of the water.
This means that these plants cannot photosynthesise and die (photosynthesis should always be mentioned at some point in the answer).
These provide more nutrients to decomposers, so these increase in number.
The decomposers will aerobically respire, using up the oxygen in the water (note aerobic respiration occurs here).
Therefore fish die as less oxygen is available