There are 6 steps for eutrophication...
If farmers use too much fertiliser,or rain washes it away, it travels into the stream/ river ect.
High nitrate and phosphate concentration in the water encourages rapid growth of algae.
These block sunlight, and plant start to die due to competition for light.
Micro-organism numbers increase as they feed on dead material.
Microbes use up all the oxygen which means that the oxygen concentration in the water decreases.
Fish and other animals suffocate and die due to lack of oxygen.