A fertiliser is sprayed onto the fields so that it will protect the crops from nasty bugs and insects. The fertiliser is made up of nitrates.
When it starts to rain the soil is quick to absorb the nitrates from the fertiliser and stores itself in the ground water where it is then taken away by the current to a lake.
When the fertiliser reaches the lake the nitrate makes the algae and other aquatic plants grow very quickly.
This leads to the blocking of any sunlight reaching the bottom of the lake where other plants and animals lay.
As no sun reaches the bottom of the lake, all the plants begin to die and decompose. The bacteria from the dead plants begin to consume all of the oxygen which leads to hypoxia and results in anoxia.
As a result of the anoxia the fish are left with no oxygen to breather and they die. From use of pesticides we see that it results in the death of all beings in a body of water, therefore we should stop using them.