The Fertilisers are washed into the lakes and rivers when it rains.
The increase of Nitrates and Phosphates in the water increases algae growth .
The Algae form a bloom over the water surface.
The sunlight can't reach the plants which then die as they can't Photosynthesise.
Bacteria break down the dead plants.
The Bacteria use up the oxygen in the water during respiration causing organisms to die.