How does the process of eutrophication work?

As we know, fertilisers are used by farmers to increase crop yield, and artificial fertilisers in particular are very high in nutrients such as phosphorus and nitrogen which stimulate plant growth. However, if some of this fertiliser enters a nearby water source such as a pond it can cause the growth of algae on the surface. This will cause what is known as an "algal bloom" which means there is such a growth in algae that it begins to block sunlight from reaching the submerged plants. This means the submerged plants can't photosynthesise and produce the sugars they need for growth, causing these submerged plants to die. Saprobionts respire aerobically, which means when they carry out decomposition on these dead plants they use oxygen. This lowers the amount of oxygen in the water to the point there isn't enough oxygen to support aerobic organisms such as fish, leading to death. This therefore creates anaerobic conditions in the water source as only organisms that can respire anaerobically, such as certain bacteria, can survive now.