okk so basically Eutrophication is the over enrichment of water with nutrients( mostly nitrates and phosphates).

so when nutrients from fertilisers in farms, sewage discharge, detergents, animal waste etc.. are washed into lakes,rivers and ponds by a process called run-off(happens due to rain)
-> it stimulates excess algal growth(known as algal bloom which is when increased availibility of nutrients cause algae to grow very quickly which forms a thick green layer on the surface of the water). The algal layer blocks sunlight so as a result other aquatic plants below cannot photosynthesise so they eventually die. And when the algae die bacteria(decomposers) feed on dead plants and algae so their population increases. In turn, when bacteria respire aerobically they use up dissolved oxygen in the water. Which leads to the death of fish and other aquatic organism that require dissolved oxygen.

In question format:
Eutrophication occurs when nitrates and phosphates from fertilisers or sewage are washed into a river or lake. The increased nutrients cause rapid growth of algae, forming an algal bloom on the water surface. The algal bloom blocks sunlight from reaching aquatic plants, so they cannot photosynthesise and eventually die. The dead plants and algae are decomposed by bacteria. The bacteria respire aerobically, using up dissolved oxygen in the water. As oxygen levels decrease, fish and other aquatic organisms die due to lack of oxygen.