Why does the increased level of nutrients in the water affect the survival of aquatic organisms?

Eutrophication is when the environment becomes enriched with nutrients. It is a natural process in lakes, occurring as they age through geological time. It causes structural changes to the ecosystem such as increased production of algae and aquatic plants, general deterioration water quality and other effects leading to the deficiency of oxygen in the water which affects the survival of aquatic organisms like fishes etc.