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

The increased in the level of nutrients in the water leads to an excessive increase in the population of algae in the water bodies. When these algae die, they serve as food for the decomposers. A lot of oxygen is utilized in this process, consequently leading to a decrease in the level of oxygen dissolved in the water body. This in turn causes fishes and other aquatic organisms to die.