Question

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

Answer 1

Incresed amount of nutrients cause unwanted organisms (line fungi, algae, etc) to grow in the water. These organisms eat up the nutrients in the water, also they use the oxygen dissolved in the water... after a while the no. of organisms increses drastically.
thus the available oxygen in the water is very less for the aquatic animals... n thus it affects their survival...