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

Increased level of nutrients in water leads to more growth of algae and plants inside the water. For example nitrogen supports the plant life in the sea.

But problem is that water at a given temperature and pressure can only dissolve a limited amount of gases like oxygen and there is then a competition between newly grown plants and algae with aquatic organisms for consuming this limited oxygen.

In these ways increased the level of nutrients in the water affect the survival of aquatic organisms.