Final answer:
Algal blooms, caused by excess nutrients from fertilizer runoff and sewage, are directly responsible for depleting the supply of oxygen in bottom waters, leading to hypoxic conditions and dead zones.
Step-by-step explanation:
The supply of oxygen in bottom waters is primarily depleted by the process whereby oxygen is consumed by the growth of microorganisms, which is a direct result of algal blooms. These blooms are caused by excess nutrients, such as nitrogen and phosphorus, entering aquatic ecosystems from sources like fertilizer runoff and sewage.
The excessive growth of algae leads to its death and decay, which in turn increases the population of decomposing bacteria, escalating oxygen consumption and creating hypoxic conditions or dead zones where aquatic life cannot survive.