Final answer:
Algal blooms deplete oxygen through the expansion and decomposition of the algae, creating dead zones. Dense growth of algae can prevent oxygen mixing in ponds. Algal death and decay also consume oxygen and release toxins, leading to the death of aquatic organisms.
Step-by-step explanation:
Algal blooms can deplete oxygen in several ways. As the algal population expands and then dies, it provides a large increase in organic matter to the bacteria in the water. This leads to the explosion of nonphotosynthetic microorganisms that consume available oxygen, creating dead zones. Additionally, dense growth of algae can cover the top of ponds, preventing oxygen-rich water from mixing with lower layers, resulting in oxygen depletion. Moreover, the death and decay of the algal organisms release toxic byproducts and consume dissolved oxygen, leading to the death of aquatic organisms.