Which process commonly leads to oxygen depletion after an algal bloom dies off?

• A. Evaporation
• B. Aeration by wind
• C. Bacterial decomposition of algae
• D. Photosynthesis by aquatic plants

Answer: (C)

When a dense algal bloom dies, the dead algae become a plentiful source of organic matter for microbes. Bacteria that decompose this material use oxygen as they respire while breaking down the algae. If the bloom was large, the amount of organic matter is substantial, so the microbial oxygen demand can exceed the water’s ability to supply oxygen through mixing and photosynthesis. In such conditions, dissolved oxygen drops, especially in deeper or stratified layers where replenishment is limited, leading to hypoxic or even anoxic conditions that stress aquatic life. Evaporation doesn’t remove oxygen from the water; wind mixing tends to add oxygen, and photosynthesis by aquatic plants releases oxygen. Hence, the bacterial decomposition of algae is the process that most commonly drives oxygen depletion after a bloom dies.