How do algae blooms reduce the oxygen available to fish?

Algae blooms, or water blooms, reduce the oxygen available to fish through several mechanisms associated with eutrophication:

• Physical Obstruction: Dense blooms can form thick layers on the water's surface, which block sunlight from reaching underwater plants. This prevents these plants from performing photosynthesis, a process that normally replenishes dissolved oxygen in the water.12
• Oxygen Consumption: As the massive population of algae dies, it is decomposed by bacteria. This decomposition process consumes vast amounts of dissolved oxygen, often leading to hypoxia—a state of extremely low oxygen concentration that can cause massive fish kills.12
• Surface Interference: The presence of a dense bloom can physically impede the absorption of atmospheric oxygen into the water column.1
• Direct Harm: In addition to oxygen depletion, some blooms can physically clog the gills of fish, leading to suffocation, or release toxins that poison aquatic life.32

These conditions are frequently exacerbated by human activities, such as the runoff of fertilizers and sewage, which provide the excess nutrients that fuel rapid, harmful algal growth.412