How can algal blooms caused by phosphorus reduce oxygen for fish?

Algal blooms, often triggered by an excess of phosphorus from agricultural runoff and fertilizers, reduce oxygen levels for fish through several processes.123

When high concentrations of phosphorus enter aquatic ecosystems, they cause explosive growth of phytoplankton and other microscopic organisms. This dense accumulation of organic matter impacts oxygen levels in the following ways:243

• Decomposition: As the massive populations of algae die and sink, they are broken down by microorganisms. This decomposition process consumes vast amounts of dissolved oxygen, leading to hypoxia—a state of extremely low oxygen that can suffocate fish and other aquatic animals.14
• Blocked Sunlight: The dense surface mats of algae prevent sunlight from reaching underwater plants, which are necessary for producing oxygen through photosynthesis.13
• Physical Interference: In some cases, the blooms themselves can become so thick that they physically clog the gills of fish, directly hindering their ability to breathe.3

These conditions can create "dead zones" where oxygen levels fall below the threshold required to support fish and other marine life.14