How does eutrophication lower oxygen levels in lakes?

Eutrophication lowers oxygen levels in lakes through a process driven by an overabundance of nutrients, such as nitrogen and phosphorus, which enter the water via runoff from fertilizers, sewage, and other human activities.12

This nutrient enrichment triggers the rapid growth of algae and other microscopic organisms, resulting in dense water blooms on the lake's surface. These blooms negatively impact the ecosystem in two primary ways:2

• Light and Absorption: The thick surface growth prevents sunlight from penetrating deeper waters and hinders the natural absorption of oxygen necessary for underwater life.2
• Decomposition: As these massive populations of algae die, they sink to the bottom of the lake. The subsequent decomposition of this organic matter by bacteria consumes vast amounts of dissolved oxygen.13

In stratified lakes, this process often leads to hypoxia—a condition of extremely low oxygen concentration—particularly in deeper waters. This depletion of oxygen can be fatal to fish, mollusks, and other aquatic wildlife, often resulting in massive die-offs and the degradation of the ecosystem.21