How does fertilizer runoff lead to low oxygen levels in lakes and harm fish?

Fertilizer runoff contributes to a process known as cultural eutrophication, which accelerates the natural aging of aquatic ecosystems and degrades water quality.12

When fertilizers containing nitrates and phosphates wash into lakes, they act as excessive nutrients that trigger rapid, dense growth of algae and other aquatic plants, often forming algal blooms. These blooms harm the ecosystem in two primary ways:312

• Light and Oxygen Blockage: Dense surface blooms prevent sunlight from penetrating the water, which inhibits the ability of submerged plants to produce oxygen through photosynthesis.1
• Oxygen Depletion: When these massive quantities of algae die, they sink and are decomposed by bacteria and other microorganisms. This decomposition process consumes vast amounts of dissolved oxygen.32

As oxygen levels plummet—a condition called hypoxia—the water can no longer support fish and other aerobic organisms, often resulting in massive fish kills. In extreme cases, the lake becomes a "dead zone" where oxygen-dependent life cannot survive.123