How can fertilizer runoff cause eutrophication in lakes?

Fertilizer runoff contributes to eutrophication by introducing excessive amounts of nutrients—specifically nitrogen and phosphorus—into aquatic ecosystems.12

When these nutrients enter a lake, they act as a fertilizer for aquatic plant life, triggering the rapid and explosive growth of algae and other phytoplankton, often resulting in dense water blooms. This process leads to ecological degradation through several mechanisms:324

• Light and Oxygen Depletion: Dense algal mats block sunlight from reaching underwater plants, inhibiting photosynthesis. As these massive quantities of algae and other organisms die, their decomposition by microorganisms consumes vast amounts of dissolved oxygen.2415
• Hypoxia and Wildlife Loss: The resulting low-oxygen conditions, or hypoxia, can suffocate fish, mollusks, and other aquatic animals.124
• Toxicity: Some harmful algal blooms may also produce toxins that poison wildlife and render water unsafe for other uses.4

By artificially accelerating the natural aging process of a lake, human-driven nutrient loading—known as cultural eutrophication—remains a leading cause of aquatic ecosystem decline.32