How did fertilizer runoff contribute to oxygen-depleted areas in the Gulf of Mexico?

Fertilizer runoff contributes to oxygen-depleted areas, or "dead zones," in the Gulf of Mexico through a process known as cultural eutrophication.12

The Mississippi River drains a vast basin, delivering nitrogen and phosphorus from agricultural fertilizers into the northern Gulf. Because fresh water is less dense than salt water, this nutrient-rich runoff forms a distinct upper layer on the ocean surface. These nutrients trigger excessive population growth of phytoplankton, leading to massive algal blooms.32

As these large masses of phytoplankton die, they sink to the bottom and decompose. This decomposition process consumes the water's dissolved oxygen, creating hypoxia—a condition where oxygen levels drop too low to support most aquatic life. Consequently, bottom-dwelling organisms such as crabs, shrimp, and starfish suffocate, resulting in a "dead zone" that can span thousands of square miles. Addressing this environmental degradation requires improved agricultural and pollution control practices to curb nutrient loading.312