How does agricultural runoff from the Mississippi River basin contribute to the Gulf of Mexico dead zone?

Agricultural runoff from the Mississippi River basin is a primary driver of the seasonal "dead zone" in the Gulf of Mexico, a phenomenon known as hypoxia.12

This process occurs through the following steps:

• Nutrient Loading: Runoff from agricultural croplands carries significant amounts of nitrogen and phosphorus fertilizers into the river system, which eventually discharges into the northern Gulf of Mexico.342
• Algal Blooms: Because fresh water is less dense than seawater, this nutrient-rich runoff forms an upper layer on the ocean surface. These nutrients fertilize phytoplankton, triggering excessive population growth known as algal blooms.32
• Oxygen Depletion: As these massive quantities of phytoplankton die and sink, they are decomposed by bacteria. This decomposition process consumes the water's dissolved oxygen.32
• Ecological Impact: The resulting low-oxygen environment prevents most higher forms of aquatic life from surviving. Bottom-dwelling organisms such as crabs, shrimp, and starfish suffocate, creating a vast area where marine life cannot thrive.32

This cycle of cultural eutrophication has led to an annual dead zone that has reached sizes as large as 22,730 square km (8,776 square miles).342