How did the Gulf of Mexico's dead zone form?

A. Hurricanes destroyed the coastline

B. An oil spill killed all the species there

C. Fertilizer runoff from farming caused populations of algae to grow exponentially

D. All the fish and invertebrate species migrated elsewhere

C. Fertilizer runoff from farming caused populations of algae to grow exponentially.

The dead zone is caused by excess nitrogen and phosphorous coming down the Mississippi River. Nitrogen and phosphorous enter the river through upstream runoff of fertilizers, soil erosion, animal wastes, and sewage. This increases algae growth, the food chain is altered, and dissolved oxygen in the area is depleted.