What causes ocean dead zones and how big have they gotten?

The answer

Ocean dead zones are hypoxic areas where dissolved oxygen falls below 2 milligrams per liter, suffocating most marine life. The number of known dead zones has grown from 49 in the 1960s to over 700 globally. The largest permanent dead zone spans approximately 60,000 square kilometers in the Gulf of Oman in the Arabian Sea. The Gulf of Mexico dead zone, fueled by Mississippi River agricultural runoff, averaged 15,000 square kilometers in recent years. Excess nitrogen and phosphorus from fertilizers trigger massive algal blooms; when these algae die and decompose, bacteria consume available oxygen. Climate change worsens the problem as warmer water holds less dissolved oxygen. The Baltic Sea, Chesapeake Bay, and East China Sea all contain major seasonal dead zones threatening fisheries worth billions of dollars.