What are ocean dead zones and how do they form?

The answer

Ocean dead zones are areas where dissolved oxygen levels have dropped so low, typically below 2 milligrams per liter, that most marine life cannot survive. They form primarily through eutrophication: excess nutrients from fertilizers, sewage, and industrial discharge fuel massive algal blooms that consume all available oxygen when they decompose. The Gulf of Mexico dead zone, fueled by Mississippi River nutrient runoff, can exceed 22,000 square kilometers in summer. The number of known dead zones has increased from 49 in the 1960s to over 700 today. Mobile organisms like fish flee these areas, while sessile organisms like mussels and worms simply suffocate.