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Answer: A dead zone is an area of hypoxia, water so low in oxygen that most marine life dies or leaves. Some occur naturally, but most human-linked ones are driven by nutrient pollution that fuels algae growth; when the algae die and decompose, they use up oxygen.

How it happens

1. Excess nitrogen and phosphorus run off farms or come from wastewater and fossil-fuel burning.
2. Algae overgrow.
3. Dead algae sink and decompose, consuming oxygen near the bottom.

Stratification adds to it: lighter fresh water sits on top of saltier water, limiting mixing that would bring oxygen down.

The Gulf of America zone

NOAA-sponsored research has tracked the largest U.S. dead zone, in the northern Gulf of America (formerly Gulf of Mexico), since 1985. It measured 4,402 square miles in 2025 and 1,332 square miles in 2026, the second smallest in the 40-year record. Water with 2 mg/L of oxygen or less is counted as hypoxic.

Not just the Gulf

Dead zones occur along coasts nationwide and in the Great Lakes.

Claims

• id: C1 claim: Dead zone is a more common term for hypoxia. source: S1 evidence: "is a more common term for hypoxia, which refers to a reduced level of oxygen"
• id: C2 claim: Nutrient pollution is the primary cause of human-created dead zones. source: S1 evidence: "nutrient pollution is the primary cause of those zones created by humans"
• id: C3 claim: The 2026 Gulf hypoxic zone was the 2nd smallest in the 40-year record. source: S1 evidence: "is the 2nd smallest ever measured in the 40-year record"
• id: C4 claim: Stratification is one natural cause of hypoxia. source: S2 evidence: "is one natural cause of hypoxia"
• id: C5 claim: NOAA-sponsored research has monitored the largest U.S. dead zone since 1985. source: S2 evidence: "Since 1985, NOAA-sponsored research has monitored the largest dead zone in the United States"

Sources

• [S1] What is a dead zone?. NOAA. https://oceanservice.noaa.gov/facts/deadzone.html . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Hypoxia. NOAA. https://oceanservice.noaa.gov/hazards/hypoxia/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_rbz5p1md,
  author = {{iinventedlight}},
  title = {{Ocean dead zones (hypoxia)}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/ocean-dead-zones/},
  note = {Permanent ID iil:note:rbz5p1md. License: CC-BY-4.0.},
}

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