Did the Dead Zone Shrink?

Second Smallest Dead Zone in Forty Years, and a Storm Wrote the Number

Tropical Storm Bertha turned the northern Gulf over on the eve of the survey, mixing oxygen down to the seabed days before the ship arrived. The nitrogen stayed where it was.

Each summer a research vessel out of Cocodrie, Louisiana runs a grid of stations across the Louisiana-Texas shelf and lowers an oxygen sensor to the bottom at each one. Where the reading falls to 2 milligrams per liter or below, the water is hypoxic. Draw a line around those stations and you have the figure reported every year as the Gulf dead zone.

This year the line enclosed about 1,332 square miles — the second smallest zone in the forty-year record. The survey ran July 24 through July 30 aboard the LUMCON research vessel Pelican, led by scientists from Louisiana State University and the Louisiana Universities Marine Consortium.

In June, NOAA had forecast 7,027 square miles.

The measurement came in five times smaller than the prediction and landed outside the uncertainty range of NOAA’s ensemble forecast. When a well-built model misses by that margin, the question worth asking is what happened to the water between the forecast and the ship.

What happened was Tropical Storm Bertha.

What a storm does to a layered sea

The Gulf dead zone is a stratification phenomenon before it is a nutrient phenomenon. Fresh river water spreads out over denser salt water and rides on top of it the way oil rides on vinegar. That density boundary works as a lid. Beneath it, decomposing algae draw down the oxygen and no new oxygen arrives from above, because the surface and the bottom have stopped exchanging. Excess nitrogen and phosphorus out of the Mississippi-Atchafalaya basin feed the algal growth. Stratification is what converts that growth into suffocation.

Bertha crossed the northern Gulf on the eve of the cruise. Tropical storm-force winds and high waves churned the water across the survey area and mixed oxygen-rich surface water down to the seabed. Someone shook the jar.

NOAA states the consequence directly in its own release: the cruise did not capture the full extent of the zone. The agency’s framing is that abnormal weather years help researchers understand how storms drive the breakup and reformation of hypoxia. Reformation is the word to sit with. The lid comes back. Late summer heat restratifies the shelf, the sediment holds its oxygen demand, and the water reorganizes into the same layers it had before the wind arrived.

A storm that scatters a fish kill has not cleaned the water. It has made the fish harder to count.

The nutrient ledger

Nitrogen loads have come down. In June, EPA reported USGS data showing a 28 percent reduction in total nitrogen against baseline, clearing the 20 percent interim target set for 2025. Twelve states running tailored nutrient reduction strategies can put their names on that.

The metric is flow-normalized. It divides out year-to-year variation in how much water the river carries — the right move for building a trend line, and a poor guide to what the Gulf actually received.

What the Gulf received in May 2026 was roughly 134,500 metric tons of nitrate, about 4 percent above the long-term average. Discharge in the Mississippi and Atchafalaya that same month ran about 36 percent below the 1980–2025 average. A third less water carrying more nitrate than usual describes water that got considerably more concentrated. The trend line moved one way. The river moved the other.

Phosphorus tells a rougher story. The interim phosphorus target was missed, loads sit about 13 percent above baseline, and the flow-normalized figure has crept upward since 2021. Point sources have been cut hard — 46 percent, nearly eight million pounds a year, per Illinois EPA’s Trevor Sample. The overage comes from nonpoint sources: runoff and erosion off the ground itself. What has been pulled out of discharge pipes has been matched and passed by what washes off fields.

Current Gulf modeling holds that cutting both nitrogen and phosphorus by 48 percent would be needed to reach the 2035 target. The five-year average size of the hypoxic zone stands at 3,745 square miles against a goal of fewer than 1,900. Roughly double, with nine years on the clock.

Where the nitrogen decline came from

A study published this March in Ocean-Land-Atmosphere Research, led out of the University of Illinois Urbana-Champaign, tracked two decades of nitrate across the basin and confirmed the decline. Then it went looking for the cause and did not find it in fertilizer management.

The authors credit two developments: reductions in nitrogen oxide emissions from smokestacks and vehicles under the Clean Air Act, and more efficient nitrogen uptake by modern corn hybrids as yields rose. Cleaner air and better seed genetics. Co-author Robert Howarth, who chaired the National Academy committee that recommended these reductions in the late 1990s, characterized progress toward the 45 percent goal as slow, with declines in Mississippi River nitrogen flows showing up only over the past decade.

The improvement is real, substantially incidental, and running behind what the target requires — while phosphorus climbs.

What a one-week measurement cannot see

The annual cruise produces an area. It does not produce a duration, a cumulative exposure, a benthic mortality count, or an account of how much dead bottom the shrimp fleet steamed across to find working ground.

The mechanism itself is well understood. Low oxygen renders critical habitat unavailable to bottom-dwelling species, and mobile animals like fish and shrimp clear out. Animals that cannot move lack that option. Whether hypoxia held for two weeks or ten this summer is a question the July snapshot was not designed to answer, and Bertha took away even the snapshot.

Two statistics arrived this season. One is a flow-normalized trend that divides out a drought. The other is an area measured days after a storm stirred the sensor’s field of view. Both point somewhere hopeful. Neither removed a pound of nitrogen from the Mississippi basin.

The other half of the problem

Reduce-at-source is the whole of current federal and state strategy, and it should continue. It is also voluntary, slow, and dependent on nonpoint behavior across a subcontinental watershed where the phosphorus figure is presently moving backward.

The River Refugium Project addresses what reduction leaves behind: intercept nutrient-loaded water inside the basin, grow the nitrogen and phosphorus into biomass, harvest the biomass, and move it out of the watershed as product. Reduction lowers what enters. Harvest removes what already has. A watershed that has been banking nutrient load since the 1950s needs both, and one of the two is currently being attempted.

Version 3.0 of the technical framework published on August 10 — a consolidated whitepaper and five appendices. Open access, no paywall, reproduction and adaptation permitted.

Project page: https://www.cernunnosfoundation.com/rrp/

Documents:

Appendix B and Appendix C are where to start if the question on your mind is where.

Next July the Pelican runs the grid again. Given ordinary weather, the number will be larger, and it will be reported as a setback. It will be a measurement taken through clear air.

Sources

NOAA / NCCOS, “NOAA, Partners Measure Smaller-than-Anticipated Hypoxic Zone in Gulf,” August 5, 2026
NOAA, 2026 Gulf of America Hypoxia Forecast one-pager (May nutrient loads, ensemble forecast)
NOAA, “NOAA forecasts an above-average summer ‘dead zone’ in Gulf of America,” June 2026 (discharge figures)
U.S. EPA, “Task Force Reports Progress Toward Pollution Reduction Goals for Gulf of America,” June 11, 2026
Environmental Council of the States, “Mississippi River States Make Strides on Nutrient Pollution,” June 18, 2026
Agri-Pulse, “Progress made toward voluntary nitrogen reductions,” July 1, 2026 (phosphorus detail; Trevor Sample, Illinois EPA)
Botero-Acosta et al., Ocean-Land-Atmosphere Research, 2026, DOI 10.34133/olar.0131
Fennel & Laurent 2018, dual 48 percent reduction requirement, cited in Hypoxia Task Force Report to Congress
LUMCON, 2026 Shelfwide Hypoxia Cruise data