The Breathless Floor
Every summer, fertilizer washed down the Mississippi feeds an algae boom off Louisiana. When the algae die and rot, they use up the oxygen in the bottom water, and the seafloor can’t support life. This year the zone came in far smaller than forecast. Here’s why one good year isn’t a fix.
2nd smallest in 40 years of measurement, against a June forecast of 7,027 sq mi. The survey came after a tropical storm passed through the Gulf (GCOOS). The five-year average is still 2.5× the goal.
What “dead” means, in milligrams
Scientists measure dissolved oxygen in milligrams per litre. Once bottom water falls below 2 mg/L it counts as hypoxic: fish and shrimp that can swim away do, and the life that can’t move dies.
Forty summers of measured suffocation
The zone’s size swings with how much water and nitrogen the river carries each spring. It runs small in drought years and large in wet ones, and in most years it has stayed far above the goal.
The Hypoxia Task Force judges progress by the five-year average, not one summer. Its goal is under 1,900 sq mi by 2035, a deadline already moved back from 2015. The record is 8,776 sq mi (2017), close to the size of New Jersey. The smallest was 15 sq mi, measured in the 1988 drought.
A local problem with a global count
The Gulf isn’t unique. Reports of coastal dead zones worldwide grew decade after decade as fertilizer use spread.
Diaz & Rosenberg (Science, 2008) found the number of reported systems had grown exponentially since the 1960s. They linked the growth to river runoff of fertilizers and to fossil-fuel burning.
Forty-one percent of a country, draining into one bay
The Mississippi/Atchafalaya basin drains about 41% of the contiguous United States, much of it Corn Belt farmland. Its nitrogen reaches the Louisiana–Texas shelf, where the dead zone forms.
USGS finds that agriculture is the largest source of the basin’s nitrogen and phosphorus, with the highest yields in the Corn Belt. Each summer, warm fresh river water sits on top of saltier water, and that layering stops the surface from resupplying oxygen to the bottom.
The whole ocean is slowly losing its breath
Nutrient pollution suffocates coastal water, and warming makes it worse, because warm water holds less oxygen and mixes less.
What you can do
The size of the dead zone is set upstream, in how much nitrogen leaves farm fields. That’s where it can be shrunk.
Feed the soil, not the sea
Fertilizer timed and dosed to what crops need keeps nitrogen out of rivers. Agriculture is the basin’s largest nitrogen and phosphorus source (USGS).
Back the buffers
Wetlands, cover crops and streamside strips catch runoff before it reaches the Mississippi.
Read the five-year line
One small year after a storm isn’t a recovery. Watch the five-year average against the 1,900 sq mi goal.
Ask where shrimp comes from
Hypoxia changes the size of the catch: big shrimp got pricier in dead-zone months (Smith et al., PNAS 2017).