Published: February 6, 2018

By Daniel Gross

Runoff into the Mississippi River can drastically affect ocean chemistry where it meets the Gulf (Source: Sagredo/Creative Commons).

Scientists think river pollution will make this year’s low-oxygen zone – often called a “dead zone” because it kills marine life – the worst yet.

By Daniel Gross

This summer could be a harsh one for organisms in the Gulf of Mexico, if low-oxygen zones there are as large as expected.

A paper from the National Oceanography and Atmospheric Administration (NOAA) says this year’s low-oxygen (hypoxic) area could be the largest ever. It will encompass a region as large as Lake Erie off the coast of Louisiana and Texas. That’s a serious threat to 9000 square miles of sea life.

Organic matter on the seafloor needs oxygen to decompose, and surface oxygen can’t always keep up. As dead cells sink, oxygen demands rise. That means overabundant life can actually cause “dead zones.”

The problem is that chemical runoff, particularly crop fertilizer, can encourage overabundant life. Nitrogen pollution in the Mississippi River acts like fertilizer for algae. When it decomposes, it sucks oxygen away from the surface.

As a result, runoff drastically affects yearly hypoxic zones. Shrinking them requires scaling back pollutants like manure and nitrogen-rich chemical fertilizers. Right now, few of the Midwest’s farming states limit runoff. The EPA hasn’t stepped in, although NOAA and the EPA have been measuring runoff since 1997.

Annual size of Gulf hypoxic zones, with this year’s predicted range in dark gray (Source: Nancy Rabelais LUMCON/NOAA).

This year, nitrogen levels in the Mississippi aren’t abnormally high – but there’s just too much water. Water discharge from the river will be the highest in decades due to the high rains that flooded the river’s floodplains starting in April. All in all, an overall unprecedented amount of nitrogen will be delivered to the Gulf.

Authors of the study say drastic or unexpected weather patterns could affect their prediction. But their model has closely approximated “dead zones” in the past.

This is particularly bad news for a fishing industry still recovering from last year’s oil spill For now, Gulf fishing may be at odds with Midwest farming – at least until effective regulations are in place.

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