All this rain: Good for drought, bad for the Gulf dead zone

Testing equipment in the blue waters of the Gulf of Mexico.

Scientists regularly forecast, monitor and measure the annual dead zone in the Gulf of Mexico. This photo shows some of that work being done by LUMCON in 2022. (Photo by LUMCON)

On the heels of one of the rainiest Minnesota springs in recent memory, many headlines have focused on drought relief. The rainfall has cleansed the colors of alarm from the drought tracker, leaving a squeaky clean state map with nary a hint of its recent parched past. 

But this unceasing precipitation is actually contributing to a negative situation far downriver. 

The NOAA predicted this year’s Gulf of Mexico dead zone — an area where oxygen levels in the water are so low that little, if any, marine life can survive there — will measure 5,827 square miles. That’s about 600 square miles larger than average, equal to roughly twice the size of the Twin Cities seven-county metro area.

Scientists at NOAA make this forecast by looking at a few key factors. That includes river discharge (how much water is flowing downstream) and nutrient loading data (the amount of nitrate and phosphorus in the water).

Which is where Minnesota’s spring soaking comes in. 

“In general, the size of the dead zone is tied to overall pollutant loads to the Gulf, which are closely correlated to river flows,” explained FMR Water Program Director Trevor Russell

A dry year, he said, usually means a small dead zone (which we saw in 2023, for example). A wet year and the dead zone typically swells. 

“There are other factors too,” Russell said, “but pollution from cropland and wastewater treatment systems, and overall river flow are key drivers of the dead zone.” 

That is what is behind the above-average 2024 dead zone forecast.

A satellite image of the continental U.S. with the Mississippi River and its tributaries highlighted in blue. Undeveloped, agricultural and developed lands in the watershed are color coded.

The Mississippi River and many of its main tributaries flow through large swaths of agricultural land, indicated by the yellow areas in the map above. (Photo by USGS via NOAA)

April and May saw rainfall levels significantly above normal throughout Minnesota, with both months making a strong push for one of the wettest springs on record, writes Mark Seeley for Minnesota Weather Talk. Areas around the headwaters of the Minnesota River — a major tributary to the Mississippi — had three times the normal amount of rainfall, he says.

Clearly there has been quite a lot of water flowing into and down the Mississippi River. That matches what the NOAA has measured, which is an above-normal river discharge.

Then there’s the nutrient loading data. The main culprits responsible for creating the conditions of the dead zone are nitrate and phosphorus. While these can reach the river in a few ways, the predominant source of nutrient pollution is agriculture

The vast majority of Minnesota’s cropland is empty all through April and well into May. When water from heavy spring rains seeps into the ground or rushes across the landscape, it carries with it the nutrients and chemicals that are left over in the unprotected soil (aka the “big brown spot”). 

That includes nitrate and phosphorus, which get swept into the creeks and streams and rivers that feed the Mighty Mississippi, and ultimately make their way to the Gulf of Mexico where they fuel this annual environmental catastrophe we call the dead zone

“If we want to address the dead zone, if we want to clean up our drinking water, if we want to protect public health, then we need to address the big brown spot,” said Russell. “The only way to do that is by providing farmers with clean-water crops that can ensure living cover on Minnesota farmland from fall through spring, and by making sure farmers can sell those crops for a profit. It’s a market-based, voluntary solution that would generate significant benefits for people and wildlife from here in the upper Mississippi River watershed all the way to the Gulf.”

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