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Marine Dead Zones in Gulf of Mexico Are Expected To Last Decades

Nitrogen-rich sediments spilled into the Gulf of Mexico have created a marine "dead zone" that could last for decades. | NOAA National Ocean Service

In the Mississippi Basin, a surplus of nitrogen from fertilizer that leaches into water systems is having a major downstream impact, creating a massive oxygen-deprived "dead zone" in the Gulf of Mexico. A new study in the March 23 issue of Science now estimates that even if the nitrogen runoff was completely eliminated, it would still take at least 30 years for the dead zone to recover.

Nitrogen runoff from agriculture around the Mississippi Basin has been steadily draining into the Gulf of Mexico for decades. While the extra nitrogen is a rich resource for algae in the Gulf, driving the growth of massive algal blooms, excess nitrogen has a more deadly impact on other marine life. When algae die their decomposition consumes oxygen, creating large areas with low oxygen levels, or hypoxic zones.

Kim Van Meter of University of Waterloo explained, "In a hypoxic zone, or dead zone, there is not enough oxygen to support marine life. Fish that would normally live in the area will migrate out of the area, but other less-mobile organisms will die. Hypoxia results in a complete disruption of the food web, which impacts not only the coastal environment, but coastal communities around the Gulf that rely on commercial and recreational fishing."

By 2017, excessive nitrogen from agriculture had created a hypoxic zone in the Gulf extending 22,729 square kilometers (8775.7 square miles) — an area larger than the state of New Jersey. This came after a 1997 task force was established to study and develop mitigation plans to reduce the impact of nitrogen in the Gulf. Despite those efforts, the hypoxic zone is now three times the size of the task force's initial goals.

"Over time, nitrogen has built up in the environment — in soils and in groundwater," explained Van Meter. "Nitrogen that we see today in the Mississippi is in many cases not the nitrogen that was applied to crops this year, but nitrogen that has been slowly making its way through the landscape for decades."

Van Meter and her colleagues sought to explore what future targets might be necessary to reduce the hypoxic zone. They used modeling to analyze a "business-as-usual" scenario, as well as 25%, 75% and 100% reductions in agricultural nitrogen levels.

The results show that, due to earlier efforts to reduce nitrogen loads, maintaining these efforts under a business-as-usual scenario would reduce nitrogen loads by an additional 11% by 2050. However, in order to come close to achieving a reduced target hypoxic zone of 5,000 square kilometers (1930.5 square miles) by 2050, nitrogen levels would have to be brought to zero — a scenario that the researchers note in their paper is "not only considered unrealistic, but also inherently unsustainable."

"We believe it is crucial to better our understanding of the inevitable delays that will occur when we try to improve water quality," said Nandita Basu, a scientist at University of Waterloo who was also involved in the research. "A better understanding of these time lags, and the role of nutrient legacies in driving water quality, will help policymakers to set more realistic goals and manage expectations for everyone trying to improve the health of our watersheds."

The team plans to extend their water pollution model to include phosphorus — another nutrient of major concern, especially for inland water bodies such as the Great Lakes. And in a new project called
Legacies of Agricultural Pollutants

(LEAP), Basu said, "we are collaborating with economists to better understand the economic impacts of nutrient legacies in human-impacted watersheds"