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Every summer, an area of low-oxygen water forms in the Gulf of Mexico. Scientists refer to this as a "dead zone" because very little marine life can survive in it.
This year, scientists predict the Gulf’s dead zone will be one of the largest on record: between 7,829 and 8,717 square miles, or about the size of Massachusetts.
Dead zones are typically caused by agricultural run-off that flows into rivers. That pollution, coupled with rising ocean temperatures (warmer water holds less oxygen), is leading to larger and larger dead zones.
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Every summer in the Gulf of Mexico, marine life starts to die. It’s the end result of a process that starts far from the ocean.
Agricultural run-off flows into the Mississippi river, raising the levels of nitrogen and phosphorus — which are abundant in fertilizers and used everywhere from farms to sewage plants — in the water. When that river water flows into the ocean, those nutrients prompt algae to grow quickly and wildly. These algae blooms then die and sink to the bottom of the ocean, depriving fish and other underwater creatures of the oxygen they need to survive.
This creates an aptly named "dead zone."
Last summer, the dead zone in the coastal waters near Louisiana and Texas was about the size of Delaware. This year, it’s expected to be far bigger.
Scientists from the National Oceanic and Atmospheric Administration (NOAA) predict that the 2019 dead zone will be the second largest on record since 1985: approximately 7,829 square miles, roughly the size of Massachusetts. Louisiana State University researchers expect it to be even larger: 8,717 square miles, just shy of the all-time record of 8,776 square miles from 2017.
Either way, fishermen in the Gulf should prepare to go farther afield for their catch this summer — which means the prices of seafood could go up.
More fertilizer run-off means a larger dead zone
In the US, 41% of land drains into the Mississippi river and its tributaries, according to the Nature Conservancy. As water drains, it picks up nutrient pollution, most of which comes in the form of nitrogen and phosphorus. The nutrient-rich river water then makes its way into the Gulf of Mexico.
This influx of nitrogen and phosphorus leads to rampant algae blooms on the surface. When the algae perish and sink into the deep ocean, they’re eaten by bacteria. The bacteria use up oxygen while consuming the dead algae, which causes seafloor to choke — oxygen levels plummet, creating hypoxic (oxygen-starved) waters. Any sea creature that can evacuate, like larger fish or even crabs that can scuttle along the ocean floor, do. Everything else eventually suffocates and dies.
The dead zone in the Gulf forms every year in late spring and early summer. That’s when there are fewer storms and calmer water, which means the oxygen-rich surface water doesn’t mix as much with oxygen-depleted water below to replenish oxygen in the depths.
In addition, temperatures of the surface water rise, which also contributes to the low rate of mixing (warmer, less dense water floats on the surface instead of sinking).
By late August or September, the dead zone gets broken up by cooler surface temperatures and tropical storms or hurricanes.
Each year, the dead zone’s total area depends on how much run-off flows into the ocean. This summer, it’s expected to be larger than average because high rates of rainfall (which we had this winter) creates more run-off. According to the US Geological Survey, the amount of nitrogen and phosphorus entering the Gulf of Mexico was about 18% and 49% above average, respectively.
Dead zones force fishermen to travel farther
This is because fish swim away from the coast to escape the dead zone, which forces fisherman to travel farther out to sea, as the Nature Conservancy has reported. So fishermen wind up spending more time, fuel, and money to haul in a catch.
According to NOAA, the dead zone reduces the overall catch size of commercial fisheries, leading to smaller harvests and more expensive seafood.
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What’s more, a 2017 study found that conditions in the Gulf dead zone slow shrimp growth, leading to lower numbers of large shrimp. So during summers with dead zones, fishermen catch more small shrimp and fewer large ones. That makes the small shrimp cheaper for consumers, and the larger ones more expensive.
Can we shrink the dead zone?
Dead zones aren’t restricted to the Gulf of Mexico. The Baltic Sea boasts seven of the 10 largest dead zones in the world, according to National Geographic. In 2018, researchers reported a 63,700-square-mile dead zone in the Gulf of Oman — the world’s largest.
In the US, the Environmental Protection Agency set up the Gulf Hypoxia Task Force, which aims to shrink the dead zone down to less than 1,900 square miles by 2035. But researchers aren’t sure that’s feasible given the upward trend in the zone’s size.
"The bottom line is that we will never reach the dead zone reduction target of 1,900 square miles until more serious actions are taken to reduce the loss of Midwest fertilizers into the Mississippi River system," Don Scavia, an ecologist who helped formulate the new prediction, said in a press release.
Beyond run-off, hypoxic water in the Gulf of Mexico’s dead zone is also linked to climate change. Last year was the hottest on record for the world’s oceans, and as waters warm, they hold less oxygen. That means the problem is only going to get worse.
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