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  • For most of the year, the Gulf of Mexico is teeming with marine life,

  • from tiny crustaceans to massive baleen whales.

  • But every summer, disaster strikes.

  • Around May, animals begin to flee the area.

  • And soon, creatures that can't swim or can't swim fast enough

  • begin to suffocate and die off in massive numbers.

  • From late spring to early autumn,

  • thousands of square kilometers along the coast become a marine dead zone

  • unable to support most forms of aquatic life.

  • This strange annual curse isn't unique;

  • dead zones like this one have formed all over the world.

  • But to explore what's creating these lethal conditions,

  • we first need to understand how a healthy marine ecosystem functions.

  • In any body of water that receives sufficient sunlight,

  • plant-like organisms such as algae and cyanobacteria thrive.

  • Clouds of algae streak the surface of deep waters,

  • and in shallower regions, large seaweeds and seagrass cover the ground.

  • Not only do these organisms form the foundation of local food chains,

  • their photosynthesis provides the oxygen necessary for aquatic animals to survive.

  • Besides sunlight and C02,

  • algae growth also depends on nutrients like phosphorus and nitrogen.

  • While such resources are typically in short supply,

  • sometimes the surrounding watershed can flood coastal waters with these nutrients.

  • For example, a large rainstorm might wash nutrient-rich sediment

  • from a forest into a lake.

  • These additional resources lead to a massive increase in algae growth

  • known as eutrophication.

  • But rather than providing more food and oxygen,

  • this surge of growth has deadly consequences.

  • As more algae grows on the surface, it blocks sunlight to the plants below.

  • These light-deprived plants die off and decompose

  • in a process which uses up the water's already depleted oxygen supply.

  • Over time, this can reduce the oxygen content

  • to less than 2 milligrams of oxygen per liter,

  • creating an uninhabitable dead zone.

  • There are rare bodies of water that rely on natural eutrophication.

  • Regions like the Bay of Bengal are full of bottom-dwelling marine life

  • that has adapted to low-oxygen conditions.

  • But human activity has made eutrophication a regular and widespread occurrence.

  • Nutrient-rich waste from our sewage systems and industrial processes

  • often end up in lakes, estuaries and coastal waters.

  • And the Gulf of Mexico is one of the largest dumping zones on earth

  • for one particular pollutant: fertilizer.

  • American agriculture relies heavily on

  • nitrogen and phosphate-based fertilizers.

  • 31 states, including America's top agricultural producers,

  • are connected to the Mississippi River Basin,

  • and all of their runoff drains into the Gulf of Mexico.

  • Farmers apply most of this fertilizer during the spring planting season,

  • so the nutrient flood occurs shortly after.

  • In the Gulf,

  • decomposing algae sinks into the band of cold saltwater near the seafloor.

  • Since these dense lower waters don't mix with the warmer freshwater above,

  • it can take four months for tropical storms

  • to fully circulate oxygenated water back into the gulf.

  • This dead zone currently costs U.S. seafood and tourism industries

  • as much as $82 million a year,

  • and that cost will only increase as the dead zone gets bigger.

  • On average the gulf dead zone is roughly 15,000 square kilometers,

  • but in 2019 it grew to over 22,000 square kilometers

  • approximately the size of New Jersey.

  • Human activity is similarly responsible for growing dead zones around the world.

  • So what can be done?

  • In the short term, countries can set tighter regulations on industrial run-off,

  • and ban the dumping of untreated sewage into ocean waters.

  • On farms, we can plant buffer zones

  • composed of trees and shrubs to absorb runoff.

  • However, long term solutions will require radical changes to the way we grow food.

  • Farmers are currently incentivized to use techniques

  • that reduce the health of the soil

  • and rely heavily on nitrogen-rich fertilizers.

  • But there would be less need for these chemicals

  • if we restore the soil's natural nutrients

  • by planting diverse crops that manage soil erosion and fertility.

  • Hopefully we can make these fundamental changes soon.

  • Because if we don't,

  • the future of our marine ecosystems may be dead in the water.

For most of the year, the Gulf of Mexico is teeming with marine life,

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B2 H-INT US algae gulf oxygen marine dead zone

Can the ocean run out of oxygen? - Kate Slabosky

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    王杰   posted on 2020/08/26
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