Name: 海は酸素不足になるのか？- ケイト・スラボスキー (Can the ocean run out of oxygen? - Kate Slabosky)
Uploaded: 2021-01-14T11:23:08.000Z
Duration: 6 min 21 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

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

from tiny crustaceans  to massive baleen whales.

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.

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

unable to support  most forms of aquatic life.

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.

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

These additional resources lead  to a massive increase in algae growth

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,

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.

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.

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

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—

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

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

and ban the dumping of untreated  sewage into ocean waters.

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

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.

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