Can the ocean run out of oxygen? - Kate Slabosky

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For most of the year, the Gulf of Mexico is teeming with marine life, from tiny crustaceans to massive 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. What's going on? Kate Slabosky dives into the lethal conditions that create dead zones.

Lesson by Kate Slabosky, directed Mette Ilene Holmriis.

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.