Can the ocean run out of oxygen? - Kate Slabosky

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.

Thirty-one 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 US 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.