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[♪ INTRO] In the 1940s, ships were equipped with a fairly
new technology: sonar.
And that sonar detected a bizarre phenomenon: The sea
floor appeared to be just 300 meters below the ship in
areas that had been documented as being much deeper.
To make things even weirder, the depth changed throughout the night.
Researchers at the time gave this creepy phenomenon a
name that sounds more like a regrettable Scooby-Doo episode than a marine biology term: The Phantom
Bottom.
Today, scientists call it the deep scattering layer, or DSL
for short.
The DSL ranges from 300 to 500 meters deep depending
on the time of day, and it’s made of a layer of marine life so
dense and massive that it mimics the bottom of the ocean
on sonar.
DSLs are found around the world, but they don’t just exist
to confuse sailors and scientists.
They play a vital role in the health of the ocean and the
climate on Earth.
You’ll find lots of creatures in this layer spanning several
kilometers in width and hundreds of meters in depth.
So it’s no wonder that many ocean animals depend on
it for their next meal.
The creatures that make up the DSL include the tiniest
microbes, crustaceans, gelatinous creatures like jellyfish,
and countless fish species.
Lanternfish may be the most common larger creature
found in these layers, maybe even the most widely
distributed fish species on Earth.
These small creatures make their own light, a handy tool
when you live in the depths of the ocean where illumination
is scarce.
And it’s the swim bladder of all the fish living here,
including the lanternfish, that strongly reflects sound waves
from sonar, creating that eerie Phantom Bottom effect.
Now, a massive group of animals all hanging around in one
spot may seem like an easy target for predators.
But these critters have a few tricks up their sleeves to keep
themselves safe.
Researchers have found that DSLs aren’t uniformly mixed,
with a shrimp here and a fish there and a squid over there.
Rather, species will group together, forming a layer within
the layer.
Scientists have observed a large patch of krill
hanging out, and then another large patch of lanternfish
swimming together, with zero mixing between the two.
Researchers believe this might make it harder for
predators to single out an individual creature.
A nifty way of hiding in plain sight.
Not only will you find massive groups of single species
hanging out together, they also move in unison up into
shallow water at night and down into deeper water during
the day.
This is called diel vertical migration.
It’s another way to stay safe from predators.
The deeper you go in the ocean, the less light there
is, making it easier for animals to hide in the depths.
But they still have to eat, and due to the lack of sunlight in
the deeper ocean waters, there isn’t much in the way of
food.
Some of these animals eat phytoplankton, microscopic algae that stay in the shallow layer of the
ocean to photosynthesize.
To get that, the DSL creatures need to swim up at night to grab a meal.
Their predators follow along, and the predators of those
animals follow, and so on until the entire DSL moves, like a
perfectly orchestrated symphony timed with the sun.
It’s like an entire layer of the ocean is doing the wave.
That’s some pretty impressive choreography.
DSLs aren’t just important as ecosystems, though.
They play a critical role in the marine carbon
cycle known as the biological pump, which keeps our planet from
overheating.
The biological pump moves some of the carbon produced
in shallow waters to the deep sea in the form of marine
snow.
Made up of tiny carbon particles, marine snow is
mostly animal poop and dead plankton.
Some of that marine snow gets munched on by the
animals living in the DSL.
This ends up moving a lot of carbon out of the shallow water and into the
food web.
For us, that’s better than it making its way back into the
atmosphere as carbon dioxide, contributing to our planet’s
rising temperatures.
Currently, DSLs on Earth are understudied, and there’s a
lot left to learn about their contributions to Earth’s climate
cycle.
However, research suggests that these ecosystems are
transforming due to climate change.
As Earth warms, so does the ocean.
And as the ocean absorbs more heat, it’s becoming increasingly
stratified, with the layers of the water unable to mix
together because the temperature difference changes the density
of the water.
This prevents nutrient-rich bottom waters from
moving to the surface and feeding phytoplankton.
On top of that, low to zero oxygen zones are becoming
more common as water temperatures rise.
These are spots in the ocean where there’s less oxygen
available to the animals living there, because warmer water
holds less oxygen than cold water.
This makes certain locations in the ocean inhospitable to life.
By the year 2100, the ocean is expected to look a lot
different than it does today, and this will affect DSLs
around the world.
In some places, there will be very little phytoplankton left
and the DSLs in those regions will decline.
In other places, DSLs are expected to become more
productive than they currently are thanks to the increase in
water temperatures fueling more life.
Overall, researchers predict that they’ll be more patchy
than they are today.
DSLs are not the Phantom Bottoms they were once
thought to be.
They’re essential ocean ecosystems and a part of the
global carbon cycle.
As scientists continue to study DSLs around the world, it’s
becoming increasingly clear that they’re changing before
our eyes.
And as they transform, for better or worse, the ocean’s
ability to trap carbon will change along with them.
One can only hope that DSLs won’t disappear entirely,
becoming another kind of phantom altogether.
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[♪ OUTRO]