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[♪ INTRO]
In our time on Earth, we humans have changed
the natural environment in not so great ways.
Some of the bigger changes include climate change,
species extinctions, and ocean pollution.
And while many of these environmental issues have been studied for decades,
that doesn’t mean we know /all/ the ways that we’re affecting the natural world.
To this day, we’re still finding ways that we’re changing our environment.
One example is a newly discovered phenomenon called coastal darkening.
Coastal darkening is pretty much what it sounds like.
Parts of the ocean nearest to the land are getting darker bit by bit.
And the darkening doesn't stop at the surface;
we're talking about the whole water column.
The entire water stretch from the surface to the seafloor gets murkier and darker,
blocking sunlight from getting through.
One of the potential explanations for this phenomenon is that fertilizer runoff
gets into water, causing algae to bloom, blocking the sunlight.
Boat traffic can also stir up silt from the seafloor
and darken the water column that way.
Climate change can also cause coastal darkening by making parts of the world
rainier, so soil and other particles wash into the ocean, again making it darker.
Basically, anything that gets particles into
the water column can make the coastline darker.
And this can happen naturally or at random with bad weather and erosion.
Like, during the rainy seasons, coastal waters can get a bit darker for some time
because the soil from the rivers gets carried into the water column.
But the coastal darkening phenomenon is cumulative
and doesn’t disappear over time.
Coastal darkening changes the average amount of
light penetration in the water column over the same amount of time.
This trend could be happening either because the number of darkening events is
increasing or because the events are coming so close together
that particles don’t have time to leave the water column.
And this isn’t happening in just some places; it’s being observed all over the world.
But since it’s still such a new phenomenon,
scientists are piecing together the environmental impacts.
The farthest back published studies have looked at coastal darkening
is only a 100 years, which means our record starts
after the Industrial Revolution and the onset of climate change.
Some of the data sets that researchers have analyzed are located along the coasts
of the North Sea, mainly just offshore from Germany and Denmark.
An important thing to point out is that scientists have been measuring
how murky the water gets since the late 1800s,
using a device called a Secchi disk, which was created
all the way back in 1865 by Italian priest and scientist Angelo Secchi.
And today, that’s still the go-to method for measuring water light penetration.
Although it sounds kind of high-tech, a Secchi disk is just a plain disc
that gets lowered into the water until it becomes invisible from the surface.
When that happens, it's called the “Secchi depth,”
which changes depending on how clear the water is.
So, in a darkened water column, the disk disappears sooner.
Historically, these measurements have been used
to check water clarity, which is one indicator of water cleanliness.
But because coastal darkening has been discovered so recently,
scientists have just started comparing historical data to modern data sets.
And because turbidity, or the amount of particles in the water,
changes from day to day, it takes time to establish a strong statistical trend.
The North Sea dataset used in a study published in 2019 shows that a section of
the North Sea coast has gotten darker over the past hundred years.
This is the first study to show this trend.
This darkening trend might not be visible to the naked eye,
but it definitely affects the surrounding ecosystem.
Ocean organisms like phytoplankton rely on sunlight for photosynthesis,
so less sunlight reaches them when coastal waters become darker.
This might mean that less phytoplankton can grow in darker water columns,
affecting the phytoplankton density in the water.
Since phytoplankton are at the bottom of the food chain,
not having enough of them affects the whole ecosystem.
And these critters aren’t the only ones affected; some marine organisms use their
sight for hunting, so they’ll also have a hard time getting food in the darker waters.
Ocean chemistry might also change with this newfound darkness.
Methyl mercury is a pollutant that can reach the sea after processes like
the burning of fossil fuels, and it’s extremely toxic.
It can cause a gamut of health problems, including muscle weakness,
loss of coordination, and congenital disabilities.
Now, usually, this chemical is broken down by the UV wavelengths in sunlight.
But, if not enough sunlight reaches this chemical, it’ll stick around,
wreaking havoc in the ecosystem and potentially our health.
There’s even a chance that coastal darkening might worsen climate change
because it affects marine plants.
These plants sequester carbon by removing carbon dioxide
from the atmosphere during photosynthesis and using it as fuel to grow.
Take kelp, for example; It doesn’t only sustain many coastal ecosystems,
it also consumes a lot of carbon dioxide.
A 2016 study suggests that macroalgae, including kelp,
can sequester on average 200 million tons of carbon a year.
And coastal darkening can reduce kelp’s primary productivity,
its ability to convert carbon dioxide and sunlight to biomass,
by as much as 95%, which could affect how much carbon kelp sequesters.
So, if kelp takes in less carbon at low light levels,
darker oceans could end up accelerating climate change.
Overall, the research team found that coastal darkening
can cause kelp to sequester up to nearly five times less carbon.
But because there might be differences in how both coastal darkening and kelp
affect each other in different regions of the world, more research is needed
to see just how much coastal darkening could impact climate change.
Still, this new phenomenon is a good reminder that we don’t fully understand
how we’re changing the natural world,
or what parts of the ecosystem we might be changing forever.
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[♪ OUTRO]