This episode of SciShow is sponsored by Endel, an app that creates personalized soundscapes  to help you focus, relax and sleep.

The first 100 people to download Endel  using our link in the description will get a free week of audio experiences. [♪ 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. Thank you for watching today’s  episode and to today’s sponsor, Endel. Endel is an environment-based app that  creates personalized soundscapes to help you focus, relax, and sleep.

They combine technology,  neuroscience, and acoustics to tailor songs in real-time with personal inputs  like location, weather, and heart rate. If you’d like to try it out, the first 100  people that click our link in the description will get a free week of audio experiences. And checking them out also helps  us, so thanks for doing that! [♪ OUTRO]