Night lights make even the seas bright

New maps show where light pollution may disturb ocean dwellers

map of light pollution in the North Sea

The glow of coastal cities, wind farms and offshore oil and gas rigs are seen near the United Kingdom (landmass at left) and Norway (upper right). In April, waters here are clear enough for artificial light to reach from 1 meter (dark blue) to 30 meters (yellow) deep.

Joshua Stevens, T.J. Smyth et al/Elementa: Science of the Anthropocene 2021

Not even the sea is safe from the glare of humans’ light at night. Researchers published the first global atlas of ocean light pollution. It shows large chunks of the sea lit up at night. And that risks confusing or disrupting the behaviors of sea life.

Coastal cities cast haloes of light that stretch over the ocean. So do offshore oil rigs and other structures. In many places, the glow is powerful enough to penetrate deep into coastal waters. And that light risks changing behaviors of the creatures that live there.

Artificial lights are known to affect land dwellers. Night lighting can prevent plant pollination and foil fireflies’ flashes. They even make it harder for sparrows to fight off West Nile virus. Bright lights near shores can spread the glow out to sea.

Tim Smyth led a research team to assess where in the water this glow is strongest. Smyth is a marine biogeochemist. That means he studies how life in the oceans interacts with the environment using biology, chemistry and geology. He works at Plymouth Marine Laboratory on the southern coast of England.

map of light pollution in the Persian Gulf and the Red Sea
Areas with heavy offshore development, like the Persian Gulf (upper right), have lots of light pollution. Light pollution near Jeddah, on the western coast of Saudi Arabia (at left), also extends deep into the waters of the Red Sea.Joshua Stevens, T.J. Smyth et al/Elementa: Science of the Anthropocene 2021

Smyth and his colleagues started with a world atlas of artificial night-sky brightness that had been created in 2016. Then they added data on the ocean and atmosphere. Some data came from shipboard measurements of artificial light in the water. Others came from satellite images that estimate how clear the water is. Particles in the water, such as sediment and tiny floating plants and animals, can affect how far downward light travels. These factors vary from place to place and may change with the seasons. The team also used computers to simulate how different wavelengths of light move through water.

Next, they wanted to know how that underwater light might affect animals. Not all species will be equally sensitive. The team focused on copepods. These common shrimplike creatures are a key part of many ocean food webs. Like other tiny zooplankton, copepods use light as a cue to plunge en masse to the dark deep, seeking safety from surface predators. Normally they use the sun or the winter moon as their cue. Too much artificial light can mess up their usual patterns.

Light pollution is strongest in the top meter (about three feet) of the water. Here, artificial light can be intense enough to confuse the copepods. Nearly 2 million square kilometers (770,000 square miles) of ocean get such intense night light. That’s an area roughly the size of Mexico.

Farther down, the light gets weaker. But even 20 meters (65 feet) deep, it’s still bright enough to bother copepods across 840,000 square kilometers (325,000 square miles) of ocean.

The team described its findings December 13 in Elementa: Science of the Anthropocene.

Carolyn Gramling is the earth & climate writer at Science News. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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