Into the blue: Understanding marine light pollution

Most of us think of light pollution as a problem affecting the skies above our cities. But what about the light that spills into our oceans?

Artificial light at night (ALAN) has long been studied for its impacts on humans and wildlife on land. Increasingly, scientists are also turning their attention to how it affects marine ecosystems, from coral reefs and seabirds to fish and zooplankton. It turns out that our oceans are far from dark. 

“Marine artificial light pollution is a growing problem that many people don’t even know exists,” says Professor Tim Smyth, head of science for marine biogeochemistry and observations at Plymouth Marine Laboratory (PML) in the U.K. “We’re now seeing clear evidence of widespread ecological impacts.”

New frontier for dark sky advocacy

For nearly a decade, researchers at PML and the University of Plymouth have been exploring the extent of ALAN in the ocean. Their studies show that some 22% of the world’s coastal waters are exposed to measurable levels of artificial light at night. That’s 2 million sq km (772,000 sq mi) in total — an area roughly the size of Western Europe or about half the size of China.

Marine light pollution can come from streetlights, ports, coastal developments, fishing boats, oil platforms, and other shoreline infrastructure. While the brightest impact zones are often limited to around 100 meters offshore, light can scatter far beyond this. Skyglow from urban centers can reach many miles out to sea, particularly under cloudy skies or where waters are very clear.

“One of our main concerns is how light disrupts the natural rhythms that marine organisms depend on,” Smyth explains. “From daily migrations of zooplankton to coral spawning cycles and turtle hatchlings, many marine species rely on darkness as an environmental cue.”

Biological clocks and blue light

Under natural conditions, light in the ocean fades rapidly the deeper you go. But ALAN, especially from white or blue-rich LEDs, penetrates much deeper into the water column, up to 40 meters below the surface in clear water. This can disturb behaviors and biological processes in organisms adapted to the dark.

Smyth and his PML colleagues have focused much of their current research on zooplankton, tiny sea organisms that usually migrate toward the surface at night. These have been observed staying deeper in artificially lit waters, and the team believes this is likely to have knock-on effects across the marine food web.

Many marine species are known to be vulnerable to ALAN. Research by Tim Smyth’s colleague, Thomas Davies, published in Nature Communications, showed that coral reefs rely on the cycles of the Moon to synchronize mass spawning events. Artificial light at night, particularly in the blue spectrum, “fills in” the natural dark periods that corals rely on to initiate spawning, such as the dark window between sunset and moonrise.

Sea turtle hatchlings become disoriented by shoreline lighting and travel inland instead of toward the sea. Seabirds such as shearwaters and petrels are drawn off-course by artificial light at night, often colliding with vessels or buildings. Even shore crabs and intertidal snails show altered foraging and camouflage behaviors under artificial light, changing predator–prey dynamics.

Coastal lighting near penguin colonies can delay or prevent adults from coming ashore, interfering with nightly feeding and chick survival. Squid are drawn to bright fishing lights — a behavior exploited by fisheries but with broader ecological consequences for marine food chains. Even jellyfish appear to respond to subtle changes in underwater light, and increased ALAN may affect their vertical movement and predator interactions.

“Blue light, in particular, penetrates deeply into seawater,” says Smyth. “And unfortunately, many newer LED fixtures are heavy on the blue spectrum, especially in urban and industrial settings.”

Mapping the light below

To support research and awareness, PML has created the first-ever Global Atlas of Artificial Light at Night Under the Sea. This online tool is similar to other light pollution maps and is based on their work, allowing anyone to explore the extent of marine light pollution by region or coastline. It builds on satellite data, in-water optical modeling, and ecological thresholds to show where artificial light may be disrupting marine life.

“We wanted to make the data accessible to scientists, policymakers, and the public,” he says. “The atlas helps identify areas of concern and guide better lighting practices.”

The atlas is part of a growing global effort to highlight marine ALAN as a serious environmental issue. In 2024, the Global Ocean Artificial Light at Night Network (GOALANN) was launched, endorsed by the United Nations Ocean Decade. GOALANN brings together researchers, conservationists, and regulators to better understand marine light pollution and inform international action.

PML’s work is also part of AquaPLAN, a four-year research project funded by Horizon Europe, the EU’s flagship science program, which studies the combined impacts of ALAN and human-made noise on aquatic ecosystems. The project brings together scientists, conservationists, and policymakers to develop strategies for monitoring and mitigating these stressors, with a focus on protecting biodiversity in European waters.

Solutions from shore to sea

Like terrestrial light pollution, marine ALAN is a tractable problem that can often be reduced quickly and affordably. For instance, coastal developments can use warm-colored, fully shielded lighting directed away from the shore. Fishing fleets can try out targeted wavelengths to avoid broad-spectrum lighting.

“We’re not saying to turn off all lights,” Smyth says, “Safety is important. But we can light smarter — reduce skyglow, use appropriate colors, and turn off lights when they aren’t needed.” To this end, Plymouth City Council has worked with Smyth’s lab to begin trialing dynamic street lighting near the coast to reduce light pollution while maintaining public safety — a model that could inform coastal planning elsewhere. 

DarkSky is currently developing a new Approved Ports program to be launched later this year, which follows the Five Principles for Responsible Outdoor Lighting. The program will address light trespass into the water as well as the sky.  

DarkSky also recently launched templates for ordinances to promote higher quality lighting with reduced light pollution in communities. The templates are modular and can be amended to fit local conditions. One such optional addition is language for sea-turtle sensitive areas, which is based on the U.S. state of Florida’s “Model Lighting Ordinance for Sea Turtle Protection.”  

Another emerging concept is the creation of certified dark sky marine places, which could be protected ocean or coastal zones where artificial light at night is limited to conserve sensitive habitats. Tim Smyth thinks dark sky marine reserves would be “a way of extending the success of terrestrial dark sky places into the ocean.” He says such reserves could play a vital role in conservation planning, particularly in places like the U.K.’s Marine Protected Areas, where vulnerable species rely on natural light cues to survive and thrive.

The impacts of ALAN on marine biodiversity are still being uncovered, but it is clear that darkness matters, even in the sea. As we work to protect the night sky above, we must also consider the waters below. DarkSky Advocates can play a role by raising awareness of marine light pollution and encouraging policymakers and local governments to include oceans in lighting regulations. So far, most lighting laws focus only on land-based impacts. 

As Tim Smyth put it: “Light pollution is not just a sky issue; it’s a planetary issue. It affects the atmosphere, the land, and the ocean. Recognizing that is the first step toward protecting the full natural night.”

To explore the Global Atlas of Artificial Light at Night Under the Sea and learn more about GOALANN, visit goalann.org. More info on AquaPLAN at aquaplan-project.eu. 

This article originally appeared in the June 2025 issue of Nightscape magazine. Written by Megan Eaves, Nightscape Editor, United Kingdom.