How Light Pollution Threatens Marine Life
66 million years ago — Late Cretaceous, Montana, USA: Darkness settles over the volcanic plains of Hell Creek. As twilight deepens, a Didelphodon—an early mammal, roughly the size of a badger—emerges cautiously from its burrow. It has embraced nocturnality, a rare evolutionary path among vertebrates, to avoid competition and predation from dinosaurs, the dominant force of the time.
Even today, humans—the heirs of early mammals like Didelphodon—remain active at twilight and even long into the night. Yet, our motives have changed. We no longer have to fear predation. We stay up by choice, driven by leisure or work. And unlike our ancestors, we no longer adapt to the night—we reshape it. With the flick of a switch, we’ve banished darkness, flooding the world with artificial light at night (ALAN).
For humans, the spread of artificial light—and the light pollution that inevitably comes with it—often means little more than a faded night sky and lesser stars. A quiet inconvenience perhaps, but little more. Yet for many others, the consequences are far more serious. Nocturnal and crepuscular animals depend on the natural rhythm of light and darkness to hunt, hide, mate, and migrate. A disruption of that cycle for them means a disruption in functionality. This is especially problematic in coastal regions, where human population growth is happening faster than anywhere else. With it comes a new tide of light that spills into the sea and disrupts marine ecosystems.
A development concerning biologists and conservationists alike. One of them is Dr. Mark Lenz, a marine ecologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany.
‘For many people, it is hard to even view light as a pollutant, because light is something positive for us. We can do things at night that we otherwise couldn’t. This shift in perspective, that light isn’t always beneficial, takes a moment—and it took me a moment too.’ For Mark, light pollution was always something he was aware of, but only at a certain point did he wonder what actually happenswhen we turn on our artificial suns:
On a beach in Florida, USA,
a sea turtle hatchling is escaping from its egg. Instinctively, it crawls towards the moonlight. This is the most vulnerable time in a sea turtle’s life. It needs to hurry to get to the moderate safety of the ocean. But as it crawls closer to the light, its flippers touch concrete instead of waves. It has confused a streetlamp with the moonlit surface of the ocean and crawled in the wrong direction.
In the waters off the coast of Cape Town, South Africa,
it is a dangerous night to be a Cape fur seal. A full moon is illuminating the water’s surface, which makes it easier for great white sharks to spot their silhouettes. Unfortunately for the fur seals, the spillage of artificial light from the nearby city means that there is a full moon every night.
In the Great Barrier Reef off the coast of Cairns, Australia,
a clutch of clownfish eggs exposed to ALAN did not hatch. Their embryos have evolved to hatch after dusk to avoid predators. Meanwhile, the skeletal architecture of the juvenile corals, that are the future of the very reef the clownfish and many other species live in, is altered by ALAN and becomes more porous, which also affects their light capture capacity.
The sun is setting over the icy waters of the Arctic,
where the Earth’s greatest migration is about to start anew. The zooplankton, such as copepods—tiny drifting crustaceans that form the base of many marine food webs—that have sunk into deeper waters during the day to avoid predators that rely on visual cues, are about to migrate to the surface to feed on phytoplankton, microscopic photosynthetic organisms that float near the surface of the ocean. But this night is different. The sun has set hours ago, yet it is still bright. The lights of the oil rig won’t set.
Mark explains that one of the major problems for organisms disoriented or otherwise influenced by artificial light at night (ALAN) is that it closely resembles sunlight. This leads to marine life confusing artificial light sources with the sun or the moon—a problem that has worsened with the shift towards LED technology.
‘The light spectrum of LEDs is inherently close to that of sunlight, which makes it even more biologically relevant. Many animals, particularly those in marine environments, are sensitive to very subtle light differences. Since LEDs emit a broad spectrum, often with a heightened intensity in the blue range, they can cause more disorientation than older sodium vapor lamps, which produce a more distinct, orange-colored light.’
With a global shift towards LED technology well on its way, it is high time to think about the ramifications that shift will bring with it—especially since ALAN is not limited to coastal regions. In our increasingly interconnected world, it now permeates the open ocean as well, emanating from tankers, cruise ships, trawlers, oil rigs, and other offshore infrastructure.
The global nature of the problem is of special interest and importance to Mark: ‘Only by testing the same conditions with different organisms at various locations,’ he explains, ‘can we uncover whether the effects of ALAN are universal or location-specific.’ This approach has revealed that responses to ALAN vary significantly—not just between species, but also depending on where the organisms live. ‘Some grazer species, for example, showed increased feeding under artificial light, while others fed less or didn’t react at all,’ explains Mark. For him, these variations highlight the complexity of ecological responses and the importance of large-scale, standardized research to grasp the bigger picture.
This awareness for the bigger picture is something Mark is keen to instill in the next generation of scientists. Since 2004, he has been the scientific coordinator of the GAME program. The GAME program is a research and education programme for students that focuses on a diverse array of topics in marine ecology. And in the last five years, one of these topics has been ALAN.
This approach of conducting comparable experiments in different regions has produced some unexpected outcomes:
‘We observed that a cnidarian was particularly common when surfaces were illuminated at night,’ says Mark. This went so far that it completely covered the area. ‘I’m not sure why a hydrozoan would benefit from the light, as it isn’t photosynthetically active or anything like that. Instead, it is a tentacle feeder. I guess that it might indirectly benefit from the light because it attracts its prey.’
‘And with the algae we studied last year, I was also surprised, because it was generally assumed that macroalgae—larger, more long-lived algae—would not be able to make use of such weak light intensities. They can’t really use it for their photosynthesis because it’s too weak. Also, the energy demand of these species is too high, since they are large and long-lived. Still, we found that some algae grew better under artificial light.’
And while it might seem insignificant to research small or overlooked marine organisms, these creatures often play foundational roles in their ecosystems. Grazers like snails and sea urchins, for example, directly affect the abundance of macroalgae, which are crucial habitat builders in many marine environments. Changes in the behaviour or feeding patterns of these small animals—triggered by artificial light—can lead to cascading effects that reshape entire communities, and in turn affect animals we might care about preserving in the world our children will live in. Furthermore, these organisms have a direct impact on us as humans. Phytoplankton, for example, produces about half the oxygen we breathe—without it, we could only take every second breath.
This underscores a broader point raised by Mark: ‘When we think about habitats, we tend to think about physical spaces. But darkness is also a habitat. Just as coral reefs support a diverse range of life, darkness enables unique behaviours and processes that are critical to many species. For some species, it offers the opportunity to hunt; for others, it’s a time to find a mate or navigate the ocean. In a bright environment, these behaviours are often impossible—whether due to the threat of predators or because the natural processes simply don’t function under the presence of light. By eliminating darkness, we’re not just altering a time of day; we’re erasing an essential habitat.’
When thinking back to the Didelphodon emerging into the dark of the Late Cretaceous, we have to imagine a vastly different world. There was no permanent ice at the poles. Lush forests stretched across areas where today there is nothing but tundra and ice. Where once sponge reefs dominated, there are now coral reefs. Temperatures, acidic levels, and even oxygen concentration fluctuated massively. Yet there always has been one constant: the rhythm of day and night, darkness and light. For all of Earth’s history, life evolved under this rhythm—a rhythm that is hardwired into the DNA of nearly all life on Earth. But in the last 150 years, we humans have begun to interfere with a rhythm so fundamental that it is no wonder this has consequences. And it is important that we keep that in mind.
We humans are often lost in the dark. But other creatures are sometimes lost in the lights.
An Article by Jona-Tristan Köhring, Photo Credits under Photos