A new study published in the journal Scientific Reports has reported the profound effect coastal cities can have on the biological processes of animals that occupy the neighboring seafloor. Their results revealed that as much as three-quarters of these habitats offshore from large cities were exposed to potentially damaging levels of light pollution. It’s the first study of its kind looking at the impact of artificial light on seafloor species and what impacts the presence of this light can have on the affected animals.

Led by researchers from the University of Plymouth, UK, the study focused on the seafloor of the Plymouth Sound and Tamar Estuary, looking at the presence of light and its infiltration through the water column. They worked across four nights in 2018 using mapping and radiative transfer modeling tools to detect how much blue, green, and red artificial light was present at the surface, beneath the surface, and at the seafloor when these were shone into the sea by the researchers.

Their results showed that green and blue wavelengths left almost three-quarters of the seafloor exposed to light pollution, with each reaching 76 percent and 70 percent respectively. Red light on the other hand exposed less than 1 percent of the seafloor.

Green, blue and red light are used as a mixture to create many of the white LEDs that are employed in cities to light the streets, meaning the more intrusive green and blue light will be abundant in coastal cities. The findings paint a worrying picture for coastal species, many of which rely on natural cues from skyglow or the Moon to orientate themselves and their biological functions.

"The areas exposed here are not trivial,” said Dr Thomas Davies, Lecturer in Marine Conservation at the University of Plymouth and the paper's lead author in a statement. “Our results focused on a busy marine area and demonstrate the light from coastal urban centers is widespread across the sea surface, subsurface, and seafloor of adjacent marine habitats. But Plymouth is still just one coastal city with a population of 240,000 people.

"Seventy-five percent of the world's megacities are now located in coastal regions and coastal populations are projected to more than double by 2060. So, unless we take action now it is clear that biologically important light pollution on the seafloor is likely to be globally widespread, increasing in intensity and extent, and putting marine habitats at risk."

Concerned as to how the global expansion of coastal cities could have a profound impact on the survival of coastal species as integral biological processes get interrupted by unnatural light sources, the researchers are now calling for a more comprehensive review of the full impacts of coastal light pollution to see if there is anything that can be done to mitigate the worst of the effects.