“Arguably, the light bulb is the most transformative invention humans have introduced to this planet,” observes National Geographic. “By flicking a switch or pushing a button, we can push back the veil that would naturally shroud our lives each night.”

Yet our nighttime addiction to electric lights of all kinds, from reading lamps to street lights to neon lights, has resulted in excessive brightness that has robbed myriad creatures of darkness and the night sky. The results of light pollution have been massive.

“For billions of years, all life has relied on Earth’s predictable rhythm of day and night. It’s encoded in the DNA of all plants and animals. Humans have radically disrupted this cycle by lighting up the night,” explains the Dark-Sky Association, an organization combating light pollution worldwide.

“Plants and animals depend on Earth’s daily cycle of light and dark rhythm to govern life-sustaining behaviors such as reproduction, nourishment, sleep and protection from predators,” it adds.

And what with growing urbanization, worse is yet to come. As cities and town continue to grow in coastal areas, more than three quarters of the seafloor in their vicinity will be exposed to harmful levels of light pollution, say experts at the University of Plymouth in the United Kingdom.

Artificial light used in everyday street lighting permeates all areas of  the sea around coastal cities, which could pose a significant threat to coastal species, the scientists have found.

Light pollution, which illuminates the sky at night, can disrupt the migration of species that rely on the moon and starlight for navigation. Yet how species underwater are affected has been little understood so the researchers set out to find that out by studying the optical properties seawater.

Over four nights with little or no moonlight, the scientists shone blue, green, and red artificial light at the sea surface under both clear and cloudy conditions and during low and high tide in the Plymouth Sound and Tamar Estuaries Marine Protected Area, which is home to various marine habitats near a busy naval port lit up artificially at night.

They then quantified the artificial light exposure at the surface of the sea, beneath the surface, and at the sea floor. “Up to 76% of the three-dimensional seafloor area was exposed to biologically important light pollution,” they report in a study. “Exposure to green wavelengths was highest, while exposure to red wavelengths was nominal.”

This means that light pollution from coastal cities is likely impacting ecosystems even on the seafloor.

“The areas exposed here are not trivial. Our results focused on a busy marine area and demonstrate the light from coastal urban centres is widespread across the sea surface, sub surface and seafloor of adjacent marine habitats,” says Thomas Davies, a lecturer in Marine Conservation at the University of Plymouth who was the paper’s lead author.

Yet Plymouth, which has a population of 240,000 people, is a relatively small city and the impacts of light pollution are bound to be several magnitudes higher near teaming coastal metropolises.

“Seventy-five per cent of the world’s megacities are now located in coastal regions and coastal populations are projected to more than double by 2060,” Davis says. “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.”

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