The global expansion of coastal cities could leave more than three quarters of their neighbouring seafloor exposed to potentially harmful levels of light pollution.
A study led by the University of Plymouth (UK) showed that under both cloudy and clear skies, quantities of light used in everyday street lighting permeated all areas of the water column.
This could pose a significant threat to coastal species, with recent research showing the presence of artificial skyglow can disrupt the lunar compass species use when covering long distances.
However, the current study found that the colour of the wavelengths shone at the surface had a marked difference on how much biologically important light pollution reached the seafloor.
Many of the white LEDs now being used to illuminate the world's towns and cities use a mixture of green, blue and red wavelengths to generate their brightness.
Green and blue wavelengths left up to 76% and 70% of the three-dimensional seafloor area exposed to light pollution respectively, while the presence of red light was less than 1%.
The research - which also involved Bangor University, the University of Strathclyde and Plymouth Marine Laboratory - is published in Scientific Reports, an online journal from the publishers of Nature.
It is the first study in the world to quantify the extent to which biologically important artificial light is prevalent on the seafloor and could, in turn, be having a detrimental effect on marine species.
Dr Thomas Davies, Lecturer in Marine Conservation at the University of Plymouth and the paper's lead author, said: "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. But Plymouth is still just one coastal city with a population of 240,000 people.
"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. 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."
The study focussed on Plymouth Sound and the Tamar Estuary which together form a busy waterway and are home to the largest naval port in Western Europe.
It was conducted over four nights in 2018, when there was little or no moonlight, and blue, green, and red artificial light was shone at the sea surface during both clear and cloudy conditions, and at low and high tide.
A combination of mapping and radiative transfer modelling tools were then used to measure exposure at the surface, beneath the surface, and at the seafloor.
The researchers are now calling for a more comprehensive review of the full impacts of coastal light pollution, to try and mitigate against the most harmful effects as coastal cities grow globally.
Dr Tim Smyth, Head of Science of Marine Biogeochemistry and Ocean Observations at Plymouth Marine Laboratory, said: "Light pollution from coastal cities is likely having deleterious impacts on seafloor ecosystems which provide vital ecosystem services. We investigated this by visiting the Tamar, Plym and Plymouth Sound for four successive nights in September 2018. The time-lapse video of our trips really highlights how bright our shorelines are at night. During the fieldwork we measured the above water light field and in-water optics as well as running in-water light modelling simulations, in order for us to map the light field across the whole of the Tamar Estuary network."