Editor’s note: Artificial light at night (ALAN) – whose undesirable effects are more colloquially referred to as light pollution – has long been known to affect sea turtles. Numerous studies document that adult sea turtles avoid nesting on artificially-lit beaches and artificial lights on land draw newly-hatched sea turtles away from the ocean, leading to increased mortality due to increased predation, dehydration and energy expenditure. But what about other marine organisms? In this article, we explore (Skimmer-style) recent research about how light pollution may be affecting many marine organisms and ultimately marine ecosystems.
Natural light cues structure a lot of behaviors and processes in marine ecosystems
- Most marine organisms in the surface ocean (from surface to 200 m depth) are adapted to natural light from the sun, moon, and stars. Cues from natural light – including its cycles, colors, intensities, and spatial orientation – help structure many organismal behaviors and ecological processes in marine ecosystems.
- These behaviors and processes include migrations (e.g., lunar compasses for long distance migration, daily vertical migrations cued by moonlight), foraging, predation, rest, and reproduction (e.g., timing of broadcast spawning) for marine reptiles, amphibians, fish, birds, mammals, and invertebrates.
- In addition to light being a source of information, light is also a resource for photosynthesis and primary production.
- These behaviors and processes include migrations (e.g., lunar compasses for long distance migration, daily vertical migrations cued by moonlight), foraging, predation, rest, and reproduction (e.g., timing of broadcast spawning) for marine reptiles, amphibians, fish, birds, mammals, and invertebrates.
- Artificial light at night (ALAN) interferes with natural light patterns and the biological processes they help control and can dramatically affect the behavior of marine organisms, species interactions, distributions of marine organisms, and food webs and can potentially restructure coastal marine ecosystems.
Coastal light pollution is global and getting worse
- ALAN in marine environments comes from coastal development (e.g., buildings, streetlights, billboards, ports, piers, docks) as well as vessels (e.g., fishing and merchant marine vessels) and offshore infrastructure such as oil rigs. It has increased exponentially over the past 150 years and has become a significant issue for the marine environment in the last 60-90 years.
- Artificial skyglow is one of the most common forms of ALAN. It refers to the diffuse luminance of the night sky from artificial lighting – both upward-directed light as well as light reflected from the ground or water or other surfaces. Artificial skyglow often forms a dome of light over cities and towns and is visible from great distances.
- Artificial skyglow can now be detected above 22 percent of the world’s coasts nightly – with Europe, Asia and Africa having the highest percentages of their coastlines affected. Coastal populations are expected to double by 2060, with an attendant increase in coastal ALAN intensity and spread.
- A recent global mapping effort (2020) found the coastal areas most threatened by ALAN to be in the Red Sea, the Persian Gulf, the eastern Caribbean, and the western Pacific regions. In the Gulf of Aqaba/Eilat, some areas are 60 times brighter than under a natural night sky, and even relatively unaffected areas of the gulfs are nearly 50 percent brighter.
- Artificial skyglow can now be detected above 22 percent of the world’s coasts nightly – with Europe, Asia and Africa having the highest percentages of their coastlines affected. Coastal populations are expected to double by 2060, with an attendant increase in coastal ALAN intensity and spread.
- Another recent study (2020) examined the extent to which artificial light from coastal cities reaches the nearby seafloor. Researchers shined artificial lights that mimicked city skyglow and measured exposure at the surface of the water column, in the water column, and on the seafloor of a local estuary. They found that on cloudy nights (which have more light reflected back to the ground than clear nights) green and blue wavelengths of light-emitting diode (LED) light reach 70 percent or more of the three-dimensional space around seafloor habitats whereas less than 1 percent of the seafloor is exposed to “biologically important” levels of red wavelengths. In addition since estuaries generally have higher levels of sediment and less light penetration than many other coastal habitats, researchers estimate that ALAN could reach 100 percent of the seabed in clearer coral reef systems. These findings suggest that marine habitats near all coastal cities could be exposed to ALAN.