How Artificial Light At Night Affects Marine Organisms
By the year 2060, the coastal human population will more than double, and as a result, marine ecosystems will be increasingly exposed to artificial light at nighttime. But how does artificial light affect marine ecosystems, and why are more research and action so important?
Because of evolutionary processes, marine organisms are adapted to detect low intensity, regular cycles, and distinct spectra of natural light. The survival, reproduction, physiology, and movement of marine fishes, corals, birds, turtles, and other invertebrates are affected by artificial light at night (ALAN). Corals, for example, align their spawning events to monthly and annual variations of lunar light intensity, and zooplankton, tropical corals, and temperate marine organisms may respond to artificial light glow and waterside street lighting to depths of 70m and 100m respectively.
Since there is very little research on the impacts of ALAN on marine species, in this study, scientists surveyed the spatial and spectral distribution of ALAN near the largest naval port in Western Europe (between Plymouth Sound and the Tamar Estuary, United Kingdom) and a city of more than 240,000 that is mostly lit by LED lights. The results demonstrate that most of the sea surface was exposed to green, blue, and red artificial light during both cloudy and clear conditions. However, cloudy conditions increased the irradiance of artificial light by 200-300% because the light is reflected back on the ground.
Furthermore, tidal retreat also amplified the seafloor exposure of artificial light by factors between 2-13. Depending on the cloud and tidal conditions, green artificial light covered 76% (low tide, cloudy), 46% (low tide, clear), 61% (high tide, cloudy), and 32% (high tide, clear) of the seafloor. Blue artificial light reached 70% (low tide, cloudy), 43% (low tide, clear), 49% (high tide, cloudy), and 23% (high tide, clear) of the seafloor, while red artificial light reached less than 0.5% under all conditions.
Unfortunately, these results show how great the extent of biologically important ALAN already is, and in regard to the rapidly growing coastal population and LED usage (approximately 80% of the lighting market share by 2022), a broader understanding of the far-reaching consequences is urgently needed. As mentioned before, marine organisms are very sensitive to light; and cloudy conditions amplify (artificial) sky glow, which extends the localized direct lighting to hundreds of kilometers (or miles). This can disrupt certain natural behaviors that are only undertaken in cloudy conditions (e.g. migration of birds and amphipods).
Nonetheless, there are some ways to improve the situation for marine ecosystems. Firstly, we need more concrete measures and mapping of territories influenced by ALAN, since satellite images mainly detect the light of the sea surface and don’t sufficiently measure what’s reaching the seafloor or sub-surface. Furthermore, the results suggest that the use of red artificial light should be increased by installations in coastal regions to decrease the exposure of green and blue artificial light in marine habitats. Only 0.4% of the seafloor was exposed to red light because it attenuates faster in water and is not as visible to marine organisms as green and blue light. However, we have to acknowledge that red light is also guiding biological processes in some marine organisms and only a broad reduction of ALAN will avoid long-term ecological effects.
Lastly, we as animal advocates have to recognize and spread awareness about less known and obvious man-made ecological impacts like ALAN. Additionally, if we live in a coastal region, we can try to directly influence and change the lighting situation by forming action groups to impact policy on a local level.
https://doi.org/10.1038/s41598-020-69461-6