The dark side of light, or undesirable ALAN

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Mehr Licht!” was allegedly called out by Goethe on the last day of his life, almost exactly 191 years ago (March 22, 1832). The veracity of these words is questioned, and the saying has more of an anecdotal value. However, it expresses the human need to strive for illumination, especially burning two centuries ago, when access to artificial light was limited to candles, cagoules or kerosene lamps. Until the invention of electric lighting, human life (and life on Earth in general) was largely regulated by a natural light-dark rhythm.

The introduction of artifical light at night (ALAN) is one of the most drastic anthropogenic changes on Earth. They are compared with toxic pollution, land use change and climate change due to increases in greenhouse gas concentrations. Due to its harmful effects on ecosystems, ALAN is considered a pollutant – “light pollution.” With the expansion of the human population and the development of urbanization, the level of electrification, and therefore the intensity of artificial light, is increasing. Scientists estimate that artificial night lighting is increasing around the world at a rate of about 3 – 6% per year, and although this growth has slowed over the past few years (lights are already installed in much of the globe), the Earth is still getting brighter. According to Falchi and co-authors[1], light pollution now affects nearly a quarter of non-polar land, including almost half of the United States and 88% of Europe.

Disruption of natural light cycles

The introduction of artificial light is primarily associated with the disruption of natural light cycles, which are due to the Earth’s movement relative to the sun and moon, as well as meteorological factors. The most obvious change introduced by artificial light is the disappearance of total darkness at night, i.e. the disruption of the diurnal cycle. It is probably the strongest environmental behavioral signal on Earth. Almost all animals can be classified as nocturnal or diurnal. If a nocturnal animal is only active in extreme darkness, in lighted areas it will not be able to survive or will have to change its behavioral pattern. Changing day length or photoperiod is a key signal for seasonal behavior (e.g., mating season) in animals and plants inhabiting extra-tropical regions. The presence of light at night can deregulate physiological mechanisms, altering the behavior, thermoregulation and hormonal functioning of animals, thereby leading to disruptions in reproduction, resting and migration.

The first studies on the effects of artificial light on trees at night were conducted as early as the 1930s. This was done in the 1970s by Edwin Matzke of the University of Columbia[2]. However, the problem of light as a pollutant was not brought to the attention of astronomers until four decades later, pointing out, among other things, its negative impact on making astronomical observations[3]. Since the 1970s. more and more reports began to appear in the literature on the effects of artificial light on organisms, but it wasn’t until 2002 that they began to appear. Two biogeographers from Los Angeles, California – Catherine Rich and Travis Longcore – brought widespread attention to the ALAN problem by holding a conference on the ecological consequences of nighttime lighting. The aftermath of their work was the first book on the environmental impact of intentional lighting Ecological Consequences of Artificial Night Lighting (Rich and Langcore, 2006). Since then, we have seen the development of this avenue of research, to which advances in imaging information techniques (mainly satellite observations) have contributed significantly.

Ecological consequences of ALAN

The effect that artificial light has on organisms is highly variable, ranging from beneficial (e.g., increased ability of predators to observe prey) to extremely harmful and even lethal. An example of the latter is the disorientation of sea turtle hatchlings, which move toward cities instead of into ocean waters, intepretting city light as that reflected from the surface of the water. Humans, on the other hand, are an example of a species for which artificial light at night has both beneficial and detrimental effects. It extends the potential functioning time indefinitely, but on the other hand, it disrupts the daily rhythm, causing harmful stress. Disrupted circadian cycles have been linked to sleep and mood disorders, obesity, diabetes and certain types of cancer, among others.

The negative impact of artificial light on individual organisms and entire ecosystems is called “ecological light pollution.” When the degree of impact of artificial light on species varies, its introduction can change the species structure in an area. For example, some species of spiders avoid lighted areas, while others are eager to build webs directly on lamp posts. Because the lamps attract many flying insects, spiders that tolerate the light gain an advantage over those that avoid it, and as a result can take over domination of the artificially lit area. Changes in the frequency of representatives of different species can have a knock-on effect, as they affect interactions between species in the ecosystem and the trophic network. For example, changes in the behavior of nocturnally active insects can alter the survival rates of flowering plants after dark, which will in turn affect diurnal animals for which these plants provide food or shelter.

One of the most important ecological consequences of light pollution is changes in predator-prey interactions. Predators, using their eyesight to hunt, gain additional time during the day to search for food, increasing the pressure on their prey. The interaction dynamics of the species involved in the process are being modified, and possibly the direction of their evolution. Exposure of migratory animals to the glow of light in the sky, especially around major cities, can have a serious impact on their migration patterns.

ALAN in aquatic ecosystems

Most research on the effects of artificial light on organisms has focused on terrestrial ecosystems, while aquatic ecosystems are much less well understood in this regard. Water provides an additional “optical filter” that changes the wavelength (color), direction and polarization of the incident light. The availability of light and its spectral range decrease with increasing water depth. In the shallowest layers red light is absorbed, in the deepest layers blue light is absorbed. The light sensitivity of aquatic organisms is dependent on the depth of the habitat. Thus, for example, fish living closer to the surface are more sensitive to red light, while fish living in the depths are more sensitive to blue light. Under unmodified conditions, the light that scatters underwater at night comes from stars, the moon and bioluminescent organisms. Light pollution changes the intensity, colors and frequencies that normally affect aquatic organisms.

The best-known and perhaps best-recognized example of ALAN’s impact on aquatic organisms is the disruption that artificial light causes to zooplankton migration. This group of animals lives in deeper, darker waters during the day, thus avoiding predation from fish that use their eyes. At night, however, it migrates to shallower waters to feed on algae. The movement is considered one of the largest biomass migrations in the world. Research conducted in suburban Waban Lake near Boston, Massachusetts, in the late 1990s. In the 1970s. by Marianne Moor and co-authors[4] showed that glow reduced the height to which zooplankton (specifically, Daphnia retrocurva) rise by up to 2 m, and the density of algae-feeding individuals decreased by 10 – 20%. This leads to a decrease in the intensity of zooplankton feeding on algae in the near-surface layers. This behavioral change can drive processes in lakes, such as the emergence of algal blooms and their secondary effects – the withdrawal of submerged vegetation and a decline in water quality.

In addition to the daily vertical migration of planktonic animals, the phenomenon of drift of animals inhabiting rivers, i.e. moving downstream, is also known. Nocturnal drift, resulting from increased animal (mainly insect) activity at night, is reduced at sites exposed to light pollution, which alters the abundance and composition of the assemblage compared to unlit sites. Under the influence of artificial light, the intensity of fish feeding on drifting macroinvertebrates increases, which can result in a decrease in the number of adult insects inhabiting the coastal zone. ALAN can therefore alter species interactions in the aquatic-terrestrial zone, working across ecosystem boundaries.

Studies on a wide range of fish species, such as perch, roach, bleak, chub, bass and Atlantic salmon, show that ALAN affects fry development, delays and alters dispersal patterns, reduces adult sex hormone production and affects biological rhythms, migratory and parental behavior[5]. Light pollution also affects the migration of some fish species. For example, artificial light slows the movement of juvenile Chinook salmon and draws them closer to the shoreline, where they are more vulnerable to predation by birds and mammals.

Most of the research on the effects of ALAN on organisms involves animals, but after all, light is an essential factor for photosynthesis. Surprisingly few studies have explored the impact of artificial light on primary producers. Under natural conditions, photosynthesis occurs during the day, when sunlight provides energy for the process. So far, it is unclear whether light pollution can stimulate nocturnal photosynthesis, although there are reports that it can occur at light levels only slightly higher than moonlight (e.g., for algae adapted to long polar nights). The minimum thresholds for photosynthesis in nature are not yet well understood.

There are examples showing the beneficial effects of ALAN on primary producers. Experimental studies, conducted by a Chinese-Danish team led by Chao Xu[6, 7], indicate that the introduction of artificial LED lighting increased the regenerative capacity of some vascular plant species shaded by zeutrophied waters with low transparency. These results suggest that artificial light, especially red light, can be used in urban lakes, for example, as a way to stimulate the regeneration of macrophytes submerged in waters with unfavorable light climates. However, further large-scale field studies are needed to fully elucidate the potential of using artificial light to improve aquatic vegetation. There may also be a question as to whether the environmental costs of such a procedure, in view of ALAN’s negative impact on other environmental components, will outweigh the benefits.

Reducing the impact of ALAN through light management

Light pollution is very uneven and depends on the distribution, intensity and characteristics of its sources. The most severe effects occur right next to streetlights and illuminated buildings, but the glow can extend for hundreds of kilometers from city centers. For aquatic ecosystems, coastal areas with developed infrastructure are most vulnerable to artificial light. Waters in urban areas are often intentionally lit, for example, for aesthetic reasons.

It is clear that artificial light is necessary for humans to function, and no one expects us to withdraw from its use. Instead, it is important to understand that the problem is not so much the light, but the way we use it. According to recent estimates, artificial lighting consumes about 20% of the world’s electricity, but up to 35% of the light emitted by streetlights (which are the largest single source of light pollution) is wasted through scattering due to poor lamp design. Therefore, it is important to use appropriate shields that direct the light to the targeted areas, prevent light scattering and glare. In many regions of the world, traditional streetlights are being replaced by LEDs. The commonly used white ones emit more radiation in the blue part of the spectrum, which is the most disruptive to the biological rhythms of many organisms, including humans. For this reason, amber LEDs or low color temperature light sources are recommended when designing lighting for species habitats. In addition, they can be designed to shine in specific parts of the spectrum, dim and turn off remotely. While this does not guarantee the complete elimination of the negative impact of ALAN, for most species it will significantly reduce this impact.

Further ALAN research

Despite the growing awareness of the negative impact of ALAN on the functioning of organisms and entire ecosystems and the marked increase in the interest of scientists in various fields in this issue, the problem is still not well recognized and requires research by many scientific disciplines. A series of international conferences(International Conference on Artificial Light at Night), held annually or biennially since 2013, is dedicated to ALAN issues. Topics include a wide range of issues of artificial lighting at night, including. sources of artificial light (e.g., technology, industry, lighting design), measurement techniques and methods (e.g., remote sensing), human and environmental effects (e.g., ecology), public perception issues (e.g., sense of safety), and how regulations can help balance the benefits and harms of artificial lighting. The upcoming eighth ALAN conference will be held this August in Calgary, in the Canadian province of Alberta.


In the article, I used, among others. From the publication:

[1] Falchi F., Cinzano P., Duriscoe D., Kyba C.C.M., Elvidge C.D., Baugh K., Portnov B.A., Rybnikova N.A., Furgoni R., 2016. The new world atlas of artificial night sky brightness. Science Advances 2(6): e1600377.

[2]. Matzke E.B., 1936. The effect of street lights in delaying leaf-fall in certain trees. American Journal of Botany, 23(6), 446-452.

[3] Riegel K.W., 1973. Light pollution. Science, 179, 1285-1291.

[4] Moore M.V., Pierce S.M., Walsh H.M., Kvalvik S.K., Lim J.D., 2000. Urban light pollution alters the diel vertical migration of Daphnia. Verh. Intern. Verein. Limnol. 27, 1-4.

[5] Grubisic M., 2018. Waters under Artificial Lights: Does Light Pollution Matter for Aquatic Primary Producers? Association for the Sciences of Limnology and Oceanography.

[6] Xu C., Wang H.-J., Yu Q., Wang H.-Z., Liang X.-M., Liu M., Jeppesen E., 2019. Effects of artificial LED light on the growth of three submerged macrophyte species during the low-growth winter season: Implications for macrophyte restoration in small eutrophic lakes. Water, 11(7), 1512.

[7] Xu C., Wang H.-J., Li Y., Xu C., Yu Q., Liu M., Zhang M., Wang H.-Z., Hamilton D.P., Jeppesen E., 2022, Can artificial light promote submerged macrophyte growth in summer? Aquatic Ecology, 56, 89-98.

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