What is the problem?

What's happening here?

Global map showing percentages of the populations of different countries that live within 100 km of the shore. Coastal cities with populations greater than 1 million and shoreline degradation levels are also shown[1].

Coral reefs are the vastest biologically diverse ecosystems on Earth[2]. They are tremendously sensitive and an intricate system composed of many different invertebrate coral species[3]. Corals are in constant degradation due to anthropogenic effects, which are effects originating in human activity[4]. One such anthropogenic problem is light pollution, specifically, Artificial Light at Night (ALAN) that negatively affects coralline organisms.

How do humans relate to this problem?

The cause of ALAN is from anthropogenic sources such as overly lit and powerful street lamps, billboards, hotels and office buildings[2]. This light pollution causes the natural biological rhythms of corals that depend on lunar illumination cues to become disrupted and desynchronized, altering their behaviours, reproduction and overall physiology[2].

World maps showing the rate of change of the area of the world that is lit (left) and the measured radiance of each country from 2012 to 2016. Warmer colours on the map correspond to greater rates of change[5].

Where in the world does this occur?

Light pollution of this sort originates from tourism, urban growth and the development of beachside resorts[6]. One such country primarily known for its incredible natural coastal beauty is Mexico, which has over 11,000 km of coastline shared amongst 17 coastal states and 46% of the Mexican population[6]. In addition to tourism, the simple act of human greed is a driving cause behind this unending development, as over 40% of the nation’s gross domestic product (GDP) is derived from the industry, commerce and services activities of all the coastal states combined[6]. In regions such as northern Africa and Bangladesh, up to half of their population live in coastal areas and in the Nile Delta, the population can be as dense as 500 to 1000 people per square kilometre[1]. Marine ecosystems belonging to coastal regions of the world with greater population density are at a higher level of risk due to the increased infrastructure level required to sustain such a population[1]. With more infrastructure present, more artificial lights are required to illuminate it, which further pollutes the marine ecosystems and corals within; in countries where corals are not native, the nesting and sleep patterns of various birds are also disrupted by light pollution[7].

How pervasive is the problem?

ALAN is a very pervasive and growing problem, as over 22% of coastlines worldwide and 35% of marine-protected areas are affected by it[8]. Incredibly diverse marine habitats such as those which corals call home are especially vulnerable to the potential impacts of the disruption of natural day-night cycles[2]. At a rate of spread of 2% per year in terms of areas lit and radiance of the light itself, something must be done about ALAN[5].


Anthropogenic activities produce significant light pollution, especially ALAN, which negatively impact coral by interfering

The earth orbits around the sun once every 365 days[9]. The four seasons are caused by the Earth’s tilted axis rotation and its position to the sun[9].

and changing the early life stages of corals[2][3]. Corals are the foundation of coral reef ecosystems and are sensitive to the slightest change in the type of light or intensity of light[3][10]. They have developed an internal biological clock which is driven by the natural cycles

The Earth rotates on its tilted axis, completing a full rotation in 24 hours[9]. A single rotation causes the natural daily light-dark cycle[9].

of the Earth's orbit around the sun, the Earth's rotation, the moon's orbit around the Earth and the lunar phases[2][9][11]. These natural cycles include the light-dark cycle, which regulates the daily rhythmic changes in various biological processes and functions, including their physiological, behavioural and reproductive systems[2][3][12][13]. However, these natural cycles can be disrupted by light pollution.

The moon reflects the sunlight, making the moon the second brightest object in the sky after the sun[9]. The moon orbits around the Earth in 27 days while rotating on its axis, which is observed as the lunar phases[9].

ALAN can block or amplify the natural light composition of the moon or stars that can cause an irregular light-dark cycle to occur[14]. Changes to the natural cycles can disrupt the biological timing of coral functions[2][3][12]. Coral depends on the lunar cycle cues for synchronization and the coordination of biological systems such as spawningduring reproduction, where coral release their eggs and sperm into the water to reproduce[3][14]. Light pollution affects the early life stages and survivorship of coral[3]. Disruption of biological timing causes gene expression changes associated with the cell and the cell cycle, including proliferation, growth, and survival[11][14].

Corals are more sensitive to white light (580-620 nm) and blue light (420-480 nm) than other light wavelengths, with blue light having the most damaging effects on coral symbionts[12]. White and blue light causes intense oxidative stress, and stress contributes to coral bleaching[12][14][15]. Coral bleaching occurs when coral expels their symbionts and increases the risk of coral death[16].

Light pollution degrades coral reefs and damages photosynthetic system[3][12]. ALAN is harmful and destructive to various coral symbionts, such as dinoflagellates, which are needed to enhance coral calcification rates[3][14][15][17]. Light pollution inhibits the repair ability of coral as symbiont photosynthetic processes require complete darkness to function[15]. Moreover, causing changes or loss in symbiont densities may result in coral death[14].

The extent of the problem

What are the measurable ecosystem changes that have occurred?

Anthropogenic light pollution has become an increasingly important stressor to be aware of over the years. In general, studies have shown that the amount of artificial light on land is continuing to increase at a rate of 6% per year globally [18]. Moreover, nearly no organisms can be found to be immune to this form of pollution [11]. This means that light pollution is a problem that is shared globally, and comes to affect all lifeforms on Earth.

Climate change and other environmental stressors, such as human interference, can also add to existing effects from light pollution and accelerate photoinhibition in coral [19]. Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Coral are also subjected to severe degradation each year, which can contribute to disrupting the food web and the biodiversity that coral reefs support. This can lead to ecosystem dysfunction, and ultimately come to impact human health as well[20].

Worldwide, there are nearly a billion people living along coasts, bringing light pollution to marine ecosystems. In fact, studies showed that about 22.2% of the world’s coastlines were exposed to light at night[20]. As the global human population grows rapidly each year, light pollution will increase as well[21].

This table shows the extent of light pollution globally, and the total percent of coastline affected.

This will likely lead to the loss of coral, and the suffering of many marine species as they lose their habitats and food sources. The many environmental changes and anthropogenic threats that coral face will continue to lead to increased rates of species extinctions, ecosystem alterations and degraded ecosystem function [21].  

What is the present status compared to the past?

Coral are essential to supporting a wide variety of organisms, many of which act as important resources for humans [22].Yet over the past few decades, as many as 75% of the world's coral reefs are threatened and as many as 95% may be in danger of being lost by mid‐century[23]. With increased light pollution, coral reef conditions have decreased, and will continue to decrease unless action is taken, like implementing light pollution policies that include human well-being, inter-related socioeconomic consequences, and most importantly, the structure and functioning of ecosystems [18]. Regardless of the wide extent and strong influence of light on aquatic organisms being well known, little research has addressed the consequences of light pollution[11].

One concern is that the rate of environmental change will exceed the evolutionary ability of coral species to adapt [11]. Developing our research with time, will help scientists better understand the disease, and hopefully help us learn how to better prevent some of the impacts light pollution has on coral.

What is the prognosis for the future if we continue on our current trajectory?

Artificial light pollution is a global environmental issue, one that should be considered a serious threat as it damages many ecosystems that can be considered as one of the world’s most biologically diverse and functionally important [8]. Coral reefs are a great source for food, new medicines, and provides many communities with income through jobs, such as commercial fishing[24]. These are just some of the many commodities that we are at risk of losing as if we continue down our current path of increased light pollution.

Coastal development has also been closely linked to human settlement, industry, aquaculture, or infrastructure can cause severe impacts on near-shore ecosystems, particularly coral reefs[24]. Additionally, the illumination from structures from coastal development, on or near the coast, support economic and social activities that can positively or negatively affect the coral reefs near the structures[15]. As coral already undergo severe degradation from various other climate and human activities, light pollution is proven to severely impact not only the health of coral, but also the future of entire coral reef ecosystems located near coastal urban and lit areas as well[15]. This is especially true, if light pollution continues to increase in the future and corals are exposed to even greater amounts of artificial light.

Given the impact, what are the solutions?

Many of the issues regarding light pollution and its effects on marine organisms such as coral reefs, have been exacerbated recently due to immigration towards new technologies like LED lights. LED lights negatively impact marine environments as they penetrate deeper than traditional lighting and across different wavelengths. [25]

Previous areas of focus:

Many studies such as the one referenced in the figure below, [26] have looked at determining which wavelengths of light penetrate the deepest and thus cause the most harm.

From this figure, we can see how the depth of penetration varies across the light spectrum. One specific trend observed from this figure, is the ability of blue light to penetrate “beyond the first few meters of the water column". [26]

Local solutions:

Local solutions must focus on providing incentives to preserve and protect areas of the ocean that rely on natural light. Programs such as dark sky initiatives have been initiated  in places like Utah and Idaho by the International Dark Sky Places Program. This program specifically focuses on educating small communities and public parks on the impacts of light pollution.[27] Further implementation of these types of programs in local communities, would be a major step forward in minimizing the effects of light pollution on coral reefs.

Global Solutions:

Globally there are many ways to remedy the issue of light pollution, including using fluorescent type light sources with longer wavelengths rather than LED lights which emit shorter wavelengths of light capable of greater penetration.[28]  In addition, since wavelengths of approximately 443 nm, correlating to blue light evidently pose the greatest ability to harm coral at significant depths, a secondary global solution would be to use LED lamps with lower Kelvin ratings. Lamps with lower Kelvin ratings, have lower intensity and spectrum and therefore reduce the “blue peak”.[28]Simple implementation of the local and global strategies discussed above would be a major step forward in minimizing the effects of light pollution on coral reefs. In order to gauge the effectiveness of these strategies however, further programs must also be implemented in accordance with the ones previously discussed. Strategies such as biological monitoring implemented by the UN convention on biological diversity, identify issues as they arise while simultaneously suggesting specific follow-up investigations in order to create a treatment plan.[29] A program such as this is an absolute necessity, as it is crucial to make sure that the implemented strategies are indeed effective while simultaneously monitoring if these strategies are impacting the environment in other unintentional ways.[29]


In conclusion, implementation of more environmentally friendly lighting technologies, careful biological monitoring and incentives to promote change at a local level are viable solutions to effectively combat and minimize the effects of light pollution on coral reefs.


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