Artificial Light Is Another Way Human Activity Threatens Coral Reefs

A new study by researchers at Plymouth University has found that coastal cities can cause enough light pollution to trick coral reefs so that they spawn outside of times that are best for reproduction.

The release of eggs is triggered by lunar cycles on particular nights of the year. These are called coral broadcast spawning events, and they are essential to the recovery and maintenance of coral reefs that have been affected by mass bleaching and other comparable events, a press release from Plymouth University said.

“Corals synchronise their spawning events to maximise the probability of reproductive contact. Our results suggest that lit and unlit reefs are spawning on different nights, reducing the probability of reproductive contact, which reduces reproductive success and genetic exchange between reef systems,” Dr. Thomas Davies, a professor of marine conservation at the University of Plymouth, who is also the study’s lead author and the main investigator of the Artificial Light Impacts on Coastal Ecosystems (ALICE) project, told EcoWatch in an email.

Through a combination of spawning observations and light pollution data, the researchers demonstrated for the first time that coral reefs exposed to artificial light at night (ALAN) spawn one to three days nearer to the full moon than reefs that are not.

The study, “Global disruption of coral broadcast spawning associated with artificial light at night,” was published in the journal Nature Communications.

Coral eggs are less likely to be fertilized and survive to produce new corals if they spawn on different nights, and new adult corals are needed to help reefs that have been affected by disturbances such as bleaching events to recover.

The research is the most recent example of the ALICE project, funded by the National Environment Research Council.

The study of ALAN’s effects on spawning corals expanded on earlier research which mapped the parts of the ocean that had suffered the greatest effects of light pollution.

The earlier study discovered that at a depth of 0.0006 of a mile, biologically important ALAN affects 0.73 million square miles of coastal ocean, which is about 3.1 percent of the Exclusive Economic Zones worldwide.

“Impacts are documented across all levels of biological organisation, all aspects of the life cycle and across multiple phyla in the sea, on land and in lakes and rivers,” Davies told EcoWatch.

In the recent study, the scientists used a global dataset of more than 2,100 observations of corals spawning and paired them with the data from the earlier study.

Using the combined information, they were able to show that ALAN may be moving spawning triggers forward due to the minimum illuminance it creates from sunset to moonrise after the full moon.

“Corals are critical for the health of the global ocean, but are being increasingly damaged by human activity. This study shows it is not just changes in the ocean that are impacting them, but the continued development of coastal cities as we try and accommodate the growing global population. If we want to mitigate against the harm this is causing, we could perhaps look to delay the switching on of night-time lighting in coastal regions to ensure the natural dark period between sunset and moonrise that triggers spawning remains intact. That would potentially raise a number of economic and safety issues, but is something we potentially need to consider to ensure our coral reefs are given the best chance of survival,” Davies said in the press release.

Coral reefs are critical for the health of the world’s ocean in part because they are home to 25 percent of all known marine species, Davies told EcoWatch.

Dr. Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at Plymouth Marine Laboratory and the study’s senior author, said the detrimental effects of ALAN have only recently begun to be understood.

“This study further emphasises the importance of artificial light pollution as a stressor of coastal and marine ecosystems, with the impacts on various aspects of biodiversity only now being discovered and quantified. A critical first step along that path was enabled with our global in-water light pollution atlas which highlighted for the first time the true extent of the problem, which hitherto had gone unrecognised,” Smyth said in the press release.

Coastal regions worldwide were examined during the study, and it was found that reefs in the Persian Gulf and the Red Sea are especially affected by light pollution.

Davies told EcoWatch that Ninh Diêm, Vietnam, is another particularly affected area.

These places have coastlines that have seen heavy development in recent years, as well as coral reefs that are close to the shore and especially at risk.

“The Red Sea and the Gulf of Eilat/Aqaba are heavily impacted by Artificial Light at Night (ALAN) due to urbanization and the proximity of the reefs to the coastline. Despite the challenges posed by ALAN, corals in the Gulf of Eilat/Aqaba are known for their thermal tolerance and ability to withstand high temperatures. However, a disturbance in the timing of coral spawning with the moon phases can result in a decline in new coral recruits and a reduction in the coral population,” said co-author of the study professor Oren Levy, who heads the Laboratory for Molecular Marine Ecology at Bar-Ilan University in Israel, in the press release. “It is crucial that we take immediate action to reduce the impact of ALAN on these fragile marine ecosystems. By implementing measures to limit light pollution, we can protect these vital habitats and safeguard the future of the world’s oceans. It’s our responsibility to ensure that we preserve the biodiversity of our planet and maintain a healthy and sustainable environment for generations to come.”

So what can people do to reduce light pollution in order to protect coral reefs and other organisms from its adverse effects?

“The simple answer is stop using artificial light at night, however the reality is more complex. Subtle interventions such as avoiding wavelengths that cause ecological harm, or avoiding times during important biological events might go a long way,” Davies told EcoWatch.

Davies said the long-term prognosis for coral reefs globally is, “Not great. Most forecasts see existing reef systems disappearing by the end of the century if carbon emissions are not drastically cut. However, transplantation, relocation and genetic modification might help to preserve coral reefs as an ecosystem, even if many existing reefs are lost.”