Coastal lights trick coral reefs into spawning earlier than they should

Newswise — A new study has discovered that the light pollution produced by cities along the coasts can deceive coral reefs into spawning outside of their natural and optimal reproduction periods.

Broadcast spawning events in corals are important for the survival and recovery of reefs after events such as mass bleaching. These events are triggered by lunar cycles and lead to the release of eggs on specific nights each year.

Researchers found that coral exposed to artificial light at night (ALAN) tend to spawn one to three days earlier than those on unlit reefs. This is significant because coral spawn according to lunar cycles, which are crucial to their survival after mass bleaching events.

If coral spawn on different nights due to light pollution, there is a higher chance that their eggs will not get fertilized, resulting in fewer new adult corals. This can make it harder for coral reefs to recover after they experience bleaching events or other disturbances.

The Natural Environment Research Council has funded the Artificial Light Impacts on Coastal Ecosystems (ALICE) project, of which this research is a part. The study has been published in the journal Nature Communications.

In December 2021, researchers published a study which showed the areas of the ocean most impacted by light pollution. This new research, published in Nature Communications, builds on that study. Both studies are part of the Artificial Light Impacts on Coastal Ecosystems (ALICE) project, which is funded by the Natural Environment Research Council.

That study found that at a depth of one metre, 1.9 million sq km of coastal ocean are exposed to biologically important ALAN (around 3.1% of the global Exclusive Economic Zones).

For the new study, researchers paired that data with a global dataset of 2,135 coral spawning observations from the 21st century.

The researchers found that ALAN could be causing the corals to spawn earlier by creating a period of low light between sunset and moonrise after the full moon.

Dr Thomas Davies, the lead author of the study and a marine conservation lecturer at the University of Plymouth, said that coral reefs are critical to the health of the ocean, but they are being damaged by human activity, including coastal city development. The study shows that it is not just changes in the ocean that are affecting them, but also light pollution from cities. To reduce the impact of light pollution on coral spawning, he suggests delaying the use of night-time lighting in coastal regions so that the natural dark period between sunset and moonrise is preserved. However, this approach may pose economic and safety challenges, but it is worth considering to ensure the survival of coral reefs.

Dr Tim Smyth, who is the Head of Science for Marine Biogeochemistry and Observations at Plymouth Marine Laboratory and senior author of the study, emphasized the significance of artificial light pollution as a stress factor for marine and coastal ecosystems. He stated that the impacts on various aspects of biodiversity are only now being discovered and quantified, and that the study highlights the importance of the global in-water light pollution atlas in enabling this research. This atlas revealed the true extent of the issue, which had gone unrecognized before.

The study examined coastal regions around the world, but coral reefs in the Red Sea and Persian Gulf are among the areas most impacted by light pollution.

These are places where the land near the ocean has had a lot of buildings and construction in the past few years, and where the coral reefs are in danger because they are located very close to the shore.

Co-author Professor Oren Levy, who heads the Laboratory for Molecular Marine Ecology at Bar-Ilan University in Israel, added: “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. 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.”