The light pollution caused by coastal cities can trick coral reefs into spawning outside of the optimum times when they would normally reproduce, a new study has found.

Coral broadcast spawning events – in which lunar cycles trigger the release of eggs on certain nights of the year – are critical to the maintenance and recovery of reefs following mass bleaching and other similar events.

However, using a combination of light pollution data and spawning observations, researchers were able to show for the first time that corals exposed to artificial light at night (ALAN) are spawning one to three days closer to the full moon compared to those on unlit reefs.

Spawning on different nights could reduce the likelihood of coral eggs being fertilised and surviving to produce new adult corals that help reefs to recover after bleaching events and other disturbances.

The research, published in Nature Communications, is the latest to be carried out as part of the Artificial Light Impacts on Coastal Ecosystems (ALICE) project, which is funded by the Natural Environment Research Council.

It builds on research published in December 2021 which mapped out the areas of the ocean most affected by light pollution.

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.

This enabled them to demonstrate that ALAN is possibly advancing the triggers for spawning by creating a perceived period of minimum illuminance between sunset and moonrise on nights following the full moon.