Light pollution from coastal cities found to alter reproduction cycles of coral reefs

New Delhi: Light pollution caused by coastal cities can trick coral reefs into spawning or laying eggs outside of their optimal 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, an international team of researchers, including those from the University of Plymouth, UK, showed 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, funded by the Natural Environment Research Council, UK.

This study built on research published in December 2021, which mapped out the areas of the ocean most affected by light pollution.

The 2021 study found that at a depth of one metre, 1.9 million square kilometres of coastal ocean, forming around 3.1 per cent of the global Exclusive Economic Zones, are exposed to biologically important ALAN.

An "Exclusive Economic Zone" or "EEZ" is an area of the ocean, generally extending 370 kilometres beyond a nation's territorial sea, within which a coastal nation has jurisdiction over both living and nonliving resources.

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

The study looked at coastal regions all over the world, but coral reefs in the Red Sea and Persian Gulf are particularly affected by light pollution, it said.

They are areas where coastlines have been heavily developed in recent years and where coral reefs are both close to the shore and at particular risk, the study said.

"This study shows it is not just changes in the ocean that are impacting corals, 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," said Thomas Davies, Lecturer in Marine Conservation at the University of Plymouth, the study's lead author and also principal investigator of the ALICE project.

"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," said Tim Smyth, the study's senior author.