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Discovery: Light pollution from coastal cities is damaging coral reefs

In a recent study, scientists have discovered that the light pollution generated by coastal cities is damaging coral reefs and leading reefs to spawn outside their ideal reproduction times. 

The research offers compelling evidence that coastal artificial light at night (ALAN) is tampering with the delicate balance of coral spawning events. These events, typically tied to lunar cycles, are crucial for the survival and recovery of coral reefs, especially in the aftermath of mass bleaching and other similar disturbances.

The ALICE project

The findings are part of the Artificial Light Impacts on Coastal Ecosystems (ALICE) project, which is under the funding umbrella of the Natural Environment Research Council. Coral spawning events are typically triggered by lunar cycles, leading to the release of eggs on specific nights of the year. 

The research team used light pollution data combined with spawning observations to demonstrate an alarming trend that is damaging coral reefs. They found that corals exposed to artificial light at night are now spawning one to three days nearer to the full moon compared to their counterparts on reefs unlit by artificial light.

This change in spawning times could have serious repercussions on the survival of coral reefs. If coral eggs are released on different nights, the chances of them being fertilized and surviving to become adult corals are drastically reduced. Adult corals play a vital role in helping the reefs recover after bleaching events and other disturbances.

The ALICE project’s research builds on a study published in December 2021, which identified the ocean areas most impacted by light pollution. This study found that about 1.9 million square kilometers of coastal ocean, at a depth of one meter, are exposed to biologically significant ALAN. This represents around 3.1% of the global Exclusive Economic Zones.

How the study was done

For this latest study, the researchers married the data from the 2021 study with a global dataset containing 2,135 coral spawning observations from the 21st century. This allowed them to propose a novel idea: ALAN could be advancing the triggers for spawning by creating an illusory period of minimum illuminance between sunset and moonrise on nights following the full moon.

Dr. Thomas Davies, a Lecturer in Marine Conservation at the University of Plymouth, led this study. He also serves as the principal investigator of the ALICE project. 

The research, published in Nature Communications, represents a significant contribution to our understanding of the profound effects of artificial light pollution on marine ecosystems. 

What the scientists learned about damaging coral reefs

According to Dr. Davies, the study’s findings could have major implications for the future survival of our planet’s coral reefs and the impact of damaging coral reefs.

“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,” said Dr. Davies.

“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.”

Dr. Tim Smyth, head of Science for Marine Biogeochemistry and Observations at Plymouth Marine Laboratory and the study’s senior author, added: “This study further emphasizes 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 unrecognized.”

The researchers looked at coastal regions all over the world, but coral reefs in the Red Sea and Persian Gulf are particularly affected by light pollution. These 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 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 study co-author Professor Oren Levy, who heads the Laboratory for Molecular Marine Ecology at Bar-Ilan University in Israel.

“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.”

More about coral reefs and importance to ecosystems 

Coral reefs, often referred to as the “rainforests of the sea,” are among the most biodiverse and productive ecosystems on Earth. They provide a multitude of vital services that contribute to the well-being of the environment and humankind. That is why humans must stop damaging coral reefs.

Biodiversity hotspots 

Firstly, coral reefs are biodiversity hotspots. They occupy less than 0.1% of the world’s ocean area, yet they provide a home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates, and other cnidarians. 

This biodiversity is not only a critical resource for scientific research and potential medical discoveries but also plays a vital role in maintaining the overall health of our oceans.

In addition to their incredible biodiversity, coral reefs also have immense economic value. They are the backbone of many local economies, especially in developing countries, due to their importance in the tourism industry and for local fisheries. It’s estimated that millions of people around the world directly rely on the resources from coral reefs for their livelihoods and food security.

Coral reefs also play a crucial role in protecting coastlines from the damaging effects of wave action and tropical storms. They serve as natural barriers, reducing the impact of waves, thereby preventing erosion, property damage, and loss of life.

Moreover, reefs play a crucial role in the carbon and nitrogen cycles. They facilitate carbon dioxide removal from the atmosphere, helping mitigate the impacts of climate change. By damaging coral reefs, they can no longer provide these benefits.

Coral reefs need our protection 

However, despite their importance, coral reefs face severe threats from various factors including climate change, ocean acidification, pollution, overfishing, destructive fishing practices, and disease. 

These threats have led to significant reef degradation globally, and there’s an urgent need for effective management and conservation measures.

Protecting and restoring coral reefs is not just about preserving these beautiful and diverse ecosystems for future generations. 

It’s also about the survival of countless marine species and the sustenance of economic activities that depend on these ecosystems. Indeed, the health of our planet’s coral reefs is a bellwether for the overall health of the world’s oceans.

Impact of climate change on coral reefs

Climate change poses a significant threat to coral reefs, impacting them in several ways. The primary threats are from increasing sea temperatures, ocean acidification, and more intense storm events.

Rising Sea Temperatures

One of the most immediate threats of climate change is the increase in sea surface temperatures. Warmer water temperatures can lead to coral bleaching, a stress response in which corals expel the symbiotic algae living within their tissues, known as zooxanthellae. 

These algae provide corals with food through photosynthesis and give them their vibrant colors. Without them, corals turn white (hence the term ‘bleaching’) and, if the high temperatures persist, the coral may die.

Ocean Acidification

The burning of fossil fuels releases carbon dioxide (CO2) into the atmosphere, a significant portion of which is then absorbed by the oceans. When CO2 dissolves in seawater, it forms carbonic acid, leading to a process called ocean acidification. 

This process reduces the amount of available carbonate ions, a key building block for corals and other marine organisms to build their skeletons and shells. Lower carbonate ion levels can slow down coral growth and even dissolve existing skeletal structures, leading to weaker reefs.

Sea Level Rise

Rising sea levels due to melting polar ice can also impact coral reefs. Although corals can grow to keep up with slow sea-level rise, rapid increases may submerge them too deeply, reducing the sunlight they receive, which is essential for the photosynthetic processes of their symbiotic algae.

More Intense Storms

Climate change is predicted to increase the intensity of tropical storms, which can cause physical damage to coral reefs. Strong storms can break apart the coral’s rigid structure, causing large-scale reef destruction.

Changes in Ocean Currents

Climate change can also alter ocean currents, which may disrupt the delivery of nutrients to reef ecosystems, further affecting their health and resilience.

The loss and degradation of coral reefs due to climate change not only impact the multitude of species that depend on them but also the human communities that rely on those species for food, income, and coastal protection. Thus, efforts to mitigate climate change and to develop strategies to help coral reefs adapt to changing conditions are vital for the survival of these critical ecosystems.


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