Photo: Pixabay/xiSerge 

Coral reefs around the tropics are under increasing stress from warming temperatures, water acidification, pollution and overfishing. Yet one ray of hope for embattled reefs can come in the form of sunshine, according to scientists in the United States.

By ensuring that the optical quality of water in which reefs marinate remains good, we can go a long way towards protecting them, the expert report in a study.

“[Our] results suggest a fundamental role of solar energy availability and photosynthetic production in explaining global-scale patterns of coral biodiversity and community structure along depth gradients,” the experts write. “Accordingly, the maintenance of water optical quality in coral reefs is fundamental to protect coral biodiversity and prevent reef degradation.”

It has long been well understood that reefs derive their energy from sunlight, but the extent of the impacts of the sun on the health of reefs has not been fully explored before, the authors say.

“Coral reefs are one of the most biodiverse ecosystems on Earth. To better understand that diversity, we looked at the role sunlight plays in the symbiotic relationship between coral and the algae that provide the oxygen for its survival,” explains Tomás López-Londoño, a postdoctoral scholar at Penn State University who was a lead author of the paper.

“We found that underwater light intensity plays a critical role in the energy expended by the coral’s symbiotic algae to maintain its photosynthetic activity,” López-Londoño adds.

“What’s new here is we developed a model that provides a mechanistic explanation for the biodiversity patterns in coral,” he goes on. “Central to that explanation is water clarity, meaning that preserving the underwater light climate should be a priority for coral reef conservation. It’s as vital as pollution mitigation, limiting ocean acidification, and reducing thermal stress.”

The researchers studied the growth of corals in an aquarium wherein they simulated depth and various gradations of sunlight. They also developed a mathematical model that can track the association between depth-dependent variations in photosynthetic energy to corals and gradients of species diversity in colonies.

They set about examining data on reefs in the wild by comparing varying degrees of water clarity and biodiversity patterns in hotspots of marine biodiversity worldwide to chart productivity and biodiversity. Based on their results, “much of the variation in species richness with depth is driven by changes in exposure to sunlight,” they report.

When they applied their model against global data sets on corals, the researchers found that variation in sunlight-driven energy produced by algae living on corals supply heavily influence “the spatial variation of species diversity within coral communities.” This means that the most highly productive underwater environments are those that have access to plenty of sunlight.

“The model is very elegant in that it takes into consideration only two things,” notes Roberto Iglesias-Prieto, a professor of biology who was a co-author on the study. “It looks at productivity, the potential that an alga has to extract energy from the sun, and the cost of living, the cost of the repair of the photosynthetic machinery. It’s a very simple notion and we found it explains the existing empirical data.”

The study points the way forward for reef conservation everywhere by highlighting the need to preserve the clarity of the seawater around coral reefs. This can be done by eliminating sedimentation and water pollutants from human causes such as coastal development and mass tourism.

“We tend to react reflexively against large-scale threats like ocean acidification and thermal stress from climate change. We say, ‘This is a serious issue, but what can I really do locally?’ In the case of mitigating optical pollution, the answer is ‘everything,’” Iglesias-Prieto stresses.

“Unlike so much of the environmental threats that corals face, this is something that can and should be managed locally,” Iglesias-Prieto adds.