Coral reefs' rescue might come straight from the sky

Although coral reefs occupy only about 1 percent of the ocean floor, they serve as a habitat for one-quarter of all marine species. Regrettably, 14 percent of global coral reefs have perished over the past decade. Furthermore, the United Nations estimates that up to 70 percent of these highly vulnerable ecosystems are under threat, with 20 percent irreparably damaged.

Reefs face a multitude of threats including climate change, ocean acidification (caused by increased air concentrations of carbon dioxide that is absorbed by water), pollution, and overfishing.

A study undertaken by scientists from Lancaster University in the UK and published in "Science Advances", proposes that seabirds living on islands near coral reefs can significantly expedite coral growth.

The international team of researchers also found that coral reefs near seabird colonies could recover from bleaching events more quickly. Bleaching, triggered by the loss of symbiotic algae in corals in heated waters, often results in mass die-offs of coral.

The research focused on the Acropora family of corals - the most prolific worldwide. The study found that corals in the vicinity of bird-inhabited islands bounced back roughly 10 months faster (at around three years and eight months), in comparison to reefs further from bird colonies (four and a half years).

Bird droppings are crucial to this process. Rain washes the guano, or bird feces, from the islands into the sea, where the nutrient-rich nitrogen and phosphorus contained in the droppings nourish the corals.

"Our study clearly indicates that nutrients from seabird guano promote a quicker growth and regeneration rate in Acropora corals," said Dr. Casey Benkwitt, a coral reef researcher at Lancaster University and a co-author of the study.

The research was carried out in the Indian Ocean. The scientists compared reefs near islands with large seabird populations, such as red-footed boobies, sooty terns, and fairy terns, to reefs by islands with fewer birds. On the latter, a rat population thrives, feeding on the eggs and chicks of the birds. These rats are invasive and harmful.

In 2015-2016, coral reefs in the Indian Ocean succumbed massively to bleaching and death due to marine heatwaves. Scientists monitored select ecosystems for three years and checked the recovery of Acropora. They traced the presence of nutrients from bird droppings based on samples of stable nitrogen isotopes.

The team also experimented to validate whether the nutrients from bird droppings indeed expedite the growth of reefs. They transplanted Acropora corals from rat-infested islands to bird-inhabited ones and vice versa. The corals moved from rat-infested waters to bird-populated areas, growing 2.4 times faster.

"The possibility that a natural solution could help enhance the resilience of coral reefs to climate change is encouraging," Dr. Benkwitt noted.

The researchers also feel that their findings support earlier studies highlighting the ecological damage caused by invasive rats on tropical islands.

"Eliminating rats and revitalizing seabird populations could really help restore the natural balance of nutrients in the coastal marine ecosystem, and spur rapid rejuvenation of coral reefs," argued Prof. Nick Graham from Lancaster University and lead author of the study.

"Moreover, the benefits of such an approach extend beyond reef regeneration. The fish growth rate on reefs near islands with large seabird colonies is faster, and in general, there is 50 percent more fish biomass there than on reefs near rat-infested islands." added co-author Dr. Shaun Wilson from the Australian Institute of Marine Science.