Throughout the world, patches of coral reef are seen surrounded by "halos" of bare sand that could stretch from hundreds to thousands of square meters.
Beyond these halos are dense fields of seagrass or algae, which makes the presence of the bald rings even stranger. It has long been observed, but until now, scientists have not been able to make sense of it.
According to the University of Hawai'i News, scientists generally attribute reef halos to marine life who live in the patches of coral and venture out to eat the surrounding seagrass and algae.
New research detailed in two published studies led by Elizabeth Madin of the Hawaiʻi Institute of Marine Biology in the School of Ocean and Earth Science and Technology sheds more light on this mysterious phenomenon that's visible from space. Ultimately, these coral reef halos could even provide scientists a new way to monitor the health of the reef and marine life from the cosmos.
Marine Reserves Influence Coral Reef Halos
In the first study published in the journal Proceedings of the Royal Society B, Madin and her team observed no-take marine reservations where fishing is not allowed to determine whether predator presence affect reef halos.
Small fish consume the seagrass and algae surrounding the coral reefs. However, scientists believe that their fear of predators keep their eating ground to a limited area and prevent them from wandering too far away from the reef. Thus, the researchers predicted that the number of predators around could influence the size of the reef halos.
"With fewer predators, you would expect the grazing fish to be less fearful and so venture further from the reef, resulting in wider halos," explained Madin in a report from New Scientist.
However, when the team used satellite imagery to measure and compare sizes of reef halos, they found that there is no difference between the sizes of the rings around reefs in marine reservations and those that are in fishing zones.
Still, findings show that halos are more likely to occur in no-take marine reserves.
Another Way Marine Life Create Halos
Besides plant-eating fishes eating the seagrass and algae around the reefs, Madin's team revealed that other types of fishes also affect the halos in the second study published in the journal Frontiers.
It turns out, fishes that eat invertebrates scour the reef halos' edges, digging the sand, pushing the algae further, and ultimately making the expanse of bare sand bigger.
In both of the published work by Madin and her team, halos are shown to be a potential indicator of certain aspects of reef ecosystem health, since it is an indirect effect of healthy, balanced fish populations.
Furthermore, the researchers made use of freely available satellite imagery along with traditional field-based experiments and observations, showing a growing need for cost and time-efficient techniques of monitoring.
"This will therefore pave the way for the development of a novel, technology-based solution to the challenge of monitoring large areas of coral reef and enable management of healthy reef ecosystems and sustainable fisheries," said Madin.