A study by University of Delaware (UD) reveals new facts about the deep sea reefs -- known as mesophotic reefs – near Bonaire in the Dutch Caribbean.
The waters around Bonaire include a marine park with some of the most well-preserved coral reefs in the Caribbean basin. At 30 to over 150 meters' depth, these reefs are largely unexplored as they are normally located beyond the reach of divers and are too expansive to be studied using submersibles.
Using autonomous underwater vehicles (AUVs) equipped with remote sensing, acoustic sonar systems and cameras, UD Professor Arthur Trembanis and his colleagues mapped nearly two square kilometers of seafloor around the leeward side of Bonaire. More than 20 scientists and engineers from six countries took part, including UD undergraduate students on study abroad. They hope the mapping effort and resultant data will help local conservation efforts.
First, they identified the location of the mesophotic reefs, then analyzed the data collected to get the depth, slope and surface roughness of the seafloor.
“So you might be able to see, hey, the slope is low but there is a big bump there, giving you the physical properties of the area. Then the backscatter from the sonar might tell you something about the nature of the seabed, like whether it is sandy or comprised of hard coral,” said Professor Trembanis, an associate professor in the College of Earth, Ocean, and Environment's School of Marine Science and Policy.
He added that some of the mapped reefs may have originated when mean sea levels were lower. Then, as sea levels rose, he theorized that such communities evolved to include only deeper water-tolerant species, providing a refuge in spawning or settling sites for both the corals and fish.
"If we want to try to study refugia connections between shallow and deep reefs, first we want to know where the deep reefs are located," he said. For instance, if there were shallow water species found in a deep water setting that had been disturbed, it is possible that they had been transported there by a significant wave event and had not grown in situ.
Two sites had evidence of reef-like structures at depths greater than about 168 meters with no other associated shallow water reefs nearby. This suggested a submerged reef, rather than a reef created from a collapsed fragment from the shallow water reef above.
More than half of the observed reef structures were outside the marine protected area (MPA), giving rise to questions on whether increasing the extent of the MPA would be necessary to protect the sources of shallow coral larvae and photosynthetic algae called zooxanthellae.
The researchers also discovered some slump features likely related either to tsunami events or hurricanes, prompting Professor Trembanis to say that the data could help in hazard risk management throughout the Caribbean.
"Neighboring islands of Aruba, Curacao and throughout the Dutch West Indies/Caribbean share a common approach to marine management and it's likely that the distributions we are seeing in Bonaire are present in these other islands. This opens the door to future projects to create baseline maps of where deep reefs are located and in what condition," he said.