Republished from Frontiers in Marine Science (CC BY 4.0) :

Angharad Brewer Gillham: Scientists in search of coral reef barriers which had been detected but not investigated in the 1960s, and were thought to be dead, discovered an apparently thriving ecosystem of deep-water corals.

Using sonar surveys to narrow down their search areas and then an underwater drone and trail cameras to investigate, they identified a mesophotic coral garden teeming with species of reef-loving fish and multiple different varieties of coral. The researchers suspect that other as-yet undiscovered reef ecosystems could still be out there.

A missing-presumed-dead coral reef has been found alive and well off the coast of Benin. 1960s surveys, focused on identifying promising fishing areas, flagged a possible deep-water coral reef but assumed it must be dead. Now, using a modern sonar survey and the Deep Sea Camera System provided by the National Geographic Exploration Technology Lab, scientists have found an apparently healthy reef.

“It was not an easy field campaign,” said Gérard Zinzindohoué of the Institut de Recherches Halieutiques et Océanologiques du Bénin, lead author of the article in Frontiers in Marine Science. “It was a mix of long days at sea, technical problems, and a lot of uncertainty. But then we saw structures on the sonar that looked like they could be reef signatures, which gave us hope that we were close.

“Next, we surveyed the structures with an underwater drone and the Deep Sea Camera System. That was a big moment for us, because suddenly we could actually see the seafloor. I still remember seeing those first images. It was a mix of excitement and disbelief, because after all this time thinking about this reef, suddenly there was something real in front of us.”

The unexpected survivor

The historical surveys had identified a possible reef more than 50 meters beneath the surface and declared that it was probably dead. Zinzindohoué and his colleagues reasoned that it could have been a mesophotic coral ecosystem.

“A mesophotic coral ecosystem lives deeper than the usual shallow coral reefs, in zones where light is much more limited,” explained Zinzindohoué. “Unlike typical shallow reefs, it does not always form a single continuous reef structure. Instead, it can appear as patchy or scattered coral communities spread across the seafloor.”

Using the historical surveys as a guide, the scientists surveyed 11.5 kilometers of seabed with sonar. This revealed two signals which could represent reefs. To investigate the most promising areas, the team used an underwater drone and the Deep Sea Camera System, like underwater trail cameras. The footage showed a seemingly healthy reef, with eight different types of coral and eight species of fish sheltering and feeding around the corals: golden African snappers, blackbar soldierfish, Guinean angelfish, West African goatfish, the Monrovia doctorfish, three-banded butterflyfish, and two species of damselfish.

This reef doesn’t seem to be a continuous coral barrier, but patches of coral occupying rocky areas suitable for their growth. However, it’s possible that further surveys will identify more reefs.

“To our knowledge, this is the first confirmed record of a living mesophotic coral ecosystem on the Gulf of Guinea continental shelf,” said Zinzindohoué. “However, I think what we found is probably not an exception. It is more likely a reflection of how little we still know about coastal regions in West Africa and elsewhere. If we were able to find this reef again after 60 years, how many others are still undocumented or simply forgotten? So yes, I do believe there are likely more coral systems out there that we have not documented yet. But we should not assume they are everywhere. Each discovery needs to be confirmed. Without going back into the field, we are really just guessing.”

Reading the archive

The scientists point out that they haven’t been able to sample the corals directly yet, which means that the species identifications still need to be confirmed. It’s also impossible to know, without more data, if the corals were dead in the 1960s and have since recovered. Much more research will be needed to learn about this reef and other possible reefs nearby.

“This is still an initial exploration,” said Zinzindohoué. “The historical reports suggested that this could be a continuous coral reef barrier extending roughly 40 kilometers parallel to the coast. We only surveyed a small section of this area and we had a limited number of visual transects. If I had all the grant money in the world, the first thing I would do is return to the site with a proper scientific diving and sampling program. These are ecosystems that are still poorly known in this part of the world, and every additional piece of information matters.

For me, the most exciting part is about understanding the reef’s history, and what it can tell us about how the ocean in this region has changed over time. It feels like we have only just started to read an archive that has been there for a very long time.”