As coral reefs continue to degrade across the globe, research efforts to better understand these complex ecosystems and their restoration efforts are becoming increasingly important. In the summer of 2021, off the southern coast of the Caribbean island of Curaçao, a team of researchers began to deploy so-called ‘coral arks’ to study their effectiveness in advancing coral restoration efforts. 

Here, PhD student Jason Baer from San Diego State University and the rest of the Rohwer lab are now building and deploying large, geodesic structures in the mid-water column to grow and study small reef communities. These floating arks are designed to function as miniature reef communities to study and restore coral reefs. An important aspect of them is that they are intended to grow coral communities away from the seafloor to give the corals and other reef organisms an environmental advantage.

“By moving corals and all the organisms that support their health off the seafloor and into the mid-water, we can provide the coral reef community with improved environmental conditions such as higher flow and light, as well as avoid some of the challenges they face on the seafloor, such as sedimentation and hypoxia,” notes Baer.

According to Baer, the team’s goal is to use the arks not only as tools to better understand the conditions corals need to thrive, but also to help restore reef sites that have been negatively impacted by hurricanes, ship groundings, beach restorations and other factors. “We hope these miniature reef communities will be used as one tool to conserve reefs by creating pockets of coral reef biodiversity, even in degraded areas, that could help reseed and repopulate the surrounding reefs,” says Baer.

“Healthy coral reefs build and maintain these beautiful and complex 3D structures and as water passes over them, it creates an equally complex flow environment. Unfortunately, as reefs around the globe degrade, that 3D structure is eroding away. A degraded coral reef loses its structure, its rich flow regimes and eventually most of its diversity. To restore coral reefs, we have to understand how all of the players on a reef, from the smallest microbes to the largest apex predators, interact with the surrounding water in ways that promote reef health,” he adds.

Once an ark is deployed, a complex mini-reef community is built which can then be moved up and down in the water column to study how an entire reef community responds to different conditions, such as differing water flow patterns. After considering numerous locations around the globe to deploy the first arks, the team chose degraded reef sites in Curaçao due to the special water flow patterns in this location. 

According to Baer, most of the Arks were anchored at around 23 m, with the structures themselves floating at around 15 m depth. Special consideration was further taken at the sites in Curaçao to avoid the Arks being caught in trawling lines from fishermen in the area, resulting in slightly deeper deployments. Baer adds: “Much of the surrounding reef at this site has been smothered by sand and sediments from beach restoration and land-based construction projects. However, while coral cover on other reefs in Curaçao is much higher, this reef does support some coral assemblages, and fish still thrive here in great numbers, making it a great site for an Ark.”

“Coral reef communities on the seafloor face threats like sedimentation and lack of oxygen. Arks and the coral reef communities they support are moved up into the water column, where higher flow speeds and light can support the health of the community,” he says. “So far, our testing with arks has shown that they provide a flow environment similar to a healthy coral reef. Higher flow on the outside of arks supports corals and other organisms that build reef, while the structural complexity of arks creates reduced flow zones on the interior that support nutrient recycling, and provides a habitat for organisms that make their home inside reefs. Current and flow measurements both inside and outside arks are a crucial piece of the puzzle in understanding how reefs respond to changes in their environment and how we might be able to help them recover.”

The hope is that the arks will not only create self-sustaining coral communities but also improve the quality of the surrounding reefs by attracting more fish and other organisms. Knowledge of the existing environmental conditions proved essential to choosing the site for the Arks in Curaçao. According to Baer, the team needed to answer three crucial questions about the water movement at the sites they were considering: what the ambient current speeds were, what the residence time of reef water was, and how current speeds differed at different depths in the water column. To answer these questions and choose an optimal site, the team used the Eco current profiler from Nortek.

Quantifying the arks’ physical environment was important because water movement can have big impacts on a coral reef community. “Flow is such an important factor when it comes to the health of marine organisms, and it’s often overlooked,” says Baer. “The complex movement of water over reefs governs the flow of resources to and from the reef, determines where certain organisms choose to live, and has huge impacts on biological processes.”

The arks team used the Nortek Eco ADCP to measure current speeds at three different potential deployment sites. For each site, the team looked at current speeds in a three-layer profile. The team used the Eco bottom mount to deploy the Eco at each site, for between 48 to 72 hours at a time, at around 18 m depth. The resulting data provided them with clear insights as to the advantages of one site over the others.

According to Baer, higher current speeds can be beneficial to coral health. “We selected one of these sites (Marriott) as a promising deployment site based on its higher current speeds than the other sites,” says Baer of the decisions they based off Eco data. The team was also able to quantify the current speeds throughout the water column at the Marriott site using the Eco and found that velocities were fairly constant throughout.

One of the team’s current research questions surrounds how and why different organisms are attracted to different types of corals. For example, small fish and crabs often seek refuge in the small holes created by branching corals. Studying these relationships, Baer says, can provide key insights as to how other organisms like fish and crabs in turn support coral health. The coral communities that develop on the Arks are being studied to determine not only which structures and conditions are most beneficial to coral growth, but also to determine the contributions of grazers, such as fish, urchins and hermit crabs, as well as other organisms that make up the Arks’ reef community.

To investigate these special relationships, the team built three-dimensional structures known as Autonomous Reef Monitoring Structures (ARMS) out of limestone and placed them on the Arks. These structures were built to offer varying sizes of spaces for fish and other organisms to inhabit, mimicking the natural structures these organisms would typically call home.