On the tiny island of Bonaire, scuba divers tend to an underwater nursery, slowly plucking off the algae that smothers young coral dangling from fibreglass trees. The island’s entire coastline is a protected marine park and a major draw for tourists, but like elsewhere in the Caribbean Sea, coral cover has declined since the 1970s due to warming sea temperatures, disease outbreaks, coastal development and pollution. A team of scientists and volunteer divers are working tirelessly to restore the dying reef in the hope that it will serve as a model for the rest of the Caribbean.
Above water, Bonaire has a semi-desert landscape dotted with giant cacti and low shrubs, which trap sediment and help prevent soil erosion. There is one problem, though: feral goats and donkeys, descendants of those once brought in by Spanish settlers, love to gobble up the scant vegetation. The loss of plant life, combined with powerful winds and hurricanes, increasingly pushes sediment and waste into the ocean.
As Bonaire is located outside of the hurricane belt, it provides a safe haven for boat owners, but this can also negatively affect the coral. “We now have more boats mooring in the town area, very close to shore. They just release their waste water into the ocean, right there, on top of the reef,” says Francesca Virdis, a marine biologist and project co-ordinator at Reef Renewal Foundation Bonaire (RRFB). Waste water introduces bacteria, viruses and disease to the shallow reef as well as nitrogen and phosphorus that can fuel the growth of algae. As algae and corals both require sunlight to survive, they compete for space on the sea bed.
Seven years ago, the RRFB started cultivating corals in underwater nurseries with the aim of outplanting them in areas that have suffered most from natural and anthropogenic threats. Today, the nonprofit takes care of eight nurseries with over 120 fibreglass “trees”. The trees can hold between 100 and 150 fragmented corals each, which means a total of 15,000 corals can be grown at any given time. “We are trying to bring the reef back to what it was, not only by restoring the corals but bringing back the fish populations that live within the corals,” Virdis says.
Each artificial “tree” (above) is attached to the sea bed with sand anchors and held up with floats that sit near the ocean surface. Fragmented corals – essentially cuttings from 200 adult corals originally collected from the reef – are hung by monofilament lines, out of reach of worms, snails, crabs and sea stars that all prey on the soft tissue of coral polyps. After six to eight months, the nursery-reared corals are ready to be moved to restoration sites. The main nursery is located at Klein Bonaire, an uninhabited island about a 25-minute boat ride west from Bonaire. But some of the restoration sites are further offshore and require a huge logistical effort: once the team of divers has harvested the coral fragments, they will transport them – 300 at a time – by boat to the new site.
When corals spawn, it means they are healthy and have reached sexual maturity. They only get one chance a year to spawn. For two or three nights after a full Moon, and for only 30 minutes at a time, an entire colony will eject capsules containing both eggs and sperm into the surrounding water. The capsules will float to the surface to mix with others along the reef. Scientists are able to speed up the reef recovery by collecting the capsules, mixing them together and releasing them back into the water.
Once common in shallow waters, staghorn and elkhorn corals are the two main species grown in the nurseries. Staghorn corals (Acropora cervicornis) are branching, reef-building corals that can live for hundreds of years. Under the right conditions, they grow 10-20 cm per year and form dense thickets several metres across – providing many nooks and crannies for reef fish to hide. Elkhorn corals (Acropora palmata, pictured above) have thick and sturdy branches resembling elk antlers. Both species can reproduce asexually through fragmentation into smaller pieces. If a branch of coral falls from the reef, it can reattach to rock and form a new colony.
Each branch of staghorn coral is fixed on to square bamboo frames for structural support and will fuse with another within a few weeks. The bamboo frame will dissolve after a few years, leaving only a coral patch. The aim is for the corals to take root on the degraded site, build a reef structure and eventually spawn together. Some 7,000 corals are “outplanted” each year, but Virdis and her team are aiming for 100,000 in the next five years.
Genetic diversity is key. The RRFB has 50 genetic strains of staghorn and 50 strains of elkhorn coral. Different strains possess different strengths: some can better withstand diseases, others are more tolerant to heat or grow faster. In the nurseries, each tree holds a single genetic strain of coral. “It’s a way for us to track the diversity,” says Virdis. To restore a degraded reef, her team will plant different coral species and strains in order to increase its resilience.
Each day, new algae is growing on the tree-like structures. If left unchecked, algae will suffocate the living corals, so a large part of maintaining the nurseries is removing the algae from the structures. The RRFB trains volunteer divers to support the weekly cleanups and monitoring at restoration sites. This involves checking for disease and damage to the coral fragments, and taking water samples, which are important in indicating reef health. Corals need clear water that lets sunlight through, and is free from sediment and nutrient runoff.
Bonaire’s reefs remain among the best in the Caribbean. But what does success look like? “The real success of the restoration project is measured once the corals are outplanted,” says Virdis. “You can be successful in the nursery but if your corals don’t survive when you plant them [on the reef], then your restoration project is not successful.” To track the recovery of a site, her team takes thousands of images and stitches them together to create a photographic mosaic of an area as big as 3,000 square metres. This allows them to monitor coral cover and mortality.
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