Rebuilding the Reef
Coral reefs cover well under one per cent of the ocean floor, yet they support roughly a quarter of all known marine species and protect thousands of kilometres of coastline from storm waves. Over recent decades, however, many reefs have been badly damaged. Rising sea temperatures cause corals to expel the tiny algae that live inside their tissues and supply most of their food, a process known as bleaching. If the heat persists for several weeks, the corals starve. Pollution washed off farmland, destructive fishing methods and outbreaks of coral-eating starfish add to the pressure. In response, a growing number of scientists, conservation groups and coastal communities have turned to an idea that would once have seemed absurd: rebuilding reefs by hand.
The most widespread technique is known as coral gardening. Divers collect small branches from healthy colonies, often pieces that have already been broken off by storms, and attach them to underwater nurseries. These may be simple frames shaped like trees, made from plastic piping, from which fragments hang on fishing line, or floating ropes anchored to the seabed. Suspended in the water, the fragments are kept away from sediment and from predators that live on the bottom, and many grow several times faster than they would on the reef itself. After six to twelve months, they are large enough to be transplanted back onto damaged areas, where they are fixed in place with marine cement or cable ties.
Coral gardening works well for fast-growing branching species, but massive corals, which build the solid boulders that form a reef's framework, may take decades to reach a useful size. A partial solution came from an accidental observation by the marine biologist Dr Tomas Ferreira, whose team noticed that when a colony was cut into very small pieces, each just a centimetre or two across, the pieces grew dramatically faster than larger ones. Ferreira believes that the cut edges trigger a kind of healing response, and that when several fragments of the same colony are placed close together, they soon fuse into a single larger colony. This method, known as micro-fragmentation, can produce in two or three years a colony that would normally take decades to form.
Both techniques, however, rely on cloning: every new coral is genetically identical to its parent. This worries Dr Ana Liu, who studies coral reproduction. A reef planted with clones, she points out, may be highly vulnerable to a single disease or heatwave. Her team instead collects the eggs and sperm that corals release into the water during mass spawning events, which take place on only a few nights each year. The resulting larvae are raised in tanks and then released over damaged reefs or allowed to settle on small ceramic tiles before being planted out. Liu's experiments show that larvae settle far more readily on surfaces that already carry a thin pink layer of a particular type of algae, apparently because it signals a suitable home.
Other projects focus on the structure of the reef itself. Where explosives or ship groundings have reduced a reef to loose rubble, young corals cannot survive, because the fragments roll around in the current and crush them. One answer has been to cover rubble fields with networks of steel frames coated in sand, which hold the loose material in place and give corals a stable surface on which to grow. Some researchers have also experimented with playing recordings of healthy reef sounds through underwater speakers, finding that the noise attracts young fish, which in turn graze on algae that would otherwise smother new coral.
Despite these successes, restoration has its critics. Dr Helena Marsh, a reef ecologist, notes that the total area restored worldwide is tiny compared with the area being lost each year, and that a single severe heatwave can kill transplanted corals as easily as wild ones. More seriously, she argues, high-profile restoration projects may give politicians and the public the false impression that the problem is being solved, reducing the pressure to tackle rising ocean temperatures, which remain the main threat. Even supporters admit that costs can be very high, particularly where divers must work at depth.
Advocates respond that restoration is not intended to replace action on climate change but to buy time, keeping reefs alive and genetically diverse until conditions improve. Some also point to social benefits. Dr Rafael Okoro, who has worked with fishing villages in several countries, argues that projects succeed only when local people are involved from the very beginning, and that paying them to maintain nurseries gives communities a direct financial reason to protect reefs. In some places, former fishermen now earn a living guiding tourists around the coral gardens they helped to build. The future of reefs will be decided largely by what happens to global temperatures, but in the meantime, the work of rebuilding them is teaching scientists a great deal about how these remarkable ecosystems function.
