The Octopus’s Gardener: Growing Coral Reefs in the Gili Islands

The Gili Islands are an ellipsis of three tiny coral cays just off the coast of Lombok, Indonesia. The islands, infamous for their full moon parties as much as their white sand, are often tacked on to the end of a trip to Bali. Gili is also a diver’s haven, surrounded by some of the most beautiful coral reefs in the Indo-Pacific, stretching in brilliant azure ribbons out to the horizon.

This acropora coral could be over a hundred years old, and the porites coral beneath it (the rounder, smoother one), grows even more slowly. Image by Liberty Lawson

For many, Gili might seem as close to paradise as imaginable, but looking a little closer, it is teetering on the edge of being lost. The white sands are strewn with straws and bottle caps. On the windy western side of Gili Trawangan, the main island, luxury resorts overlook a sun that sets over waves bobbing with plastic debris. Further offshore, like most of the world’s coral, Gili’s fringing reefs are struggling under human pressures. For decades, it was common practice for fisherman to use dynamite bombs, placed on reefs to blast coral into fragments and stun the fish, who would float to the surface of the water to be scooped up and sold to fish markets. This practice is now illegal, but overfishing, plastic pollution, agricultural runoff, increasingly severe storms and damage from boat anchors have all taken a severe toll on the reef’s ability to bounce back from over fifty years of  underwater warfare.

This baby soft coral was found growing on a plastic bag off the shore of Gili Meno. Image by Liberty Lawson.
Fish, turtles and crustaceans, particularly juveniles, rely on reefs for shelter. When corals are damaged, the whole ecosystem is affected.The tiny black and white fish living in this Acropora coral are Damselfish. They are extremely territorial, and rarely venture more than half a meter from their specific coral head, and are unable to survive without it. Image by Matthew Oldfield

In addition to their unique biodiversity, coral reefs serve an important geological function: protecting coastlines from wave damage, erosion and flooding. The Gili Islands, like most coral archipelagos, are low lying, with the majority of the population living at sea level close to the shores. Without strong offshore reefs, storms can wipe the sand clean off a beach, and villages are prone to severe flooding. Increased monitoring and marine park zoning has meant that fish populations are beginning to return to the area, and while artificial reefs and breakwalls can be made from everything from concrete blocks, old tyres and even sunken ships, the toxic chemicals that leach from these materials makes it impossible for corals to grow on them.

Using pH neutral cement to build reef-walls, which approximates the limestone substrate that corals naturally produce, doesn’t guarantee success either — even in perfect conditions, corals in the wild only grow between 0.3 – 5cm per year, and the slow-growing hard corals that build the solid foundation of the reef are quickly outcompeted by seaweed.

With the ocean conditions that climate change has precipitated, growing corals and restoring reefs successfully requires more than just patience.”

So, what actually are corals? Corals aren’t quite a plant, nor are they an animal — technically, they are both, a symbiosis of millions of tiny polyps, side by side. Inside the cells of each polyp are even tinier organisms: coloured algae called zooxanthellae. These ‘zoox’ provide the polyp with energy and food from the sun, via photosynthesis, in exchange for living in the polyp’s tissues. Using this energy, the polyps grow, secreting a chalky-white calcium carbonate skeleton. Over millennia, this substrate builds up to create islands like the Gilis.

It’s incredible that such tiny organism can create structures that are visible from space, but, they only do so when the conditions are right.”

The conditions that corals need seem fairly simple: clear water and stable temperatures, but they are very sensitive to change. When sea temperatures fluctuate, the zoox are unable to produce as much energy, and the polyp (assuming that it is being taken advantage of by a lazy tenant) expels the colourful zoox. This results in the coral ‘bleaching’, and turning a ghostly, luminous white. At this point, the coral can recolonise its cells with new algae, but, if temperatures remain too high, eventually the coral will begin starving, and its white skeleton starts to erode, as was dramatically demonstrated in the mass bleaching events in 2016 and 2017 that devastated the northern Great Barrier Reef.

It’s impossible to control the clarity of the water, and the stability of the temperature, but, there is a third variable that corals rely on: pH, or acidity levels. The job of the coral polyp is to change its internal pH, taking in bicarbonate ions from the ambient seawater to make its calcium carbonate skeleton, which then forms the limestone bedrock of the reef.

Coral conservation initiatives in Belize and Florida maintain coral nurseries, where fragments of coral are tied on to delicate mobile-like trees. In clear waters, the corals can grow and be monitored until they are big enough to be transplanted back into the wild. Image by the Coral Restoration Foundation, Florida
Image by Fragments of Hope, Belize

Back on Gili Trawangan, a few streets back from the beach is a dusty, colourful building – the headquarters of the Gili Eco Trust. Lead by the incredible Delphine Robbes, Gili Eco is spearheading the island’s war on waste. They run regular beach clean ups, organise medical clinics for the island’s stray cats, and they maintain the world’s largest collection of Biorock artificial reefs.

Biorock  is one of the most successful methods of growing corals and restoring reefs, first pioneered by Tom Goreau and Wolf Hilbertz. Based on some ingenious yet relatively basic principles of chemistry, the idea is that if we can’t change the quality or the temperature of the water, then we should focus on changing the pH.

It’s loosely analogous to the idea that if we are trying to grow vegetables in poor quality soil, we might as well use the best fertiliser.”

A biorock reef begins on shore as a huge steel scaffold. Metal bars are bent and welded together into a huge cage-like structure, anywhere from 3 to 10 meters in diameter. The structure is taken out and sunk offshore, where it is wired up to a low voltage electric source back on the land. By the magic of electrolysis, the whole steel structure becomes ionically charged, changing the pH. Essentially, this means the metal acts kind of like a magnet in the saltwater, drawing in nutrients and minerals which coat the bare metal and start building up rock layers made of the coral’s favourite substrate: limestone.

Once the structure is in place, scuba divers will pull on their neoprene and dive onto the reefs further out from the islands. With each large storm that passes through, the wave action often damages corals on the outer reefs, breaking them off. Boat anchors often do the same, and so divers take big baskets down with them to collect the fragments of coral tumbling around on the bottom, which would otherwise be lost under the sand. Even if the coral has already bleached, as long as it hasn’t been covered by algae, it can be brought back to the biorock, and tied onto the steel frame.

With less energy being wasted on regulating their internal pH, the corals are free to concentrate on producing strong skeletons. Corals on the biorock grow an average of 3 to 5 times faster, depending on the species of coral. Thanks to the increased nutrients available to help mitigate the energy loss from heat stress, the transplanted corals were found to be more resistant to bleaching events in 2009 and 2010. During the 2016 El Nino, the biorock corals bleached at a slower rate, and recovered much more quickly than the corals on the natural reefs.

Occasionally, sponges and seaweeds can over-colonise the biorocks, so divers have to continue checking up on them. The biorocks also need to be kept close to shore, so they can be connected to their electric source. Gili Eco has experimented with solar panels, wind turbines and wave powered sources, but sadly the unattended equipment is usually stolen. However, the biorocks use very little electricity, less than a lightbulb, and they accrete substrate at a rate of a few centimetres per year, until the whole structure is encased in limestone, like a natural reef. Once established, the electric current can be switched off, and the corals will continue growing as they would in the wild, returning reefs and biodiversity to previously damaged and barren coastlines.


Photographs of the Pemuteran Biorocks are by Matthew Oldfield. If you want to visit these reefs in person, check out ZuBlu, his sustainable dive travel company.  To learn more about Biorock reef restoration programs and support the work of Gili Eco Trust, check out their website here.

More than 120 biorock structures have been built around the Gili Islands, each a unique shape. This one is a manta ray, others have been built to look like giant dolphins, whales, turtles, pyramids and archways. Image by Matthew Oldfield