Author: Dr Ines Lange

The coral reefs off South Sulawesi in Indonesia are stunningly beautiful, with an incredible variety of colourful corals covering the entire ocean floor – in some places… 

Large areas of formerly thriving reefs were destroyed by destructive blast fishing 30-40 years ago. The remaining rubble fields have not recovered naturally since, as dead coral fragments rolling around the seafloor prevent the survival of settling coral larvae. These degraded sites are very sad to witness, devoid of colours, fishes, and other marine life.  

Luckily, the damage is not irreparable. Around Pulau Bontosua, a small Indonesian island, the determined and skilful Mars Reef Restoration team plants healthy coral fragments onto hexagonal, sand-covered steel frames, termed “Reef Stars”. These Reef Stars are then installed over large, degraded areas where they stabilize the rubble and kickstart new reef growth.  

The Mars Reef Restoration Programme in action. In collaboration with local communities, healthy coral fragments are attached to “Reef Stars”, which are then installed over degraded reef areas (credit: The Ocean Agency and Indo-Pacific Films) 

With a team of Indonesian and UK-based researchers, I went out to South Sulawesi to study the recovery of these restored reefs. Our new study published in Current Biology shows that the restoration efforts do not only bring back coral cover, but also some important reef ecosystem functions.  

One of the most important functions of a coral reef is to create a complex, actively growing framework structure, which provides a habitat for marine animals and protects adjacent shorelines from waves and storms. We measure the overall reef growth as the reef carbonate budget – the balance between reef framework production (by calcifying corals and coralline algae) and erosion (e.g., by grazing sea urchins and fishes). This metric gives us an indication of overall reef health and function. 

Diver rolling out a transect tape over a restored reef and me surveying carbonate budgets using a flexible tape to measure every coral colony under a transect line (credit: Ines Lange, Tries Razak) 

We monitored 12 sites that were restored a few months to four years ago, as well as 3 healthy and 3 degraded control sites. Using this data with local coral growth rates, we assessed reef carbonate budget recovery on restored sites over time. 

Our survey data shows that in the years following coral transplantation, coral cover, coral colony sizes and carbonate production rates tripled, and after 4 years, restoration sites were indistinguishable from nearby healthy reefs in all investigated parameters. This means that after only four years, restored reefs are growing at the same speed as healthy reefs, provide similar habitat for marine life, and efficiently protect the adjacent island from wave energy and erosion. We did not expect to see such a fast recovery of reef functions, and it gives us hope that local management actions can meaningfully contribute to the future survival of reefs.  

Recovery of reefs from dead, degraded deserts to a healthy functional ecosystem over four years (credit: Ines Lange) 

However, not everything is perfect. The corals used to construct the restored reefs are predominantly a mix of different branching coral types – these are chosen by the restoration team because they are less damaging to source from wild coral colonies and easier to attach to the metal frames. This means that the coral communities on the resulting restored reefs are somewhat different to      healthy reefs, which in addition to branching corals harbour boulder-like and encrusting coral types. 

These differences in restored coral communities are important. They may affect habitat provision for some marine species, which prefer larger caves created by big coral boulders, rather than the small spaces found between branching types. They may also impact the reef’s resistance to future heat waves, because branching corals are notoriously sensitive to bleaching. As such, this progress in restoration is encouraging, but continued efforts are also needed to regrow the full range of coral types. 

Pictures of a restored reef (left) and a healthy reef (right) in South Sulawesi. Both have high coral cover but the healthy reef shows a higher diversity of boulder-like and encrusting coral types in addition to branching corals (credit: Ines Lange). 

Global warming is the most severe threat to coral reefs worldwide, leading to large-scale bleaching events when sea temperature rise above a certain threshold, followed by coral mortality and reef degradation. Does this mean that reef restoration efforts are pointless? 

We think that giving up on coral restoration is the wrong option. Rather, restoration strategies should be adjusted to accommodate planning for warming waters. In some cases, this will look like prioritising restoration efforts in thermal refugia areas, where transplanted corals are less likely to encounter lethal environmental conditions I the near future. This approach was taken is the studied Mars Reef Restoration Programme in South Sulawesi, where naturally high variability in temperatures helps the corals to withstand periods of heat stress and reefs haven’t experienced severe bleaching in over a decade. 

As is so often the case, there is no one-size-fits-all solution, and what will happen in any given location around the world will depend on many factors, including environmental conditions, natural coral larvae supply and restoration techniques. Nevertheless, this positive example from Indonesia can be used as inspiration for other reef restoration projects around the world and gives us the encouragement that if we can rapidly reduce emissions, we have effective tools to help regrow functioning coral reefs.  

Diver swimming over a degraded reef area that was destroyed by dynamite fishing (left), and over a reef that was restored over 4 years ago (credit: The Ocean Agency).