Article body copy
Decommissioned ships, concrete waste, military tanks, sculptures, and even cremated human remains mixed with cement have all been purposefully sunk over the years to form artificial coral reefs. Other human-made structures have, sometimes inadvertently, contributed too, such as oil rigs, jetties, and sea walls. Given time, life has colonized the cracks and crevices, often producing diverse and abundant ecosystems. However, as new research suggests, when it comes to artificial coral reefs, these stand-ins are no replacement for the real thing.
This realization came to Claudia Hill, a graduate student at the University of Groningen in the Netherlands, after she and her colleagues surveyed the coral diversity of a 200-year-old artificial reef that lies beneath the tropical coastal waters of Sint Eustatius, in the eastern Caribbean. The reef comprises stone ruins of an 18th-century jetty and 19th-century breakwater that, by 1834, had been washed into the sea by hurricanes. It is the oldest artificial reef that has ever been studied, the researchers say.
Comparing the artificial reef with a nearby natural reef, the team discovered that the artificial reef was dominated by different coral species, had fewer species, and had less species interactions. The finding corroborates previous studies that have compared artificial and natural reefs. But, given the age of the reef, Hill was surprised. “I expected the cover and abundance to be higher due to the structure being so ancient, and thus having had sufficient time for a mature community to establish,” she says.
Danwei Huang, a reef biologist at the University of Singapore who was not involved in the study, says the discovery is important. It suggests, he says, “that the succession of species recruiting and growing on the reef is a process that takes more than two centuries.”
“So any artificial reef we would set out to create today, without further interventions, will take more than that amount of time to recover the same functions and diversity as a natural reef.”
By showing that even two centuries is not enough for an artificial reef to grow to approximate the bounty of a natural reef, the study stresses the importance of other variables, such as local environmental conditions and the structure of the surface underlying the reef, for encouraging coral diversity. Hill found that the natural reef had more of the crevices and overhangs that usually help corals to colonize and grow, which could explain the difference in biodiversity.
“It raises the stakes for us to prioritize the protection of natural coral reefs all over the world because, once lost, artificial reefs may not naturally grow back to recover reefs that have been lost,” says Huang. And while intentional artificial reefs are often designed with structures that mimic natural reefs, Huang says that scaling these structures to the level of an entire reef remains challenging, and that more is required than just innovative designs.
“There is a need for interventions on artificial structures,” Huang says, “such as assisted evolution and improved restoration techniques that could accelerate the recruitment and growth of corals and other reef organisms.”
Even so, Hill highlights that although the Sint Eustatius artificial reef comprised different species than the nearby natural reef, it still hosted a diverse and healthy community that enhanced the marine life in the vicinity. Artificial reefs that are not identical to local natural reefs can be valuable for diving tourism, too, which can benefit local economies and remove pressure from natural reefs.
However, nonnative coral species have previously been shown to use human-made structures, including docks, pontoons, oil platforms, and shipwrecks, as stepping stones to expand their range, potentially upsetting surrounding ecosystems.
“Precautions should be taken in advance of deploying artificial reefs due to the potential of attracting invasive species,” says Hill. “But artificial reefs are usually better than no reefs at all.”