Corals bounce back quicker on artificial structures

Asahi Shimbun

10-05-2025

Corals growing on artificial breakwaters (Provided by the Port and Airport Research Institute)
Artificial structures like concrete breakwaters to protect shorelines seem to offer a quicker fix than natural reefs for corals devastated by bleaching events, say Japanese researchers studying how climate change affects the sea floor.
Bleaching refers to the whitening of corals due to rising seawater temperatures. Severe or frequent bleaching events can kill the underwater ecosystems.
It turns out that corals tend to recover more quickly on artificial structures.
Typically when scientists study the phenomenon, they spend up to 10 years collecting field data.
For the latest study involving the Port and Airport Research Institute (PARI) and other entities, researchers in Okinawa Prefecture relied on 29 years of field data along three breakwaters, as well as in surrounding natural reefs in Naha Port.
The data in question covered the period from fiscal 1989 through fiscal 2018.
A large-scale bleaching event occurred in the surrounding waters in 1998.
Corals in natural reefs were found to have covered only 5.2 percent of the seabed in 2001, and 5.6 percent in 2004. But when breakwaters were examined, corals were found to have recovered more rapidly, from 8.8 percent in 2001 to 25.4 percent in 2004. Moreover, the cover rate remained high in subsequent years.
DIFFERENT CORAL COMPOSITIONS
The corals did not have the same taxonomic composition on the breakwaters that they had in the natural reefs.
Corals of the genus Acropora, which are more likely to inhabit sites exposed to pounding waves, accounted for a large part of the rapid post-bleaching recovery on the wave-dissipating blocks, whereas relatively slow-growing corals, including those of the genus Porites and the family Faviidae varieties, increased gradually in proportion in the natural reefs.
The researchers also found that coral recovery was accelerated by grooves on the surface of the artificial structures.
They discovered that corals were more abundant in shallow waters where the sunlight on moderately sloped substrate was strong.
When part of a breakwater was modified into zones that met those conditions, the area covered by coral increased by about 40 percent.
PRECIOUS LONG-TERM DATA
The study relied on data gathered by the Okinawa General Bureau, a government body that supervises Okinawa's promotion and development that discovered coral-covered breakwaters in the 1980s and subsequently began making observations in surrounding areas.
Past studies around the world have looked at using artificial structures as substrate for corals, but the research periods never lasted longer than 10 years, and none covered such a long span as the latest one, said Toko Tanaya, a PARI senior researcher who was on the team.
But why coral regeneration occurred faster on the artificial structures is a subject for future studies, Tanaya said.
However, she said it seems likely that artificial breakwaters, given their sturdiness and location in areas exposed to pounding waves, attract fast-growing coral varieties, such as species of the genus Acropora.
'Of course, the best thing would be to make sure that natural coral reefs survive,' seeing as different ecosystems can result from restoration in natural reefs and on artificial structures, Tanaya said.
'Climate change has made bleaching so frequent in recent years that corals no longer have enough time to recover before the next bleaching event occurs,' she added.
INFRASTRUCTURE NOT INTENDED FOR CORALS
When bleaching occurs, algae and other creatures may attach themselves to the dead coral with the result that the limestone skeletons collapse and pile up like rubble.
When that happens, the skeletons are too wobbly and unstable for new larvae to stick to and survive, Tanaya said.
However, breakwaters and similar coastal erosion defenses are not put in place for the sake of corals, she noted.
Still, a few modest improvements made while they are being built or repaired 'could help ensure the availability of shallow-depth zones that are suitable for the growth of corals,' Tanaya added.
The research results were published online in the journal Scientific Reports (https://doi.org/10.1038/s41598-025-93531-2).