Experts in ocean ecology are investigating the health of coral reefs, put to the test by the increase in natural and anthropogenic disturbances. While there is growing concern about their overall future, certain reefs resist repeated attack, but however lose their diversity.
Not all coral reefs are going to disappear overnight or in the next few decades. “Some of them, such as the Moorea reefs in Polynesia, even demonstrate great vivacity and an unexpected ability to withstand the increasing number of disruptions that affect them,” reveals marine ecologist Mehdi Adjeroud. “This resistance however is at the expense of a substantial decline in their diversity. 1 ."
Coral reefs have always been subjected to natural stress: cyclones, seasonal warming of water or outbreaks of predators. But over the past four decades, the intensity and frequency of these events have soared, not to mention damage caused by Man: pollution, overfishing, hypersedimentation 2 etc. To assess the impact of these repeated attacks, scientists closely monitor the health of corals.
Very exposed reef
The Moorea barrier reef is well suited to this observation and has been carefully monitored by experts from the Centre de recherches insulaires et observatoire de l’environnement (CRIOBE or Centre for Insular Research and Observatory of the Environment 3 ) since 1991. This island near Tahiti, which is relatively protected from anthropogenic damage because of its distance from the mainland, is very exposed to the increasing number of natural disruptions. Over the past twenty years, it has been subjected to a series of events particularly damaging to corals: a cyclone in 1991, followed by a significant bleaching event, both linked to a strong El Niño episode, then from 2006 to 2009 an outbreak of Acanthaster planci – the coral killing starfish - and finally, in early 2010, another particularly devastating cyclone. This almost uninterrupted succession of environmental disasters could have resulted in the demise of the reef. However, the reef demonstrated its formidable restorative capacity.
And re-colonisation processes were even more surprising in 2010. The coral that was initially decimated by starfish was eventually eradicated by Cyclone Oli. The cover rate dropped to 0%. However, in the space of four years, the coral re-colonised the depleted reef. The high density of herbivorous fish around Moorea, which limited the proliferation of algae on reefs, may have been instrumental in this strong resistance.
“This has more to do with recovery than resilience, as the coral community changed, shaped by events”, Mehdi Adjeroud opines. “This rapid re-colonisation is primarily the result of resistant species able to reproduce quickly and profusely, to the detriment of more sensitive corals.” Researchers will now attempt to determine whether coral resistance is always synonymous with a decline in diversity.
1. ↑ Adjeroud M., Kayal M., Iborra-Cantonnet C., Vercelloni J., Bosserelle P., Liao V., Chancerelle Y., Claudet J. & Penin L., Recovery of coral assemblages despite acute and recurrent disturbances on a South Central Pacific reef , Scientific Reports ; 26 juin 2018 ; doi: 10.1038/s41598-018-27891-3
2. ↑ Accumulation dans les écosystèmes aquatiques de sédiments drainés par les cours d’eau, souvent en raison de la déforestation