Some coral species have found a haven in hard times, one that could keep them alive through the multiple threats posed by the twenty-first century. The roots of mangrove plants in the Virgin Islands have been found to provide protection from the twin threats of global warming and ocean acidification.
Coral reefs represent one of the planet's most endangered ecosystems, with at least three-quarters considered under threat. Overfishing and run-off from land account for much of the problem, but even when local protection is provided, high temperatures and changing ocean chemistry can be enough to destroy reefs on their own.
With a quarter of marine biodiversity on the line, suggestions as radical as covering entire reefs in shade cloth or releasing alkaline substances have been proposed. However, at Hurricane Hole in the Virgin Islands Coral Reef National Monument, the corals have found their own answer.
The roots of red mangrove (Rhizophora mangle) plants reach down to the seafloor in the brackish waters of tropical estuaries. In Biogeosciences, a team from the US Geological Survey report that more than 30 coral species are growing among the roots. The authors call this, “The first natural, non-reef coral refuge from thermal stress and ocean acidification.”
Caroline Rogers, USGS. Corals grow on and under the roots of red mangroves.
The mangroves provide shade during hot spells that cause corals to bleach, a sign of stress that can often precede death. “Corals were thriving in low-light from mangrove shading and at higher temperatures than nearby reef tract corals,” the authors note. “A higher percentage of C. natans [boulder brain coral] colonies were living shaded by mangroves, and no shaded colonies were bleached.” Meanwhile unshaded colonies of the same species nearby experienced severe bleaching.
The benefits extend beyond shade however. The water around the mangrove roots are more alkaline than those in open water, allowing coral larvae to settle and form calcium carbonate skeletons.
Mangroves provide no assistance to offshore reefs, and are under widespread threat themselves. Nevertheless, their conservation—already valued for protection against flooding and carbon storage—could help maintain coral species that might one day be a seed to reestablish destroyed reef populations. However, the researchers note, “It is not known how many other mangrove areas in the world harbor such a high diversity of corals, as most people do not look for corals growing in these areas.”