As Oceans Warm, Tropical Corals Seek Refuge in Cooler Waters
Due to soaring temperatures, tropical coral reefs are facing a bleak future. But recent research shows that some of these corals are migrating to cooler subtropical seas, offering a measure of hope that these ecosystems can survive the existential threat of climate change.
From the shores of Florida to the islands of Japan, from the Midway atoll to southern Australia, an unheralded ecological regeneration may be underway. Reef-building coral, which has become ever more threatened in the superheated waters of the tropics, is making a bid for survival by migrating to subtropical climes that meet its temperature requirements.
The coral reefs of the tropics have looked doomed. Bleached by marine heatwaves, suffering mass die-offs, and stuck to the sea bed, they have no obvious escape as the oceans warm. Some experts say they will be gone by mid-century, the first great ecosystem casualty of the climate emergency.
But the news is not entirely grim. It turns out that young corals can be surprisingly mobile, able to move in ocean currents, if their homes become inhospitable, and relocate to more equable waters. “I do believe there is a glimmer of hope for them,” says Nichole Price of the Bigelow Laboratory for Ocean Sciences in Maine, the lead author of the first global study of their sporadic recovery.
Marine ecologists are reporting migration of tropical coral into subtropical regions, part of a wider “tropicalization” of ocean ecosystems as species seek cooler waters away from the Equator. While struggling in their former habitat, they are proliferating between 20 and 35 degrees north and south of the Equator, with young refugee corals creating new reefs hundreds of miles from home.
With coral on the move, the map of the world’s reef-building tropical coral is being redrawn.
Price cautions that the migration does not come close to compensating for tropical decline. The percentage increase in the establishment of new corals in the subtropics, while encouraging, is from a low level. “Many more corals are being lost near the equator than are migrating to the subtropics,” she says. So overall, there has been a global decline in recruitment of 82 percent over the past four decades. But, even so, wipe-out may not loom.
In southern Japan, for example, at 33 degrees north, more than 70 coral species now occupy most of Tatsukushi Bay. In the United States, staghorn and elkhorn corals are extending their ranges northward up the Atlantic coast of Florida. And in Australia, coral appear to be migrating south from the Great Barrier Reef to the coast of New South Wales around Sydney, at 30 degrees south.
This has been going on for a while, but until now nobody had assembled the complete picture. Price’s six-nation study, published last month, finds that with coral on the move, the map of the world’s reef-building tropical coral is being redrawn.
Her study has two headline findings. First the bad news: The recruitment of new young coral on tropical reefs has declined by a staggering 85 percent since the 1970s. Even reefs that appear healthy are simply not reproducing. But the good news is that there has been a 78 percent increase in recruitment on sites studied outside the tropics. Coral recruitment has been “shifting poleward,” Price concludes.
Tropical coral reefs are among the most complex ecosystems in nature. Submerged within a few feet of the sea’s surface, they are drenched in sunlight, which drives their ecology. Most reefs host a dazzling array of creatures: worms and snails, limpets and conches, crabs and eels, sea cucumbers, and sharks, all consuming and being consumed.
Reefs grow either as platforms extending from shorelines, or upwards from submerged mountains on the seabed. The Great Barrier Reef off Australia is more than 1,400 miles long, and took an estimated 500,000 years to reach that size. The Enewetak atoll in the Marshall Islands extends down more than half a mile and is the product of some 60 million years of growth
The corals that make these reefs are soft-fleshed creatures, related to sea anemones. But they produce hard exoskeletons that accumulate in the billions and fuse to rocky seabeds, creating reefs. Their famous kaleidoscopic color is provided by microscopic algae that live inside the translucent corals in a symbiotic relationship. In return for shelter, the algae provide the corals with their main source of nourishment.
Coral reefs occupy only about 1 percent of the world’s oceans — from the Caribbean to the “coral triangle” centered on Indonesia in Southeast Asia — but they are home to about a quarter of all marine species. They have survived destructive fishing, pollution, mining for construction materials, the anchors of cruise ships, and even nuclear bomb tests. But climate change has proven to be their nemesis.
If water temperatures rise as little as 2 degrees Fahrenheit above normal for periods of more than a few weeks, the stressed coral expel their algae, and the multi-colored reefs turn a ghostly white. This “bleaching” has reached epidemic proportions and is among the most unambiguous signs of ecological decline from global climate change.
Bleaching need not be lethal. But if the waters do not cool enough to allow the symbiotic algae to return within a few weeks, the corals starve to death. And death may come more quickly if the temperatures are sufficiently high, as happened on the Great Barrier Reef in 2016, says Tracy Ainsworth of the University of New South Wales.
Tropical corals could be gone as early as mid-century. But research published in recent weeks suggests that all may not be lost. Christian Voolstra, a geneticist at the University of Konstanz, in Germany, showed that some corals can adapt to changing environments by expelling one lot of symbiotic algae and adopting a new set better attuned to new conditions. Price’s findings of mass migration by refugee coral from overheated tropical reefs suggest a whole new route to survival.
Subtropical regions were until recently seen as marginal environments at best for reef-building coral, which generally require water with temperatures above 64 degrees F. But as temperatures rise, these waters are increasingly attractive.
The critical stage for migration is during reproduction, before young coral attach themselves to reefs. Some larvae are able to float in the water for months. Their migration routes generally follow persistent warm ocean currents that wash along north-south coastlines. These include the Gulf Stream in the North Atlantic, a similar flow on the Asian side of the North Pacific called the Kuroshio Current, and the East Australian Current. Coral migrations have also been recorded along the Suez Canal, which connects the Indian Ocean and the Mediterranean via the Red Sea.
There may be limits to how far from the Equator tropical coral can successfully migrate.
To form reefs in their new locations, the young coral need a suitable rocky sea floor close to the surface, where light is abundant all year round. They also need symbiotic algae to accompany them.
There may be limits to how far from the equator tropical coral can successfully migrate, however warm the waters become in the future. One is the amount of light that penetrates to the reefs, particularly during winter. And there is the looming threat across all latitudes of the increasing acidification of ocean surface waters, as they absorb ever more carbon dioxide from the atmosphere. Increased sea-water acidity dissolves the skeletons of corals, which are composed of calcium carbonate.
But for now, the signs of coral migration are promising.
Price and her coauthors reviewed 92 local studies of coral recruitment in the tropics and subtropics. Most of the studies involved placing ceramic tiles on the sea bed to see if new coral colonies would form on them. In all, they uncovered more than 1,200 records of coral recruitment, mostly in the subtropics and often within a half-a-year of the tiles being installed.
Japan is one hotspot of the renaissance. Tatsukushi Bay, on the southern tip of the southern island of Shikoku, is bathed in the warm waters of the Kuroshio Current. At almost 33 degrees north, it is at the same latitude as Charleston, South Carolina and the farthest from the Equator recorded by Price. The Tatsukushi reef is not new, according to Vianney Denis, now of the National Taiwan University. But in recent decades, with new arrivals, it has grown much denser and more biodiverse. More than 70 coral species now cover 60 percent of the bay’s rocky bed.
At nearby Tosa Bay, reef building has undergone “rapid expansion in response to increasing coastal water temperatures,” with tropical fish moving in to populate the reef, says Yohei Nakamura of Kochi University.
Hiroya Yamano of the National Institute for Environmental Studies in Tsukuba, dates the expansion of reef-building coral species in Japanese waters to around 1930. Since then, they have spread along the coastline at a rate of up to 8.5 miles a year, he says. With reef fish following in the coral’s wake, he says that “fundamental modifications of… coastal ecosystems could be in progress.”
On the east coast of Australia, coral on the Great Barrier Reef off Queensland is suffering from bleaching and mass deaths. But refugee corals seem to be migrating south into New South Wales, along with the reef fish that live among them. Researchers reported in 2012 that four new reef-building species had shown up in the state’s Solitary Islands. The extent of the influx of tropical corals into the area remains unclear. But Andrew Baird, a coral reef ecologist at James Cook University and co-author of the 2012 paper, says there’s no doubt that sub-tropical species are increasing in abundance at the edges of their range in east Australia.
In Florida, reefs of staghorn and elkhorn coral, once found only south of Miami, are expanding northward along the state’s Atlantic coast. Elkhorn coral has also established new reefs in the northern Gulf of Mexico off the coast of Texas, where they’ve never been seen before, according to William Precht, director of marine and coastal programs at Dial Cordy and Associates and Richard Aronson of the Florida Institute of Technology. This is despite coral decline in the face of disease in the Caribbean and elsewhere in the Atlantic.
The great coral migration is part of a much bigger story of marine adaptation to climate change.
History suggests that the potential for reef expansion outside the tropics in the 21st century may be substantial. At the height of the Holocene 6,000 years ago, when sea temperatures were about 3.5 degrees F warmer than today, coral reefs extended much farther north and south than in recent times, says Denis. The coastlines of Florida and southern Japan, which both feature prominently in the current migration, were among those that benefited.
The great coral migration is part of a much bigger story of marine adaptation to climate change, says Adriana Verges of the University of New South Wales, Australia. The warm ocean currents flowing out of the tropics are creating hotspots that enable the spread not just of coral but also of the tropical fish species that graze on them. She calls it a “global tropicalization” of ocean ecosystems.
The herbivorous fellow travelers include rabbitfish, which have moved in large numbers into the eastern Mediterranean via the Suez Canal, as well as unicorn fish, parrotfish, and surgeonfish.
The newcomers can be voracious. The parrotfish that are invading Florida’s shores consume local seagrasses five times faster than native grazers. And here the concerns grow. On tropical reefs, such hungry herbivores perform a valuable role, suppressing seaweed growth and so helping sustain the dominance of the native coral. They may also aid coral recovery after catastrophic events such as bleaching. But the grazers’ arrival outside the tropics can be destructive of existing ecosystems.
Notable among these vulnerable ecosystems are kelp forests – hugely productive coastal habitats made up of seaweed that can grow as large as trees. Verges says that in southern Japan, invading parrotfish and rabbitfish have eaten kelp forests across thousands of acres, clearing the sea bed for reef-building corals. Japanese fishers have coined a name for the decline in kelp and seaweed beds: isoyake.
Should we regard such changes as the destructive invasion of alien species, or a fruitful migration of a threatened ecosystem in the face of climate change? There is no easy answer. As Satoshi Mitarai of the Okinawa Institute of Science and Technology Graduate University in Japan, a coauthor of Price’s paper, puts it: “The lines are really starting to blur about what a native species is, and when ecosystems are functioning or falling apart.”
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