Night has fallen in the tropics. Beneath the ocean surface, a soft body and swaying tentacles reach up from a skeletal cup. This is a polyp, the individual of the coral reef. Its tentacles are armed with stinging tips, ready to bring any passing plankton into the body. The polyp is not alone. Its colony of identical swaying polyps may have been around for centuries. Panning further out, the colony is part of a reef that could be thousands of years old and more than a thousand kilometres long.
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| Credit: NOAA/Julie Bedford |
A generation ago, coral reefs were thought to be immutable. Colonies might occasionally perish at the force of a hurricane or predator, but the overall reefs would carry on, providing a quarter of marine life with food or shelter, providing developing countries with a quarter of their fish catch, providing shorelines with storm protection, and providing jobs in tourism.
Then in 2006, two common corals were listed as threatened. In little more than the decade since, 80 percent of Caribbean corals have died. Along Australia’s Great Barrier Reef, almost half the corals died in 2016 and 2017 alone. Today, no group of creatures is moving towards extinction at a faster rate than corals.
To understand the biggest killer of the past few years, we can go back to those polyps trying to catch passing plankton in the night. The water in the tropics is clear, precisely because plankton numbers are low. Coral food is in short supply, and the nightly foraging expeditions don’t supply enough to sustain even slow-growing corals. The shortfall is made up during the day, not by the corals themselves but by algae called zooxanthellae living within their soft bodies. Unhindered by murky plankton-filled waters, the algae produce food from the sun’s energy by photosynthesis, and share the food with their coral hosts. It is these algae that give coral reefs their vibrant and varied colours.
The coral-zooxanthellae relationship has stood the test of time, but it is failing the test of climate change. While ocean temperatures vary naturally from one year to the next, human activity has been pushing that natural range upwards. Some years are now warmer than the reefs have ever experienced, triggering the zooxanthellae to produce toxic levels of oxygen. At risk of being poisoned, corals respond by expelling the algae, resulting in the colourless ghost towns characteristic of coral bleaching. Without their supply of carbohydrates, the corals don’t survive long.
Coral bleaching is just one of the threats to the reefs. Another is ocean acidification. As carbon dioxide from the atmosphere is absorbed into the oceans, the water becomes less alkaline (more acidic), which makes it more energy intensive for polyps to lay down their calcium carbonate skeletons. In good times, most colonies will only grow a few centimetres each year. As the oceans become less alkaline, they stop growing at all.
Corals that survive these global threats might still perish from local activities, including overfishing or destructive fishing, coastal development, and pollution. Taken together, the threats have prompted UNESCO to warn that its World Heritage Reefs may not be functioning at all by 2100.
Ideas are aplenty among people determined to save the reefs. A review in the journal Nature recently argued that it is too late to restore corals to their former glory, but that efforts should now focus on protecting species that are essential for the functioning of the reef. Meanwhile the 50 Reefs initiative is already trying to identify the reefs that have the best chance of surviving to eventually repopulate other reefs. And the Reef 2050 Plan was drawn up in 2015 to protect and manage the Great Barrier Reef over the next 35 years.
Under any plan, coral reefs can only be saved if conditions in the oceans stabilise. The question is not whether coral reefs can survive extreme climate change - they can't. The question is whether we, as a species, can curb our earth-destroying habits before it is too late for both the corals and their marine life.
Explore further:
International Year of the Reef 2018
Coral Reefs: We continue to take more than we give, UN Environment, February 2018
Corals and Coral Reefs, Smithsonian Ocean Portal
To understand the biggest killer of the past few years, we can go back to those polyps trying to catch passing plankton in the night. The water in the tropics is clear, precisely because plankton numbers are low. Coral food is in short supply, and the nightly foraging expeditions don’t supply enough to sustain even slow-growing corals. The shortfall is made up during the day, not by the corals themselves but by algae called zooxanthellae living within their soft bodies. Unhindered by murky plankton-filled waters, the algae produce food from the sun’s energy by photosynthesis, and share the food with their coral hosts. It is these algae that give coral reefs their vibrant and varied colours.
 |
| Bleached branching coral, CC BY 3.0 |
Coral bleaching is just one of the threats to the reefs. Another is ocean acidification. As carbon dioxide from the atmosphere is absorbed into the oceans, the water becomes less alkaline (more acidic), which makes it more energy intensive for polyps to lay down their calcium carbonate skeletons. In good times, most colonies will only grow a few centimetres each year. As the oceans become less alkaline, they stop growing at all.
 |
| A diversity of corals, by Toby Hudson, CC BY-SA 3.0 |
Ideas are aplenty among people determined to save the reefs. A review in the journal Nature recently argued that it is too late to restore corals to their former glory, but that efforts should now focus on protecting species that are essential for the functioning of the reef. Meanwhile the 50 Reefs initiative is already trying to identify the reefs that have the best chance of surviving to eventually repopulate other reefs. And the Reef 2050 Plan was drawn up in 2015 to protect and manage the Great Barrier Reef over the next 35 years.
Under any plan, coral reefs can only be saved if conditions in the oceans stabilise. The question is not whether coral reefs can survive extreme climate change - they can't. The question is whether we, as a species, can curb our earth-destroying habits before it is too late for both the corals and their marine life.
Explore further:
International Year of the Reef 2018
Coral Reefs: We continue to take more than we give, UN Environment, February 2018
Corals and Coral Reefs, Smithsonian Ocean Portal
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