Coral reefs have suffered degradation from human activities associated with overexploitation and pollution; degradation that has accelerated over the past ~50 years. ~ Australian marine biologist John Pandolfi et al in 2011
Coral are colonial marine invertebrates, related to sea anemones. Coral colonies are populated by individual polyps that grow together via excreted, calcareous exoskeletons anchored to the seabed. Each polyp is a sac-like sessile animal a few millimeters wide and a few centimeters long. A set of tentacles surrounds a mouth at the top of the exoskeleton.
Corals catch small fish and plankton using the stinging cells on their tentacles, but most of their nutrition comes from endosymbiotic algal protists that photosynthesize, and thereby contribute energy to the polyp, along with aiding in the calcification of the polyp’s exoskeleton. In return, the algae get comfortable living quarters, and benefit from the CO2 and nitrogenous waste that polyps pump out.
Coral provide the base for reefs which may extend for many kilometers. These reefs are critical buffers preventing shoreline erosion.
Coral reefs can effectively protect shorelines because of their ability to cause waves to break offshore, thus limiting the energy impacting the coastline. ~ Australian coral reef ecologist Michael Cuttler
The Great Barrier Reef off the eastern coast of Australia is the world’s largest coral reef system. Comprising over 2,900 individual reefs, the Great Barrier Reef extends covers an area of 344,400 km2: roughly equal to all of the British Isles, and only slightly smaller than Japan.
It took millions of years for the Great Barrier Reef to grow to its 20th-century size. Then human pollution began to take its terrible toll. From 1985 to 2012, the Great Barrier Reef lost 50% of its coral cover. The losses have only accelerated since then. Following a prolonged heatwave, 50% of the remaining coral died from heat stress in 2016–2017.
Mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining. ~ Australian coral reef ecologist Andrew Baird
Coral reefs are mainly in shallow tropical and near-tropical seas offering tepid temperature waters. Reefs are sensitive to light, temperature, and water acidity (pH).
Coral reefs are the rainforests of the sea, forming some of the most diverse ecosystems on Earth. Though they occupy less than 0.1% of the world’s ocean surface, coral reefs are home to 25% of all marine diversity. That makes coral one of the most important keystone species on the planet. Many human societies rely upon the seafood bounty that need coral reefs to thrive.
Coral reefs are a major player in the biosphere: helping regulate global temperature. Making calcified shells absorbs carbon dioxide, thus acting as a modest sink for this greenhouse gas.
Oceans absorb 30% of the CO2 emitted into the atmosphere: a process which makes seawater more acidic. Rising concentrations of carbon dioxide in the water make calcification harder for coral. Ocean acidification has been slowing coral growth.
Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today. ~ American marine ecologist Rebecca Albright in 2016
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Corals that are stressed can experience a breakdown of the symbiosis with their photosynthetic partners, resulting in a bleached appearance. ~ American marine biologist Don Levitan et al
As oceans warm, corals lose their colorful algae, which can only cope within a narrow range of temperatures. Reefs are reduced to bleached skeletons, vulnerable to disease. Ocean acidification also engenders bleaching.
Before the 1980s, large-scale coral die-offs were virtually unheard-of. Since then, coral bleaching has happened so frequently that reefs are unable to recover.
It typically takes 10–15 years for the fastest-growing coral to recover from a severe bleaching event. Larger corals that provide shelter for bigger fish take even longer.
5 severe bleaching events have occurred in the recent past. The 1st was during the 1982–1983 El Niño.
A 2nd bleaching event coincided with the 1997–1998 El Niño, which caused ocean surface water temperatures to soar. A forceful La Niña followed a strong El Niño, bringing warm waters to the western Pacific. 16% of the world’s corals died as a result.
A 3rd, terrible coral-bleaching event occurred in 2005, in the western Atlantic Ocean. Over 40% of the corals in the Caribbean Sea died.
At the population level, a reduction in spawning persisted for several years following each bleaching event. Corals that recover from bleaching events can experience long-term reduction in reproduction. This may be catastrophic for the long-term maintenance of the population. ~ Don Levitan
The 4th bleaching event happened worldwide in 2010. All told, 25% of the world’s corals died from 1960 to 2010.
The frequency of mass bleaching events are going up because of global warming. We are hitting the corals, then we are hitting them again, and then again. ~ American biological oceanographer Mark Eakin
A 5th worldwide coral-bleaching event started in mid-2014, with a developing El Niño. An especially strong warm-water current took a grievous toll.
El Niño was over by the end of 2016, but global coral bleaching carried on through 2017. It was the longest bleaching event during human existence, and the most severe.
Coral bleaching is caused by global warming full stop. It’s not due to El Niño. We’ve had thousands of El Niño prior to 1983; none of them caused bleaching. Bleaching is caused by the rising baseline temperatures due to anthropogenic global warming. La Niña periods today are actually warmer than El Niño periods were 40 years ago. ~ Australian marine biologist Terry Hughes
Historically, La Niña events have meant cooler ocean waters; no longer.
The Great Barrier Reef was grievously affected. In 2016, half of the corals in the worst-hit areas died. By 2017, the toll had risen to nearly 70%.
We didn’t expect to see this level of destruction to the Great Barrier Reef for another 30 years. ~ Terry Hughes in 2017
Unsurprisingly, the reefs in the worst shape are those off of the most-crowded beaches, where suntan lotion and other wastes deteriorate coral. Nearly 1/2 billion people in 109 countries live close to coral reefs.
The prospects for coral reefs are put into proper perspective by understanding that the shallow waters where reefs have historically thrived are now much hotter than the surrounding oceanic shelves. In June 2015, El Niño led to a 2°C warming in the South China Sea: something not expected to cause significant coral damage. But, at an atoll in the northern part of the sea, the water temperature soared by 6°C above average, killing 40% of the coral there. This spike occurred because the atoll’s shallow water amplified the heating effect. Further, unusually weak winds slowed the diffusion of heat into surrounding water, thereby trapping the hot water within the atoll.
While coral are slowly migrating to beat the heat, a warming ocean does not spell much opportunity for expansion. Corals are confined to shallower depths at higher latitudes. A constraining factor is the amount of sunlight for sufficient photosynthesis, especially during the winter months.
As sea level rises, coral face a harder time growing toward the ocean surface to get the sunlight they need to survive.
A healthy coral reef ecosystem exists in a constant tug-of-war. As corals build their skeletons up toward the sea surface, other organisms – mollusks, worms, and sponges – bore into and erode the skeletons to create shelters. ~ American marine biologist Thomas DeCarlo
Bioerosion requires coral to constantly build. On the relatively few healthy reefs today, calcium carbonate production for exoskeleton construction barely exceeds erosion loss. This delicate balance spells slow-growing coral reefs. The combined stresses of sea-level rise, ocean acidification, and loss of nutrients will tip the balance against otherwise industrious polyps. Coral growth rates in the Great Barrier Reef plummeted 40% 1975–2012. Growth has declined since.
There’s fewer coral adults because of the high rates of mortality after the back-to-back bleaching in 2016 and 2017, and dead coral doesn’t make babies. ~ Terry Hughes in 2019
The tougher environment is just the start of the woes. Natural predation and man-made havoc bode ill for the future of coral reefs.
The crown-of-thorns starfish eats coral polyps. Prodigious predations from periodic population explosions of the starfish chewed into coral populations in the early 21st century.
Humans indirectly inflict damage on coral reefs via fertilizer runoff from farms that lead to algae blooms which block the light that corals need. Further, sedimentation that makes its way into the sea from soil erosion smothers coral.
Elkhorn coral is named for its resemblance to elk antlers. It provides a valuable marine habitat and was once the Caribbean Sea’s most abundant reef builder. But this redwood of the coral forest declined 90% from 2000 to 2010, in large part to highly contagious white pox disease, which causes large lesions that kills polyps and bare the coral’s white skeleton. The coral pathogen was a cross-species transfer from human excrement dumped into Caribbean waters.
Fishing near reefs reduces the number of herbivorous fish which keep the vegetation on reefs in check: another way that coral are deprived of sunlight and die.
Air pollution, which is particularly thick in the coastal cities of India and China, also deprives coral of needed light, and so slows their growth.
The indirect threats caused by humans are insult on top of direct injury. Despite their ecological importance, men wantonly destroy coral reefs; almost always for shortsighted commercial gain. Fishers in Central and South America and Africa use cyanide and dynamite to kill fish and bring them to the surface. These techniques decimate coral. Asian coral reefs face an additional destruction from reckless Chinese.
The Chinese in the South China Sea
In the South China Sea, Chinese boatmen gather giant clams that live deep in the coral reefs; clams that can live for over a century and grow more than a meter wide.
Large specimens might fetch the equivalent of thousands of US dollars. Besides their meat, clam shells adorn trinkets such as jewelry.
The Chinese harvest clams by having ship propellers chew through coral reefs. Once the murk clears, divers bring up any exposed clams. It is a one-time operation, as the reef is destroyed.
This plunder is illegal in China. Trade in giant-clam shells is banned by international treaties. But Chinese clam snatchers operate in broad daylight.
You guys do a great job! ~ Xi Jinping to South China Sea pillagers in 2013
Enforcement became lax since the Chinese started contesting territorial rights to waters claimed by neighboring countries. Seeking a stable base for runways and other installations which physically promote China’s militaristic assertions, dredgers suck debris off the seabed and spray it on reefs, smothering them. Such muck-spreading not only kills many kilometers2 of coral, it also obliterates any chance of recovery.
The Americans in South Florida
This Florida reef is as important to our country as the sequoias of California, and we are losing it. There are a lot of stressors that are impacting and killing coral reefs, but this is something we could easily have prevented. ~ American environmental scientist Rachel Silverstein
South Florida has the only coral reef in the continental United States. 93% of it has been lost by human negligence.
The large-scale dredging of Miami’s port in the 2010s to accommodate larger ships carelessly killed local coral reefs by burying them in sediment. Then the US Army Corps of Engineers lied about what it had done – your tax dollars at work.
I am proud to say that Port Miami is ready for the big-ship era. ~ Port Miami CEO Juan Kuryla
Evolution can buffer populations from environmental change, or it can accelerate species decline. For corals specifically, adaptability is complicated by symbiosis.
The weak link among the coral mutualists is the algal symbiont, which suffers greater mortality from thermal stress than does the host. The generation times of corals are orders of magnitude greater than those of algae, so symbionts may show faster adaptations to changing conditions.
As with many other species, acclimation is like to be overrun by the accelerating rate of climate change, especially ocean warming, but compounded in severity by increasing acidity from greenhouse gas absorption and human activities which directly contribute to coral decline.
Current trends indicate that coral reefs, and everything that depends upon them, will not survive to 2060. The world’s oceans are inexorably becoming a marine desert, as coral reefs are essential to the oceanic food web.