In October 2016, a peculiar “obituary” published in Outside magazine became a widely-broadcasted internet sensation. The proclaimed ‘death’ of the Great Barrier Reef, after more than 25 million years of thriving on earth, became the subject of melancholic social media posts that flooded the web. This article cited the phenomena of ocean acidification and climate change as the killers of the Great Barrier Reef, and predicted the destruction of global reefs by 2025. For once, coral reefs dominated the news. But as the public fumed, scientists stood by and shook our heads.
The Great Barrier Reef is dying, but it is far from dead. Coral reefs will become less common in our lifetimes, but they probably won’t die off completely. Ocean acidification and climate change do contribute to coral death, but they are not the only killers at play. These are only a few of the rumors circling around the minds of the public right now, and, in an age where we are facing real, drastic changes to our environment, it is imperative that the knowledge that is being spread tells the whole story.
In this two-part article, I will attempt to give a more accurate picture of the status of coral reefs in our world today, as well as a prediction for how they may look in the coming years. It is my hope that this article will offer some optimism, some clarification, and a reminder that life itself is wildly resilient, and should not be underestimated. It is also my hope that it emphasizes our role in this massive shift in our world’s ecosystems, and our responsibility in preserving what life exists.
See you on the other side.
There are few places on earth that rival coral reefs in terms of life, or color. On a healthy reef, thousands of species of marine organisms swim, crawl and glide through the avenues and alleyways created by ‘scleractinian’, or stony corals. These organisms, which are actually little animals in the same family as jellyfish, secrete limestone as they grow and over time, build three-dimensional structures so massive that they can be seen from space. Although tropical coral reefs are only found in less than 1 percent of the ocean, they actually harbor more than a quarter of all life found in it; on the Great Barrier Reef alone, more than 1500 species of fish, 400 species of stony corals, 130 species of sharks and rays, 30 species of marine mammals, and 6 species of sea turtles can be found, and this doesn’t even begin to describe the things that crawl.
It is estimated that up to 9 million species make their homes in tropical coral reefs; many of which are not able to live anywhere else; as coral around the globe are faced with death, many of these species will die with them. Preserving coral reef biodiversity is critical, as these ecosystems offer food, protection, jobs, and cultural and aesthetic value that are invaluable for communities all over the globe. First, let’s talk about some of the challenges coral reefs face.
Global temperature averages have been on the rise in the last century, and it is no coincidence that the amount of carbon dioxide pumped into the atmosphere is on the rise as well. Carbon dioxide is a greenhouse gas, and as it’s pumped into the atmosphere as pollution it traps the heat from the sun instead of letting it escape back out to space, slowly but efficiently making our world hotter.
As air temperatures get warmer, we see changes in global weather patterns: in particular, during the past fifty years or so we have seen intense increases in the strength of El Niño Southern Oscillation events, or El Niño for short. El Niño describes the cycle of warm and cold temperatures across the air and sea, but what is important here is that during El Niño events, some areas of the ocean get much warmer than usual: this presents a huge problem for coral reefs.
The first “mass bleaching event” in recorded history took place in 1981. It was unprecedented, confounding, and in one fell swoop, destroyed massive sections of tropical reefs through the Indopacific and Caribbean. Sadly, these events began happening frequently in the years following: 1997, 2001, 2005, 2010, 2015 and 2016 all saw bleaching crises that resulted in the combined destruction of almost half of the world’s reefs.
Divers and locals were stunned by the change in the once-colorful reefs; where once there was vibrant forests of purple, orange, yellow and red, there was now only bright white covering the sea floor, and many of the fish seemed to have disappeared without a trace. We would soon learn that these reefs had bleached, and many of these ecosystems would never recover.
Bleaching is a phenomenon that is seen in stressed corals, and can be the result of several factors, including changes in temperature, light, disease, or exposure to toxins. In this process, coral polyps expel the algae living in their tissues that give them their beautiful colors. The symbiosis between these algae, or zooxanthellae, and coral is what allows both species to thrive: the algae photosynthesize and convert sunlight into food for the coral, while coral provides nutrient-rich waste products for the algae to feed on.
This symbiosis is incredibly important in the construction of these reefs, as it is energetically expensive for corals to make and deposit the limestone that creates such massive reef structures. This symbiosis is what has allowed corals to create and maintain a thriving ecosystem for more than a fourth of all life in the ocean over the past 500 million years.
Unfortunately, the success of coral reefs is dependent on this partnership; without it, corals will die.
Corals expel their algal symbionts only as a last resort: for this to happen, conditions have to be pretty bad. When waters heat up past a certain threshold (usually a 2-3 degrees Fahrenheit increase) we see a change in the zooxanthellae: they start to produce toxins. The corals expel these symbionts in self-defense, and without their algal partners to give them color, the coral becomes transparent. The bleached corals we see are bone white because we are seeing their skeleton underneath.
However, bleaching doesn’t automatically mean death for the coral. If environmental conditions improve sometime within the weeks following bleaching, the coral can take up more zooxanthellae to repopulate their tissues and continue to grow. Sometimes, coral even bleach on purpose, and selectively uptake new zooxanthellae with different characteristics, which may be better suited to new oceanic conditions.
However, if conditions don’t improve and the coral is unable to bring in more zooxanthellae, it will starve and die. Its branches will quickly be covered by turf algae, a form of fleshy, aggressive algae that grows on ocean bottoms, and over time it will crumble into rubble. When all that remains is nothing but algae-covered debris, and the animals that once called the reef home leave seeking new shelter, the reef becomes a graveyard. In all of the areas in the oceans that used to be dominated by coral reefs, more than half are now are graveyards. Bleaching events, increasing in severity and frequency due to increased ocean temperatures, may be the greatest threat that reefs faces today.
Another major reason for coral decline is the phenomenon of ocean acidification, or put simply, the fact that the ocean is slowly becoming more acidic. This happens because the ocean acts as a large-scale regulator for the planet and absorbs much of the carbon dioxide we release into the atmosphere. Carbon dioxide reacts with water through a series of reactions to produce carbonate and H+ ions, which are what make an acid acidic.
Unfortunately, calcium carbonate (that substance that corals, crabs, tiny plankton and other marine animals make their shells and skeletons out of) dissolves readily in acid. As the ocean becomes more acidic, corals and other marine organisms simply are not able to create new skeletons.
We have already measured decreases in calcification in corals and other marine invertebrates, which slows their growth and acts as a barrier to recovery following bleaching events. As the pH continues to decrease, this phenomenon will get worse, and corals will be unable to efficiently heal following other stress events. Eventually, when the pH drops below a level called the “saturation state”, it will even start dissolving the corals and shelled creatures that already exist. At this stage, very few of these organisms will survive.
Although we have not reached this point yet, we can expect (with our current emissions levels) to reach it in the next 50 years. This will mark a huge tipping point in our ocean’s biological framework, and organisms at all levels will be affected. It is difficult to predict exactly how the biology of marine systems will be impacted, but we can expect high rates of extinction of species (some of them keystone, regulating species to the ecosystem), dramatic changes to the trophic web and food chain, and the evolution of new species due to genetic bottlenecks.
This will be a very hard phenomenon to stop or even slow, and will depend heavily on how much we can reduce our greenhouse gas emissions in the coming years.
Marine pollution comes in many forms, each of which has different (but devastating) effects on coral reefs and the ecosystems they support. Corals thrive in clean, low-nutrient waters; unfortunately, our heavy industrialization along tropical coastlines in the past century has ensured that the surrounding waters are anything but clean. Sediment runoff buries reefs, smothering and killing everything that lives on the seafloor, and excess nutrients from fertilizers and agricultural runoff provide a nutrient-rich environment for disease-causing bacteria to proliferate and become virulent, spreading like wildfire. If disease wasn’t enough, the nitrogen and phosphorus from these fertilizers encourage massive blooms of algae, and as these algae grow they use up all the oxygen in the water, causing the coral to suffocate and even blocking the sunlight that provides the corals’ partners with food.
Chemicals from sewage and mining operations prevent the processes of fertilization and settlement of coral larvae on the reef, and in concentrations high enough, kill adults outright. Toxic metals such as copper are painted on the underside of boats as antifouling paints to prevent the growth of barnacles, and as this copper rubs off it kills the offspring of all organisms that make the reef their home. It has even been shown that plastic pollution increases the risk of disease in corals, although it is not yet fully understood how this happens.
One thing should be clear: marine pollution is an issue that requires action now. Unlike ocean acidification and increasing temperatures, which will require the long-term decrease of fossil fuel usage to mitigate, reducing marine pollution is a goal that we can, and should, enforce at local scales in the next several years. Improving wastewater treatment plants, appropriately disposing of waste from cruise ships and ports, developing agricultural methods that reduce runoff, and directing shipping routes away from coral reef zones are only a few of the ways that this goal can be accomplished, and their implementation will be critical as all marine ecosystems and the infrastructure they provide are threatened in the near future.
Coral reefs provide a source of protein-rich food for more than a billion people around the globe, and provide the economic and social foundation for most island communities. And while coral reefs are home to truly astounding numbers of life, over the past century we have gotten really good at catching fish.
Overfishing on coral reefs has resulted in the elimination of key species that help the reef thrive, such as grazing fish that feed on algae and apex predators that control the populations of fish below them on the trophic web. Fish that group together to breed at certain times of year in spawning groups are prime targets for fishermen, who are able to catch incredible numbers of adult fish, but decimate huge portions of their population in the process.
To make matters worse, these practices do not give these species time to replenish their numbers, further limiting their populations in the long term. The use of non-selective fishing methods like nets and traps result in massive amounts of by-catch, or unintentional, non-target organisms such as sea turtles, dolphins, sharks and herbivorous fish.
Many communities even still use practices like blast fishing and cyanide fishing, in which explosives and poison (respectively) are used underwater to stun and kill fish, which are then easily collected on the surface. Blast and cyanide fishing completely decimate marine life on large sections of reef, leaving nothing but lifeless piles of rubble and can sterilize sections of reefs in the long term, making it impossible for these reefs to recover.
Of all direct human threats to coral reefs, poor fishing practices are the most destructive.
Ultimately, the result of overfishing is the breakdown of coral reef ecosystems through the elimination of their keystone species, upsetting the balance of the environment. As fish populations dwindle and expire, fishermen in local communities often have to choose new, ecosystem-critical fish species to catch to maintain pace with increasing demand from a growing human population.
The fates of corals and the organisms that live in proximity to them are intimately linked, and with little oversight, the overfished reef becomes a dead zone in which coral and fish alike disappear entirely. Fishermen lose their jobs, their ways of life, and their food, and the economies of local communities suffer severe hits.
This is happening every day, all over the world; in Southeast Asia, 95% of reefs are threatened by overfishing, and very few reefs around the world exist that do not suffer from some form of detrimental fishing practice. Developing sustainable fisheries, improving management and enforcement of catch laws, and utilizing less harmful fishing technologies is more essential now than it ever has been, because it is no longer just the reefs that are threatened, but our own livelihoods as well.
Disease is not a new threat to coral reefs: like all organisms, corals must constantly adapt to the spread of pathogens, developing resistances to some and vigorously fighting others. In a healthy ecosystem, disease acts as nothing more than a disturbance, and after a brief period of stress the ecosystem once again finds balance.
In recent years, however, coral disease outbreaks have been much more frequent than usual. Warming ocean temperatures have been identified as a cause for the increased disease in corals, and they do this in two ways. First, as corals bleach due to increased temperatures, their ability to fight infection and engage defense mechanisms against harmful bacteria is impaired, making them easy targets for opportunistic pathogens. Second, warmer temperatures encourage the growth of harmful bacteria that normally would exist in low concentrations in the water. Mass bleaching events are usually followed by major epidemics of disease that often deal the final blow in vulnerable coral populations, leading to death.
Unfortunately, it is not only temperature that has contributed to the occurrence of new and dangerous diseases in the coral realm. Inputs of fertilizer and urban runoff enrich the water surrounding reefs with nutrients, bringing in new bacterial strains and allowing them to thrive and spread in the rich waters. In the past 20 years, never-before-seen diseases such as black-band and white-band disease have appeared and devastated Caribbean corals, impacting 82% of reefs in the Caribbean.
To make matters worse, new diseases have also impacted other reef organisms that promote coral reef health. In 1983, a rare and unidentified disease spread rapidly and killed 98% of all long-spined sea urchins, a major grazer of turf algae that helps keep corals clean. Without the urchins tirelessly keeping algae cover down, corals in the Caribbean suffered drastic losses in the war against turf algae, and have not recovered.
As of 2016, an estimated 40 percent of all corals in the world had been destroyed beyond repair; that number is likely higher today. In the Caribbean alone, 85-90 percent of all coral reef cover has been lost permanently, and algae now dominates the sea floor. In the 2016 bleaching event that inspired the morbid Great Barrier Reef Obituary, 93 percent of the northern Great Barrier Reef was reported to have bleached, leading to death in about half of the ancient, living structure.
More bleaching events, working in tandem with all the other threats to coral reefs, present a dismal future for reefs around the globe. Climate change projections suggest that sea surface temperatures and seawater acidity may reach thresholds that corals can no longer tolerate as early as the 2030s. 15-20 years might be all tropical coral reefs have left.
Although the threats listed above are the most significant to coral reefs, they are unfortunately not the only ones: tourism, most sunscreens, the aquarium trade, invasive species, and even outbreaks of predatory starfish also pose a threat to these ecosystems. When discussing how to help coral reefs, though, we have to focus on the big issues. In Coral Reef Futures part 2, I will discuss how the scientific community has begun to repair some of the damage, and how we move forward from here. Stay tuned.