Ocean heat waves are on the rise — and killing coral | Science News for Students

Ocean heat waves are on the rise — and killing coral

But some coral species may be survivors, new research shows
May 18, 2018 — 6:45 am EST
staghorn coral

Staghorn coral may be able to last 100­ to 250 years thanks to its ability to adapt to warming waters, a new study shows. Here, a healthy staghorn grows off Magnetic Island in the central part of Australia’s Great Barrier Reef.

M. Matz et al/PLOS Genetics 2018

The world’s oceans are sweltering. Over the last century, marine heat waves have become more common and are lasting longer. Two recent heat waves have had a devastating effect on corals in the Great Barrier Reef, off the coast of Australia. But corals might not be as doomed as scientists once thought.

A marine heat wave isn’t just any bout of hot water in the ocean. Scientists define it as lasting at least five consecutive days. The water also must be unusually warm for that ocean region or season. These temperature extremes can be killers for marine species such as corals, kelp and oysters. They also can wreak havoc on fisheries and aquaculture.

In a new study, researchers wanted to know whether the frequency of these heat waves has changed. They searched for such events in data on sea-surface temperatures dating as far back as 1900. They also looked in satellite data that have been collected since 1982.

Those data suggest that the number of days each year that some part of the ocean experiences a heat wave has increased by 54 percent between 1925 and 2016. Over that time, heat waves also have become 34 percent more common — and last about 17 percent longer. Researchers shared their new findings April 10 in Nature Communications.

That trend is mostly influenced by climate change. Excess greenhouse gases in the atmosphere have trapped heat close to Earth’s surface. That has warmed the ocean’s surface waters. The researchers were able to rule out that large atmosphere-ocean climate patterns caused this trend. Such patterns include the periodic warming and cooling of waters in the equatorial Pacific. That’s called ENSO. It’s short for the El Niño-Southern Oscillation.

In coming decades, such marine heat waves may become even more common, the researchers now suspect.  

Coral devastation

It’s no secret that warming ocean waters have devastated many of the world’s coral reefs. For instance, a new study finds, a 2016 marine heat wave killed almost one-third of the coral in the Great Barrier Reef.

“What we’ve just experienced [in the Great Barrier Reef] is one hell of a natural-selection experiment,” says Terry Hughes. He’s a coral-reef expert at James Cook University in Townsville, Australia. In total, about half of the reef’s corals have died since 2016, he says. On the bright side, maybe: “The ones that are left are tougher.”

His team published its findings online April 18 in Nature.

What happens to coral reefs affects vast underwater ecosystems. It also will affect the hundreds of millions of people who depend on those ecosystems for fishing, tourism and more. So scientists want to understand how corals might fare as climate change triggers longer and stronger marine heat waves.

These heat waves can cause coral bleaching. That’s when corals eject the symbiotic algae that live with the corals. These algae are known as zooxanthellae (ZOH-zan-THEL-ay). They provide corals with both nutrients and color. If the algae don’t return, the corals eventually bleach — turn bone white — and die.

In extreme cases, marine heat waves kill corals directly. Essentially, they roast them alive. That happened to some on the Great Barrier Reef, Hughes and his colleagues found. They gathered data from satellite images and ocean surveys. It took less heat stress to kill off corals than had been expected, they showed. (Heat stress refers to how hot the water was and how long it stayed hot.)

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Aerial photo of Great Barrier Reef
This is an aerial view of part of Australia’s Great Barrier Reef. Heat waves affecting the region in recent years have killed off almost half of its corals since 2016.

A well-adapted species

Mikhail Matz is a biologist at the University of Texas at Austin. He and his colleagues have studied staghorn corals (Acropora millepora). These corals appear very vulnerable to marine heat waves. Still, this species may turn out to be one of the more resilient ones.

This branching, fast-growing coral is a key reef builder. And new analysis shows it is genetically diverse enough to survive for another 100 to 250 years. This is encouraging. Other studies had suggested that many coral reefs might not last another 80 years.

In fact, the corals on this reef show signs of adapting quickly enough to keep pace with warming waters. Or, at least they appear to for now, Matz’s study suggests.

In the northern part of the Great Barrier Reef, the water can be more than 5 degrees Celsius (9 degrees Fahrenheit) warmer than in the south. Yet staghorn corals live throughout the region. That means there already are members of this species with the genes to handle such heat. It appears that those in the north have become “locally adapted,” Matz explains. As the southern area warms, some of the offspring of the heat-resistant corals could migrate there.  

This could let some reefs beat the heat by recruiting corals from more heat-adapted sites. These animals would not need to undergo genetic changes — mutations — to become heat resistant. And that would be fortunate. After all, developing such mutations can take a lot of time.

Matz and his colleagues reported their findings April 19 in PLOS Genetics.

If corals must move to adapt, people should help make the migration corridors that the corals must travel comfortable, says William Cheung. He’s a marine biologist at the University of British Columbia at Vancouver. He was not involved in either study. Reefs along the way might be protected, for instance, from destruction by fishers and others.

That may be prove a challenge, though. Hughes’ team has already documented widespread destruction in the Great Barrier Reef. Even so, Matz believes that “there is still time for us to act before corals actually start going extinct.”   

Power Words

(more about Power Words)

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

aquaculture     The process of raising fish and other aquatic species for human consumption. These activities are sometimes referred to as “fish farming.”

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

biology     The study of living things. The scientists who study them are known as biologists.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague     Someone who works with another; a co-worker or team member.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

coral bleaching     Corals depend on certain algae to help nurture them. When the corals’ water gets too hot or infected with disease-causing agents, those algae may flee, leaving the corals a stark white. This is a bleaching event. If the algae don’t soon return, the corals can die.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

El Niño-Southern Oscillation      Also abbreviated ENSO. This refers to long-term swings in sea-surface temperatures in the tropical Pacific lasting several years. Water temperatures in the eastern equatorial Pacific tend to vary from periods that are normal to ones that are either unusually cool (La Niña) or are warmer than normal (El Niño). These changes can affect climate across the planet, leading to times that are wetter or drier than normal. ENSO changes tend to track with the depth — or boundary — above which the ocean water varies greatly in temperature (a thermocline). When the thermocline is closer to the surface, it causes an upwelling of cold, nutrient rich water from low in the ocean. This tends to cool local temperatures at the ocean’s surface.

extinct     An adjective that describes a species for which there are no living members.

frequency     The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

Great Barrier Reef     Some 2,300 kilometers (1,430 miles) long, this natural coral habitat is the largest living structure on Earth. In coastal waters off of northeastern Australia, It’s big enough to see from space. It’s home to some 3,000 coral reefs, 600 islands, and hundreds of types of 600 types corals, more jellyfish, mollusks, worms and fish. It’s also patrolled by more than 30 species of whales and dolphins.

greenhouse gas   A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

kelp     A type of large seaweed that is usually a type of brown algae. They grow underwater and form large forests, providing habitat for many organisms. Some kelp forests are so large they can be seen from space.

larva     (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)

marine     Having to do with the ocean world or environment.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

migrate     To move long distances (often across many countries) in search of a new home. (in biology) To travel from one place to another at regular times of the year to find food or more hospitable conditions (such as better weather). Species that migrate each year are referred to as being migratory.

migration     (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

mutation     (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

natural selection     This is guiding concept underlying evolution, or natural adaptation. It holds that natural mutations within a population of organisms will create some new forms that are better adapted to their environment. That adaptation makes them more likely to survive and reproduce. Over time, these survivors may come to dominate the original population. If their adaptive changes are significant enough, those survivors may also constitute a new species.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

reef     A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

strategy     A thoughtful and clever plan for achieving some difficult or challenging goal.

stress     (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem.

symbiosis     (Adj. symbiotic) A relationship between two species that live in close contact. A species that lives this way, offering substantial help to the other species, is sometimes called a symbiont.

survey     (v.) To ask questions of many people or to glean data from a broad area or over a long period.

variant     A version of something that may come in different forms. (in biology) Members of a species that possess some feature (size, coloration or lifespan, for example) that make them distinct. (in genetics) A gene having a slight mutation that may have left its host species somewhat better adapted for its environment.

zooxanthellae     Certain photosynthetic algae that live in most reef-building corals. These algae provide both food and color to the corals. The corals protect the algae. Meanwhile, the algae feed their host, provide oxygen and help the corals get rid of their wastes. This mutual support by unrelated species is known as mutualism.


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Journal:​ M. Matz et al. Potential and limits for rapid genetic adaptation to warming in a Great Barrier Reef coralPLOS Genetics. Published online April 19, 2018. doi: 10.1371/journal.pgen.1007220.

Journal:​ T. Hughes et al. Global warming transforms coral reef assemblagesNature. Published online April 18, 2018. doi: 10.1038/s41586-018-0041-2.

Journal:​ ​​E.C.J. Oliver et al. Longer and more frequent marine heatwaves over the past centuryNature Communications. Published online April 10, 2018. doi: 10.1038/s41467-018-03732-9.

Journal:​ A.J. Hobday et al. A hierarchical approach to defining marine heatwavesProgress in Oceanography. Volume 141, February 2016, p. 227. doi: 10.1016/j.pocean.2015.12.014.