Corals in Hot Water

Evan Lubofsky
Thursday, November 13, 2014

Read this story in Oceanus Magazine

A time bomb is ticking in the ocean, and faster than you might think.

The oceans are warming, and in the next 20 to 30 years many coral reefs around the globe will reach their temperature threshold, a tipping point at which they will likely yield to weakened immune systems, bleaching disease, and in many cases, death.

This race against time fuels the work of Hannah Barkley, a graduate student in the MIT/Woods Hole Oceanographic Institution (WHOI) Joint Program in Oceanography.

“I study how reefs respond to increasing water temperature, decreasing pH, and other factors brought on by climate change,” she said. “Coral reefs are not predicted to have a great prognosis for the future, and if oceans get as warm as they are predicted to in the next few decades, very few healthy reefs will be left. The things I find most compelling are the urgency of the situation, and the magnitude of the threat to ecosystems that are so important to so many people.”

The economic impacts of coral are huge. The commercial value of U.S. fisheries from coral reefs is more than $100 million annually, and about $5.7 billion worldwide, according to the National Oceanic and Atmospheric Administration. Add in revenue from tourism and jobs, and coral reefs generate $9.6 billion annually in net benefits. And that doesn’t include other societal and ecological benefits of corals, such as providing barriers protecting shorelines from storm damage and habitats for 25 percent of all marine species.

Over the past few decades, the science community has become alarmed as many corals around the globe have gone white, a phenomenon known as bleaching. Corals’ tissue is clear and their skeletons are white. Their brilliant colors come from tiny photosynthetic plants that live in their tissues and provide food to the corals in exchange for a good place to live. But stressful conditions—higher water temperatures, for example—can disrupt the delicate symbiotic relationship between algae and corals. The colorful algae either depart or are expelled, leaving the corals looking like once-red shirts that accidentally got into the laundry load with hot water and bleach. If conditions don’t shift and the algae don’t return, the corals can’t survive.

Such was the case in 1998, when unprecedented bleaching and coral death occurred in Maldives, Seychelles Islands and other areas of the Indian Ocean, affecting up to 90 percent of the region’s corals. And in mid-to-late 2005 significant bleaching occurred in the Caribbean’s Virgin Islands, stemming from record-breaking water temperatures at reef depths.

“The frequency and severity of bleaching events have been increasing over the past few decades, starting with the massive event from El Niño in 1998, which caused 16 percent of the world’s coral to die,” she said. El Niños are shifts in Pacific Ocean conditions that occur every few years: Easterly trade winds die down and allow a flood of warm equatorial waters piled up in the western Pacific to flow eastward across the Pacific toward South America.

Earlier this year, Barkley set off on an 8,500-mile journey to the Micronesian island of Palau to prepare for an El Niño that is expected to take shape at the end of 2014. Some climate models predict this coming winter’s El Niño will generate huge spikes in seawater temperature—which, in turn, could lead to significant bleaching.

“For the first time, we want to be able to capture and visualize the full cycle of an El Niño event and very precisely relate coral responses to the temperatures they are experiencing as the event happens,” she said. “That is something that hasn’t been available to us before.”

Testing the waters
Barkley’s interest in coral dates back to her childhood, when she and her family would travel to tropical locales such as Hawaii, the Galapagos Islands, and the Caribbean.

“We saw a lot of amazing places around the world, which gave me a great deal of exposure to coral,” she said. “I just loved being in the water and seeing all the beautiful marine life. I was scuba-certified at 12 years old.”

In her freshman year at Princeton, Barkley attended a seminar called “Signals, Yardsticks and Tipping Points of Global Warming,” which took her to the Bermuda Institute of Ocean Sciences (BIOS) to study the effects of temperature on coral. “I was thrilled by the idea of snorkeling for lab, and after that first experiment we did on how coral responded to temperature, I was hooked,” she said.

So much so that she spent the following summer at BIOS examining the effects of elevated temperature and other changing ocean conditions on the growth of juvenile corals, which became the subject of her senior thesis project a year later.

One of her advisors at the BIOS summer program was Anne Cohen, a coral scientist at WHOI.

“Our initial interaction was brief,” Cohen said. “There were several interns and high school helpers in the lab at the time, and Hannah was very focused on her experiment. When I got a chance to talk with her, I was immediately impressed by her ability to communicate both the details of her project and why it was important. Her questions to me indicated that she was thinking deeply about her results and had read widely.”

“At the end of that summer, I came to WHOI for three weeks to analyze my samples,” Barkley said. “During that time, I fell in love with WHOI, and Anne was here, so decided that I wanted to continue working with her and researching coral reefs and climate change in graduate school.”

Monitoring in Palau
Soon after, Barkley got the nod to head to the West Pacific to help study reef resiliency in the Palau archipelago. Working in conjunction with The Nature Conservancy and the Palau International Coral Reef Center, the research aimed to look at how corals respond to warmer ocean temperatures and another looming threat related to climate change: ocean acidification. As a result of fossil-fuel burning, a surge of carbon dioxide added to the atmosphere is dissolving into the ocean, where it reacts with seawater to make it more acidic. One of the goals of the Cohen Lab researchers is to identify corals most likely to survive the impacts of climate change.

“Many reef conservation efforts in place are geared toward protecting the strongest reefs rather than the most susceptible because these reefs likely have the best chance of surviving the impacts of climate change,” Barkley said. “So it’s important to find the most resilient reefs—though identifying them can be really difficult. It requires long-term monitoring of a range of parameters, including carbonate chemistry, pH, dissolved oxygen, and, of course, water temperature.”

“Ocean warming affects coral reefs in at least two ways,” Cohen said. “First, warm water just 1°C above normal can break down the relationship between corals and symbiotic algae they host in their tissues. When this happens, corals can starve and die.”

Warming also stratifies the ocean into warmer surface layers and denser, cooler, deeper layers, Cohen said. The two layers don't mix easily, which hinders the delivery of nutrients such as nitrogen, phosphorus, and iron from the ocean depths, which corals and other reef organisms need to grow.

To measure changes in ocean temperatures, scientists often rely solely on measurements from satellites. According to Barkley, there are limitations with that approach.

“Satellites are great for open-ocean measurements, but not so much for near-shore environments where corals are living,” Barkley said.

To collect fine-scale temperature data, Barkley and her colleagues in Cohen’s lab began establishing what has become an extensive network of temperature sensors around Palau. The sensors, which take temperature readings every 30 minutes around the clock, were deployed around the main barrier reef and several interior lagoon reefs at depths of five meters or less.

At each reef, the researchers took water samples to analyze water chemistry. They conducted surveys to observe how reefs are responding. And, they collected coral skeletal cores, which contain records of past ocean conditions. All this data, combined with the detailed view of the temperatures corals are experiencing over time, open the door to better understanding of the interaction between climate and corals. The data are also used to ground-truth satellite data, which according to Barkley is not always reliable.

Through the monitoring initiative, Barkley and colleagues have discovered that Palau's lagoon reefs are consistently one degree warmer than Palau’s barrier reefs over the past few years. And, water temperatures have peaked at more than 32°C (90°F)—hotter than expected. Despite this, some of the reefs—lagoon reefs in particular—have held up just fine.

“Certain reefs in Palau aren’t bleaching even when temperatures get really hot,” Barkley said. “When we see things like this, it gives us a sense for which reef communities could be most able to deal with climate change. It’s been a relief to see evidence that corals can in fact adapt to environmental threats—even when the threats are really severe.”

Capturing El Niño
With nearly 30 measurement points spanning 10 sites throughout Palau, it would seem the archipelago was well covered from a data collection standpoint. But the prospect of capturing the effects caused by this year’s impending El Niño spurred Barkley and colleagues to think about expanding the coral monitoring across a wider range of environments, she said. “Getting the sensors in early would enable us to get baseline data”—before the predicted big El Niño materialized.

So in August, Barkley and her colleagues headed back to Palau to “put a bunch more sensors in.” Unlike previous visits to Palau where access to remote areas was difficult, the team boarded a larger ship, the M/V Alucia, to get to the most remote atolls.

“We visited the northernmost and southernmost islands of the Palauan archipelago,” Barkley said. “These very small, sparsely inhabited atolls were among the hardest hit by both El Niño-related bleaching events in 1998 and 2010 and super-typhoons in 2012 and 2013. However, their remote locations—each over a day’s sail away from the busy population center of Koror—have made ongoing monitoring efforts extremely challenging.”

According to Barkley, very little is known about the temperatures experienced on these reefs.

“This is particularly exciting because this was the first time that extensive environmental monitoring has been conducted in conjunction with ecological monitoring at any of these sites. It will give us an unprecedented look at the temperature regimes and thermal stress levels experienced by these reef communities. And, it will allow us to connect bleaching responses to temperature levels during extreme events. As far as El Niño goes, we are so ready!”

Sharing a passion
Despite how this year’s El Niño plays out, Barkley is certain of one thing: her passion for coral research and conservation. Knowing that her research feeds into real-world conservation efforts is a huge motivator, and the fact that the people of Palau approach reef management with a similar degree of passion and enthusiasm makes her work that much more rewarding.

“The Palauans are doing an amazing job in terms of leading the world in conservation and commitment to their reefs,” she said. “I love the sense of pride they have in their reefs, and admire the fact that they have recently moved to outlaw commercial fishing despite it being such a big part of their economy. The ethic of protection pervades their entire culture.”

“Climate change-related factors, including elevated temperature, are a significant threat to the health and function of coral reefs,” she said. “If we can find reef communities that can tolerate these impacts, this knowledge will heavily factor into conservation planning in Palau and beyond.”

This research is funded by the National Science Foundation (NSF), Dalio Foundation Inc., The Nature Conservancy, and The Tiffany & Co. Foundation. Barkley is supported by an NSF Graduate Research Fellowship.