Karl Castillo was in his early 20s when he sat with his stepfather in a small boat off the coast of southern Belize, prepared to examine the coral below.
What he saw shocked him. In stark contrast to the blue-green sea that insulated the coral, these foundations of the Mesoamerican Barrier Reef had been robbed of their color.
A native of Belize, Castillo had grown up snorkeling among the coral near his seaside home. But in 1995, these bleached creatures were slowly starving to death - and he needed to find out why.
"Most people down there didn't know what was happening," Castillo said. "We were just beginning to understand."
The culprit, he later learned, was a mass of warm seawater that moved in from offshore, killing the algae that provides coral with color and nutrients.
"To me, this is hugely important. I grew up living right next to the ocean and seeing not only how much I enjoyed it, but how important it is," Castillo said. "So I tried to dedicate my entire career to seeing if I could assess the different things we see happening to the ocean."
About 15 years later, Castillo returned to the Gulf of Honduras as a postdoctoral researcher at UNC-Chapel Hill with several colleagues to do just that. By measuring the growth of coral up to 100 years old, they suggest in the journal PLoS One that the creatures farthest from shore are more sensitive to environmental threats than their shoreside peers.
"One hundred years ago, they had one of the highest rates of extension. Now it's the lowest," said Justin Ries, an assistant professor of marine science and co-author of the study. "It's not like it's always been low and they just got hit harder."
Although not all coral experts are convinced by his findings, Castillo hopes this will be another step toward understanding how factors such as climate change, pollution and run-off affect coral in different areas of the reef - and how conservation groups can organize their efforts to protect it.
Drilling for data
Massive starlet coral, the hardy species Castillo studied, show their age a lot like trees. Every year, they add a 4- to 6-millimeter layer to their calcium carbonate skeletons in a consistent pattern. Off the coast of Belize, those bands roughly correlate to the wet and dry seasons. Just like with an ancient redwood, scientists can count the rings in a cross-section of a coral skeleton to determine its age.
By studying 13 samples of coral from 19 to 98 years old, UNC researchers were able to measure the size of those rings to gather data on how fast they grew in different stages of their lives.
But collecting coral isn't quite as easy as chopping down a tree and hauling it back to the lab. Using specialized underwater drills attached to compressed air tanks, Castillo's team gathered fragile, 3-foot-long core samples that would yield accurate data.
"The coral is like a dome, so if you drill from the side, your bands might be slanted," Castillo said. "We drilled from the center of the core because we wanted to make sure they were aligned."
Although the original plan was to collect seven cores from each of three regions they sampled - nearshore, backreef and forereef - they ended up with fewer from the two areas closer to shore.
"These are marine protected areas, and in the nearshore, this is closer to where people are, and they don't necessarily want you to take too many samples," Castillo said. "So we just tried to get as much as they would allow us."
Back in Chapel Hill, Castillo's team took X-ray photos of the cores to make the bands easier to detect. They ran those photos through special software to measure distances between the bands, comparing each region's findings with similar cores to ensure they were aligned.
Their findings show that from the 1930s to 2008, forereef coral - farthest from shore - went from the fastest-growing of the three regions to the slowest. Meanwhile, skeletal growth of the coral in the nearshore and backreef zones - those closest to shore - remained relatively stable. That period included documented coral bleaching events in 1995, 1998 and 2005.
Castillo and his team say they believe shoreside coral may be better equipped to tough out new environmental strain because they experience minor stresses such as runoff and changes in temperature throughout their lives, just by virtue of being in shallower water close to the mainland.
"For some reason, I see that they are able to recover after the thermal stress period, while we see that in the forereef, they continue to decline," Castillo said.
No 'nail in the coffin'
But Alina Szmant, a coral expert and professor of marine biology at UNC Wilmington, isn't so sure.
Although she said there's no doubt that reefs in the Caribbean and around the world are in a "terrible state of decline," she's worried there's too much variation in the coral growth rate data to draw conclusions about individual reef zones.
"I don't think the paper puts the nail in the coffin that the forereef corals are doing worse," said Szmant, who was not involved with the research.
In particular, she said the banding patterns of massive starlet coral tend to be messy compared with other species. Outliers in the earlier historical data, such as measurements from 1935 to 1949 showing a huge jump in growth for seaward coral compared to their brethren, also muddy the waters. She said she's not surprised to see forereef coral growing more slowly than it once did, but she's just not convinced by the comparative decline.
"Like most things in science, it's always more complicated," she said. "The differences need to be more substantial before we interpret them."
In the paper itself, Castillo recognizes some of its relative weaknesses - its small sample size, for example. But he hopes further research will confirm his findings and pinpoint the causes of slumping coral health - and how to stop it.
"If they find what we're seeing is true, I think the important thing is we can make recommendations to marine protected area managers," Castillo said. "They have limited resources, so they can allocate their available resources to maybe more rangers out there to minimize additional stresses on forereef coral."