On coral reef, location seals fate

UNC-CHAPEL HILL (US) — Off the coast of Belize researchers have identified a troubling change in coral growth patterns on the world’s second largest barrier reef system.

About 100 years ago, seaward-side corals on the Mesoamerican Barrier Reef grew faster than the corals closer to the shore. They now grow slower.

The finding suggests that reef-building corals closest to the open ocean—and furthest from land-based threats traditionally faced by such organisms over the ages—are having the most trouble weathering the effects of environmental stress from sources such as climate change and pollutants.

The study, carried out by University of North Carolina at Chapel Hill researchers, appears in the journal PLoS One.

The researchers took core samples from 13 massive starlet corals, a species that can grow to sizes of more than 1 meter across. Then, in much the same way that land-based biologists analyze tree rings to determine the age of trees and the impact of past environmental events, the marine scientists measured the thickness of growth bands in the coral samples.

Finally, they compared samples from three zones across the reef system—the forereef (on the seaward side), the nearshore (closest to shore), and the backreef (in the middle)—to look for patterns.

They found that over the last 90 years, growth rates for corals in the forereef zone shifted from being the fastest of the three zones to the slowest, while the skeletal extension rates of corals closer to the coast remained relatively stable.

“Massive starlet corals are like old-growth trees in a forest, and the annual extension bands in these core samples tell a cautionary tale,” says the study’s lead author Karl Castillo, a postdoctoral research associate in the marine sciences department. “The forereef corals used to have the greatest linear extension, but since early last century their growth has clearly been stunted.”

While the researchers have not determined the exact causes of the change, the findings suggest corals in the forereef zone may be the most vulnerable to anthropogenic or human-influenced sources of stress, Castillo says.Forereef corals are located closest to the open ocean and are therefore typically exposed to cooler, more stable seawater temperatures. In contrast, landward side corals inhabit shallower waters and have historically experienced warmer and more variable seawater temperatures.

“This suggests that backreef and nearshore corals may be accustomed to stressful conditions because of their regular exposure to high environmental stress,” he says. “But forereef corals—which have probably been less conditioned by baseline natural stressors—appear to be more susceptible to recent human-made impacts such as ocean warming.”

Along with seawater temperature changes, another possible cause of the forereef corals’ slower growth may be sediment plumes, nutrients and pollution that have been carried long distances by ocean currents from the more densely populated watersheds of Guatemala and Honduras.

While nearshore corals have likely adapted to similar exposures from nearby sources over a long period of time, colonies in the traditionally pristine, open waters of the forereef have not, Castillo says.

Marine scientist Karl Castillo applies finishing touches to a repair plug used to fill a core sample hole in a coral on the Mesoamerican Barrier Reef off the coast of Belize in the western Caribbean. (Credit: Justin Ries, UNC-Chapel Hill)

Study senior author Justin Ries, assistant professor of marine science, says it remains to be seen whether the findings apply to other coral species, and within other reef systems around the world.

However, the slow-growing, comparatively hardy massive starlet corals are an important and relatively untapped natural archive of environmental change that can shed light on how the oceans have changed—and how corals have responded to these changes—since humans began releasing massive amounts of carbon dioxide into the atmosphere following the Industrial Revolution, Ries says.

“These findings improve our understanding of not only how corals from different zones of the Mesoamerican Barrier Reef System responded to anthropogenic stressors over the past century, but how they’re likely to respond in the future,” he adds. “This information will hopefully prove useful to policymakers and coral reef managers as they allocate resources to protect and maintain coral reef ecosystems.”

The work was partially funded by a grant from the National Science Foundation.

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