A new study just published in PLos One (Castillo et al 2010) indicates massive corals (Siderastrea sideria) on the outer reefs of the Belizean Barrier Reef are growing more slowly than they did during the early and middle 20th century.

Lead author Karl Castillo (a post doc in my lab) is from southern Belize near the reefs where the study was conducted.  He and my UNC colleague Dr. Justin Ries took core samples from 13 massive starlet corals, a species that can grow to sizes of more than 1 meter across. They then measured the thickness of growth bands in the coral samples.

[youtube https://www.youtube.com/watch?v=MLDdEOTwKzw?rel=0&w=480&h=390]

Core sections represent the most recent years of skeletal extension for Siderastrea siderea from the (A) forereef [core FR-12], (B) backreef [core BR-06], and (C) nearshore reef [core NS-14]. Numbers correspond to year of paired high-low density annual growth bands. Asterisks correspond to the annual growth bands formed during the 1998 coral bleaching event.

Background

Natural and anthropogenic stressors are predicted to have increasingly negative impacts on coral reefs. Understanding how these environmental stressors have impacted coral skeletal growth should improve our ability to predict how they may affect coral reefs in the future. We investigated century-scale variations in skeletal extension for the slow-growing massive scleractinian coral Siderastrea siderea inhabiting the forereef, backreef, and nearshore reefs of the Mesoamerican Barrier Reef System (MBRS) in the western Caribbean Sea.

Methodology/Principal Findings

Thirteen S. siderea cores were extracted, slabbed, and X-rayed. Annual skeletal extension was estimated from adjacent low- and high-density growth bands. Since the early 1900s, forereef S. siderea colonies have shifted from exhibiting the fastest to the slowest average annual skeletal extension, while values for backreef and nearshore colonies have remained relatively constant. The rates of change in annual skeletal extension were −0.020±0.005, 0.011±0.006, and −0.008±0.006 mm yr⁻¹ per year [mean±SE] for forereef, backreef, and nearshore colonies respectively. These values for forereef and nearshoreS. siderea were significantly lower by 0.031±0.008 and by 0.019±0.009 mm yr⁻¹ per year, respectively, than for backreef colonies. However, only forereef S. siderea exhibited a statistically significant decline in annual skeletal extension over the last century.

Conclusions/Significance

Our results suggest that forereef S. siderea colonies are more susceptible to environmental stress than backreef and nearshore counterparts, which may have historically been exposed to higher natural baseline stressors. Alternatively, sediment plumes, nutrients, and pollution originating from watersheds of Guatemala and Honduras may disproportionately impact the forereef environment of the MBRS. We are presently reconstructing the history of environmental stressors that have impacted the MBRS to constrain the cause(s) of the observed reductions in coral skeletal growth. This should improve our ability to predict and potentially mitigate the effects of future environmental stressors on coral reef ecosystems.

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,” said the study’s lead author, Karl Castillo, Ph.D., a postdoctoral research associate in the marine sciences department in the UNC College of Arts and Sciences. “The forereef corals used to have the greatest linear extension, but since early last century their growth has clearly been stunted.”

“This suggests that backreef and nearshore corals may be accustomed to stressful conditions because of their regular exposure to high environmental stress,” he said. “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.”

Listen to an interview with Glen De’ath about similar findings on Australia’s Great Barrier Reef here.