Staghorn corals at Carysfort Reef in 1976 (above) and 2016 (below). The extensive thickets of staghorn corals are gone today replaced by a structureless bottom littered with the decaying skeletons of staghorn coral. Photo: Chris Langdon, Ph.D.

MIAMI, 1 May 2016 () —In a new study, University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science researchers found that the limestone that forms the foundation of coral reefs along the Florida Reef Tract is dissolving during the fall and winter months on many reefs in the Florida Keys. The research showed that the upper Florida Keys were the most impacted by the annual loss of reef.

Each year the oceans absorb CO2 from the atmosphere and become more acidic, a process called ocean acidification. Projections, based largely on laboratory studies, led scientists to predict that ocean pH would not fall low enough to cause reefs to start dissolving until 2050-2060.

For two years, the researchers collected water samples along the 200-kilometer (124-mile) stretch of the Florida Reef Tract north of Biscayne National Park to the Looe Key National Marine Sanctuary. The data provide a snapshot on the health of the reefs, and establish a baseline from which future changes can be judged.

The results showed that reef dissolution is a significant problem on reefs in the upper Keys with the loss of limestone exceeding the amount the corals are able to produce on an annual basis. As a result these reefs are expected to begin wasting away leaving less habitat for commercial and recreationally important fish species. Florida Keys’ reefs have an estimated asset value of $7.6 billion.

In the natural scheme of things in the spring and summer months, environmental conditions in the ocean, such as water temperature, light and seagrass growth, are favorable for the growth of coral limestone. While, during the fall and winter, low light and temperature conditions along with the annual decomposition of seagrass, result in a slowing, or small-scale loss of reef growth.

However, as atmospheric CO2 is absorbed by seawater, ocean pH declines. The result is that the natural summer growth cycle of coral is no longer large enough to offset the effects of dissolution from ocean acidification.

“We don’t have as much time as we previously thought,” said Chris Langdon, UM Rosenstiel School professor of marine biology and ecology, and a senior author of the study. “The reefs are beginning to dissolve away.”

“This is one more reason why we need to get serious about reducing carbon dioxide emission sooner rather than later,” said Langdon.

The data for the study were collected in 2009-2010. The researchers suggest that a more recent analysis should be conducted to see how the reefs are faring today.

“The worst bleaching years on record in the Florida Keys were 2014-2015, so there’s a chance the reefs could be worse now,” said Langdon.

The study, titled “Dynamics of carbonate chemistry, production and calcification of the Florida Reef Tract (2009-2010): evidence for seasonal dissolution”, was published in the May 2 issue of the journal Global Biogeochemical Cycles. The co-authors include Langdon, UM Rosenstiel School alumnae Nancy Muehllehner and Alyson Venti, and David Kadko, now at Florida International University. National Science Foundation funded the study.

New Study Found Ocean Acidification May Be Impacting Coral Reefs in the Florida Keys


ABSTRACT: Ocean acidification is projected to lower the Ωar of reef waters by 0.3-0.4 units by the end of century making it more difficult for calcifying organisms to secrete calcium carbonate while at the same time making the environment more favorable for abiotic and biotic dissolution of the reef framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reef carbonate production. This study represents the first broad scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009-10 across seven onshore-offshore transects spanning the upper, middle and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m-2 d-1, respectively, and negative during the fall/winter at -33 ± 6 and -7 ± 2 mmol m-2 d-1. The most significant finding of the study was that the northern-most reef is already net erosional (-1.1 ± 0.4 kg CaCO3 m-2 y-1) and mid-reefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m-2 y-1) but erosional during the fall and winter. Only the two southern-most reefs were net depositional year-round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.

Dynamics of carbonate chemistry, production and calcification of the Florida Reef Tract (2009-2010): evidence for seasonal dissolution

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