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by Ilsa Kuffner, Ann Tihansky, Hannah Hamilton, and Helen Gibbons

An innovative weight-based approach to monitoring coral growth in the wild has been developed by U.S. Geological Survey (USGS) researchers and is expected to lead to more definitive answers about the status of coral reefs. 

Corals and other marine organisms build their skeletons and shells through calcification, the biological process of secreting calcium carbonate obtained from seawater. The new weight-based approach to measuring coral calcification rates can provide finer scale resolution than traditional linear measurements of coral growth. It is a valuable tool for researchers and managers seeking to understand the global decline in coral reefs.

Using the weight-based approach, USGS scientists established a new calcification-monitoring network in 2009 to study coral growth in the Florida Keys. They published their results in June 2013 in the journal Coral Reefs, and their article is available for free download.

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The researchers measured growth rates of the reef-building coral Siderastrea siderea (massive starlet coral) at four sites in the Florida Keys Reef Tract (see map). At each of the four sites, the researchers securely fastened 10 concrete blocks to the seafloor. For each block, a sample of massive starlet coral collected near the study site was epoxied to a plastic disc fitted with a 10-centimeter (4 inch)-long stainless-steel threaded bolt. The researchers slid the bolt through a hole drilled into the concrete block and fastened it with a wing nut. Thus the disk and the coral sample epoxied to it were firmly attached to the block. Every 6 months, the corals-plus-disks were detached from the concrete blocks, transported by boat (while submerged in seawater), buoyantly weighed on land, and returned to their blocks later that day. Staining the outer layer of the samples at the start of the experiment and at springtime site visits allowed the researchers also to measure linear growth of the corals—the traditional way to monitor coral growth. Comparison of the weight data and linear-growth data confirmed their expectation that weight measurement would prove a more precise and efficient way to calculate calcification rates.

“A coral may grow two millimeters in height on the left side of the colony and five millimeters on the right, so linear measurements are inherently variable and require sampling hundreds of corals to detect changes in growth over time…our method requires only 10 corals per site,” said Ilsa Kuffner, USGS scientist and lead author of the study.

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Using the weight-based approach, Kuffner’s team has provided important baseline information on spatial and seasonal variability in calcification rates of reef-building corals in the Florida Keys. For example, they found that colonies of the massive starlet coral calcified about 50 percent faster in summer than in winter. They also discovered that, during the study period, the coral grew about 50 percent faster in the remote Dry Tortugas National Park than at three other outer-reef tract sites offshore of Miami, Key Largo, and Marathon, Florida (see map). The reasons behind this surprising pattern are not clear, leaving a mystery sure to pique the interest of many reef managers.

The new approach could be highly useful to managers because it can detect small changes in growth rate over space and time due to its high level of precision. Measuring changes in growth is becoming increasingly important as corals face challenges from ocean acidification (decreasing pH caused by increasing atmospheric CO2) and other climate-change and land-use impacts. Additionally, the method uses inexpensive and easy-to-find materials, and no corals are harmed in the process.

“This tool provides the kind of scientific information needed to manage coral reefs at the ecosystem scale by looking at the relationships between coral health, climate change, and water quality. It provides partners and reef managers with better, more sensitive metrics to assess coral growth, identify the most important variables, and prioritize strategies to protect and preserve these valuable ecosystems,” said Acting USGS Director Suzette Kimball. “It is also one of the ways USGS science is advancing the National Ocean Policy by supporting a number of on-the-ground priority actions.”

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A next step in understanding declines in coral growth is discerning the different components of water quality that are driving calcification rates, and this can only be achieved through the cooperation of reef managers and scientists around the world. The real power in the new approach will be realized if it is applied across many reefs that naturally have different temperature regimes, water quality, and pH conditions.

“The study results suggest that we should pay more attention to different aspects of water quality if we hope to understand and predict what will happen to coral reefs as oceans continue to change,” said Kuffner.

According to Kuffner, managers already know that coral reefs are in decline, but they want to know why. They need a linkage between cause and effect that explains why reefs are not growing like they used to or are not recovering from disease or die-off events. Correlating finely measured coral growth rates with water quality and other environmental information is an important step toward making these linkages so they can inform management decisions.

Coral reefs are in decline globally, with the National Oceanic and Atmospheric Administration currently proposing to list 66 reef-building coral species under the Endangered Species Act. Identifying the cause of the decline is not straightforward. Oceanographic instruments have confirmed that the ocean is warming, acidifying, and changing in other aspects of water quality. Warming and acidification are results of altered carbon distribution due to burning of fossil fuels; other changes in water quality stem largely from land-use changes. Laboratory studies demonstrate that all three of these environmental stressors can hinder coral growth, but linking the causative agents to reef decline in the natural environment requires dependable, precise methods to detect change over time.

This study is part of a larger USGS Coral Reef Ecosystem Studies (CREST) project aimed at understanding the status, construction, and resilience of shallow-water reef environments around Florida and U.S. Caribbean islands, and forecasting future change to inform reef-management strategies. Current areas of research include the Dry Tortugas, U.S. Virgin Islands and Biscayne National Parks, and selected areas of the Florida Keys National Marine Sanctuary.

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The coral-growth study was conducted under scientific permits from the Florida Keys National Marine Sanctuary and the National Park Service. The USGS Coastal and Marine Geology Program funded the majority of the study, and the U.S. Department of the Interior [DOI] Southeast Climate Science Center provided supplementary funds.)