Coral Calcification Rates in South Florida During Times of Changing Ocean Conditions

Above: A curious reef squid hovers over a calcification-monitoring station at Fowey Rocks Light reef in Biscayne National Park. [larger version]

Above: CREST researchers (left to right) Adam Brame, Don Hickey, Kristen Hart, and Keith Ludwig assist with coral alizarin red staining off East Key, Dry Tortugas National Park. [larger version]

At several reef sites in South Florida, among the colorful corals and rich marine life, lie concrete cinder blocks. Each of these blocks is firmly attached to the reef and holds a transplanted coral on top. The transplanted coral is fixed onto a disc with a bolt that can easily slide into a hole at the top of the block and just as easily be removed to weigh the coral. This device, designed by U.S. Geological Survey (USGS) scientists with the Coral Reef Ecosystem Studies Project (CREST), is used to measure baseline calcification rates for one of the most abundant reef-building corals in South Florida, the massive starlet coral Siderastrea siderea. The goal of the study is to monitor coral growth as ocean conditions change.

With ever-increasing amounts of carbon dioxide (CO₂) entering the atmosphere, oceans are absorbing more and more CO₂. Part of this CO₂ becomes carbonic acid when it dissolves in oceans, lowering pH levels. Recent research shows that many marine organisms may be at risk in conditions of declining pH.

This phenomenon of lowered pH in the oceans, known as "ocean acidification," is predicted to directly affect calcifying organisms such as corals, algae, and phytoplankton, as well as the multitudes of marine life that depend on them for food and habitat. Experimental work has demonstrated decreases in calcification rates of corals and other calcifying organisms related to changes in ocean chemistry. (For example, see "Coral Reef Builders Vulnerable to Ocean Acidification," Sound Waves, March 2008.) The changes are expected to occur during the present century. CREST scientists are studying and establishing baseline rates for coral and algal growth in Florida by conducting in-situ field measurements of calcification rates.

"We are making measurements to establish these baseline rates for calcification now because it has not yet been done in a systematic, direct manner," said USGS marine scientist Ilsa Kuffner. "To do this, we have four calcification-monitoring sites at reefs offshore from Miami, Key Largo, Marathon, and in the Dry Tortugas."

The corals are periodically removed from the monitoring stations and weighed using the buoyant-weight technique. The method involves suspending the coral in a cooler of seawater while the coral hangs from a balance overhead. The difference in buoyant weight between each weighing is used to calculate the mass of calcium carbonate gained per unit time.

While the corals wait in specially modified buckets to be weighed, they are stained with a dye, alizarin red, that becomes incorporated into the outermost skeletal layer of the coral. The stain provides a reference mark because corals, like trees, grow by accreting annual layers. "Knowing the date of the staining and later measuring coral growth above the stain line give us the linear extension rate of newly calcified material," said Kuffner.

The stain line also helps with another aspect of CREST research. The modern-day calcification study contributes directly to a CREST study to reconstruct paleoclimate by using coral cores collected from the Dry Tortugas. Historical coral-core records are valuable tools for comparing calcification rates of the past with those of the present or future. The stain line on each coral confirms the time period when material above it was deposited. When coral skeletal composition and instrumental data are calibrated properly, cores can provide temperature and environmental records predating the instrumental records, revealing coral responses to past ocean conditions. Density, thickness, and elemental composition of the growth bands are influenced by such factors as temperature, rainfall, nutrient variability, and water clarity. Studying these skeletal characteristics as they have changed through time can help scientists predict how future coral growth will respond to decreasing ocean pH and warming temperatures.

"We are hypothesizing that corals on the Florida reef tract will slow their calcification rates noticeably between now and 20 or so years from now," said Kuffner. "Depending on the trajectory that CO₂ emissions end up taking, experimental research indicates we could see a 30-percent decline in calcification by 2050 or 2100."

This is year 2 of the CREST 5-year plan for studying processes that affect the status of shallow coral-reef resources in three Federally protected areas: the Florida Keys National Marine Sanctuary, Biscayne National Park in Florida, and Virgin Islands National Park on St. John. CREST scientists hope to contribute scientific knowledge that can inform reef managers on best-management strategies for coral reefs and associated ecosystems.

Additional information about this topic is available in the following technical papers:

Kuffner, I.B., Andersson, A.J., Jokiel, P.L., Rodgers, K.S., and Mackenzie, F.T., 2008, Decreased abundance of crustose coralline algae due to ocean acidification: Nature Geoscience, v. 1, p. 114-117 [http://dx.doi.org/10.1038/ngeo100].

Jokiel, P.L., Rodgers, K.S., Kuffner, I.B., Andersson, A.J., Cox, E.F., and Mackenzie, F.T., 2008, Ocean acidification and calcifying reef organisms—a mesocosm investigation: Coral Reefs, v. 27, p. 473-483 [http://dx.doi.org/10.1007/s00338-008-0380-9].