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Growth Rates in Deep-Sea Stylasterid Corals off the Southeastern U.S.

Project Goal

To assess growth rates and patterns in deep-sea stylasterid corals ("lace corals") from off the southeastern U.S.

Geographic Location

South Atlantic Bight, off South Carolina and Georgia

Approach

Coral and water samples collected during cruises to the Charleston Bump in 2003 and 2004 are being analyzed at the University of Alabama, Department of Geological Sciences. Colonies are being sliced into thin sections, along the axis of growth. Isotope ratio analysis is being done at different points across the sections, from the oldest (core) to the newest part of the coral skeleton. The isotope profiles generated from these corals will be compared to environmental data. As environmental conditions oscillate seasonally on the Charleston Bump, the isotope record can be interpreted as a time series to measure growth rate.

Results

Coral growth rates vary with environmental conditions. As corals grow, they incorporate elemental isotopes (such as oxygen isotopes) in proportion to what is in the surrounding seawater. Therefore visible growth bands and isotope distributions in lace corals are functions of environmental variability and may represent archives of past environmental conditions. If environmental conditions fluctuate on a regular seasonal or annual basis, the deposition patterns of isotopes can indicate coral age. However, this approach is complicated in deep-sea corals due to irregular variation in oxygen isotope concentrations in seawater. Oxygen and carbon isotope concentrations were found to oscillate up and down across the sections of coral skeleton but the patterns are not consistently in tandem with visible bands in the sections. In some portions of a colony, isotope lows and highs often occurred at points between visible bands, whereas in other parts of the colony there did not seem to be such consistent relationship. The variables that control these geochemical distributions are not well understood. In addition, as lace coral grows larger, natural spaces in the coral structure (canals) are filled with minerals that the rest of the coral skeleton is made of. This process obscures evidence of the original growth structures of the coral and mixes structure formed at different times throughout the colony. While the canal structures were sometimes noted to occur in ring patterns that superficially resembled growth rings (as in a tree trunk) such patterns were difficult to detect in instances where the canals were completely filled in.

Importance to Management

While fishing gear that comes in contact with the seafloor has the potential to cause coral damage, our preliminary work indicates that rapid changes in bottom temperature in this area (10 degrees Celsius over a few days) may also cause periodic coral die-offs, leading to building of large mounds of mostly dead coral along with small, young corals that may have developed since a die-off. The mounds of dead coral, as well as living colonies, may be important habitat for fishes and other organisms and this work, while not able to determine precise coral ages, has helped improve the understanding of growth patterns of these corals, and their recovery from natural or man-made damaging events.

Links

Descriptions of field work and methods can be found at:
http://oceanexplorer.noaa.gov/explorations/03bump/welcome.html
http://oceanexplorer.noaa.gov/explorations/04etta/welcome.html
http://sanctuaries.noaa.gov/missions/2010coral_east/welcome.html

Fiscal Year 2010 Funding

$50,000

Point of Contact

George R. Sedberry, George.Sedberry@noaa.gov

Oxygen (solid) and carbon (dashed) isotope concentrations for: (A) upper transect of a coral colony; (B) lower transect of the same coral; and (C) a different coral colony. Vertical bars represent growth bands visible in sections. Distances (x-axis) are in microns from the edge of the colony perpendicular to incremental growth banding.

Credit: Fred Andrus, University of Alabama