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About 30 percent of the world's coral reefs are severely damaged, and as much as 60 percent may be lost by 2030, according to the report "Status of Coral Reefs of the World: 2002" (see Global Coral Reef Monitoring Network Web site). Recent articles in the journal Science suggest that the trajectories of decline are similar worldwide, although the damage is most severe in the western Atlanticwhere coral cover has declined by 80 percent over the past 30 years. These documented coral declines, in combination with predicted global climate change, make it imperative that management of coral-reef systems take into account the processes that control their resilience.
Resilience is a key trait of reef corals in the National Park of American Samoa on Ofu Island in the South Pacific, where the corals undergo extreme environmental stress: temperatures in some pools reach 35.5°C and can fluctuate daily by more than 6°C, dissolved oxygen ranges from 23- to 212-percent saturation, and pH varies as well. Such conditions might cause excessive mortality on other reefs, owing to thermal bleaching, yet a diverse assemblage of more than 100 coral species persists on Ofu.
In August 2004, we began fieldwork for a 3-year project to examine the intrinsic and extrinsic factors that make Ofu corals so resilient. Intrinsic factors include acclimatization (changes in the corals' biochemical properties, the type of symbiotic zooxanthellae, or the composition of the microbial community), adaptation (differential survival of larvae or recently metamorphosed spat with different genetic traits, differential recruiting of larvae with different susceptibilities, local recruitment from adapted populations), and (or) susceptibility to disease. Extrinsic factors, such as water motion, dissolved-oxygen levels, sunlight intensity, or sediment abrasion, could also affect the resilience of corals under thermal stress.
Project leader Charles Birkeland (USGS Hawaii Cooperative Fishery Research Unit, University of Hawai'i, Honolulu) and his graduate students Lance Smith and Dan Barshis are spearheading transplant studies to test the response of corals to different microhabitats. Corals were transplanted between the forereef and the reef moat, or between different pools in the reef moat. One set of experiments uses staining with alizarin (a red dye with an affinity for calcium) to measure coral growth under different conditions. A second series of experiments will monitor the biochemical characteristics of the corals, including the production of heat-shock proteins and mycosporine-like amino acids (MAAs), which control temperature- and light-induced bleaching by protecting corals from thermal stress and ultraviolet-light (UV) exposure. In addition, polymorphic nuclear markers will be developed to investigate the fine-scale genetic structure of the coral colonies, in order to determine whether some corals are more genetically suited for thermal tolerance than others.
Ginger Garrison and Christina Kellogg, of the U.S. Geological Survey (USGS) St. Petersburg Science Center in St. Petersburg, FL, are characterizing the microbial communities associated with three coral genera (Pocillopora, Porites, Acropora) in two environmental regimes over a 15-month period. They will use both mucus (surface community) and tissue samples to investigate changes in microbial communities in response to temperature and, possibly, bleaching. The intent is to determine whether nondestructive sampling (of mucus) can be used to predict changes in the microbial communities and survival of coral colonies. The data on the microorganism-community changes will be integrated with other data collected from the coral colonies: genetics, molecular markers, colony survival, and environmental variables.
Greg Piniak (USGS Pacific Science Center, Santa Cruz, CA) and Eric Brown (University of Hawai'i) are using a diving pulse-amplitude-modulated (PAM) fluorometer to study the fluorescence yield of the corals' symbiotic zooxanthellae. This study seeks to determine species-specific bleaching vulnerability by comparing seasonal changes in the daytime and nighttime fluorescence of 10 common coral species in different reef-moat microhabitats. A related study will examine the relation between photosynthetic electron-transfer rate and ambient light conditions, and whether seasonal temperature regimes affect that relation.
This multidisciplinary project is funded by the USGS Biological Resources Discipline. Other research collaborators outside the USGS include Peter Craig (National Park of American Samoa); Ruth Gates, Michael Rappé, and Rob Toonen (Hawai'i Institute of Marine Biology); Dan Barshis, Lance Smith, and Jonathan Stillman (University of Hawai'i); Andrew Baker (Wildlife Conservation Society); and Rob van Woesik (Florida Institute of Technology).
in this issue:
American Samoa's Resilient Coral Reefs
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