You are here:
INDICATORS OF UV EXPOSURE IN CORAL AND THEIR RELEVANCE TO GLOBAL CLIMATE CHANGE AND CORAL BLEACHING
Citation:
Anderson, S. L., J. Machula, G. Cherr, R G. Zepp, D L. Santavy, L J. Hansen, AND E. Mueller. INDICATORS OF UV EXPOSURE IN CORAL AND THEIR RELEVANCE TO GLOBAL CLIMATE CHANGE AND CORAL BLEACHING. Presented at US Environmental Protection Agency Symposium on Indicators in Health and Environmental Risk Assessment, Research Triangle Park, NC, June 5-6, 2000.
Impact/Purpose:
The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic resources to global change. The task will contribute experimental and modeling tools for assessments of the interactions of global climate and UV changes with coral reefs and selected watersheds and estuaries in the U.S. These activities are contributing primarily to two APGs in the ecosystems component of the Global Change Research Multiyear Plan: the 2006 APG (APG 3) on building the capacity to assess global change impacts on coastal aquatic ecosystems, including coral reefs and estuaries and the 2004 APG (APG 2) on building capacity to assess and respond to global change impacts on selected watersheds. One major task objective is to assess interactions of global warming and UV exposure that are contributing to the observed coral bleaching and disease. Our lab is working with scientists at the NHEERL Gulf Ecology Lab to characterize UV exposure and effects at several coral reef sites. Other research in this task is examining the interactions between UV-induced breakdown of refractory organic matter in estuaries and coastal areas that enhance UV penetration into the water and concurrently form biologically-labile nitrogen-, phosphorus- and carbon-containing substances that stimulate productivity and microbial activity. This task also involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). The objectives of this project are to assess the impacts of land use and climatic changes on soil nutrient cycles and microbiota, trace gas exchange and water quality in the Brazilian cerrado. This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. Other objectives of this task are to assess the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the Piedmont region of the southestern U.S.
Description:
A compelling aspect of the deterioration of coral reefs is the phenomenon of coral bleaching. Bleaching can destroy large areas of a reef with limited recovery or recruitment, and it may be induced by a variety of stressors ranging from exposure to temperature and salinity extremes, sedimentation, pollution, and ultraviolet light. Under conditions of ocean warming, often associated with calm, stratified waters, photobleaching of UV-absorbing chromophoric dissolved organic matter (CDOM) is increased, and penetrance of both UV-B and UV-A is greatly enhanced. Indices of UV- specific effects in coral tissue are needed to test whether UV increases, associated with global climate change, are harmful to coral. To address this challenge, we have evaluated UV-specific effects in coral and have characterized factors that alter penetrance of UV radiation over coral reefs. An immunoblotting assay was developed to examine UV-specific lesions (thymine dimers) in coral and zooxanthellae DNA. We observed dose-dependent increases of thymine dimers in purified coral (Porites porites) DNA exposed to UV-C in the laboratory and with intact P. porites in a Suntest solar simulator. However, with whole coral, effects were not strictly proportional at the lowest exposure times. Next, the assay was used to determine whether thymine dirners are detected under realistic field conditions (Maryland Shoals, Florida Keys, July 1999). Thymine dimers in P. porites collected at 0830 hrs and 1300 hrs differed significantly (p=0.04). A diurnal variation experiment was conducted in July, 1999 at Eastern Sambo Reef (Florida Keys). Sampling was conducted at multiple timepoints for three days to determine whether thymine dimers follow hypothesized diurnal cycles. Chlorophyll, tissue protein, zooxanthellae counts, and carotenoid pigments will also be analyzed. We are now developing immunofluorescence techniques that will allow us to determine where dimers are localized within coral tissue and zooxanthellae. Results obtained to date indicate that UV-specific effects can be discerned under field conditions and related to exposure dose. Studies now underway will allow us to determine whether these effects are linked to coral bleaching and other indicators of stress in coral. UV measurements were made as part of the experiments conducted during July 1999 at Eastern Sambo reef and nearby sites including profiling along transacts from reef to shore. Results of these analyses indicate that the coral reefs at Eastern Sambo (at 3-4 meters) were receiving UV-B radiation that was equivalent to 25-30% of surface UV irradiance. However, the water just inside the reef in Hawk Channel and closer to land was considerably more opaque to UV. This water photobleached with loss of UV absorbance and fluorescence when it was exposed to simulated solar radiation. These results indicate that photobleaching of the DOM and transport of near-shore water out over the reefs may play a key role in controlling UV penetration to the reef surface.