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UV EXPOSURE OF CORAL ASSEMBLAGES IN THE FLORIDA KEYS
Citation:
Zepp, R G. UV EXPOSURE OF CORAL ASSEMBLAGES IN THE FLORIDA KEYS. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-03/095 (NTIS PB2005-101445), 2003.
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:
Recent studies have indicated that solar radiation can be a significant stressor of coral assemblages in tropical and subtropical marine environments. Here we review the scientific literature related to the interactions of solar radiation with coral reefs, with emphasis on harmful effects of solar UV radiation. Then, we present results of a case study of corals' UV exposure in the Florida Keys. UV exposure was quantified using diffuse attenuation coefficients that were determined using downwelling vertical profiles of UV and visible radiation from sites located at the Upper, Middle and Lower Keys and the Dry Tortugas. For comparison, absorption and fluorescence spectra of the filtered water samples from these sites were measured. Absorption and diffuse attenuation coefficients were highly correlated in the UV-B (290-315 nm) spectral region and ratios of absorption to diffuse attenuation coefficients were > 0.9 throughout this spectral region. Absorption coefficients in the 300 to 500-nm spectral region could be closely described by a nonlinear exponential function. These results indicated that the penetration of solar UV into waters over the coral reefs in the Florida Keys is controlled by the chromophoric component of dissolved organic matter (CDOM) in the water. Analyses of the dependence of underwater UV irradiance on changes in atmospheric ozone or in the UV attenuation coefficients of the water over the reefs indicate that: (1) the dependence on ozone or UV attenuation coefficients can be quantified using radiation amplification factors (RAF); RAFs can be computed using a power relationship for direct UV damage to DNA or for other UV damage to the photosynthetic system of the zooxanthellae associated with corals; (3) UV damage of both types is more sensitive to changes in water UV attenuation coefficients than total ozone, especially damage to the photosynthetic system; (4) RAFs for direct DNA damage to corals caused by changes in ozone are reduced by UV light attenuation in the waters overlying the reefs. Continuous measurements of solar UV-B irradiance (305 nm) at a location close to the reef tract (Sombrero Tower) demonstrated that diffuse attenuation coefficients undergo large diurnal and seasonal variations in response to fluctuating CDOM concentrations that are linked to currents and CDOM transformations. Other results further indicated that light exposure in the waters around the Florida Keys strongly varies with time and location. Generally, diffuse attenuation and absorption coefficients increased sharply along south-to-north transects from the deep bluewaters of the Florida Straits into Hawk Channel, the shallow coastal shelf region between the reef tract and the Keys. The largest change occurred over a narrow region that represented the interface between the green-yellow waters in Hawk Channel and the blue Atlantic water. Analyses of the results obtained at the deep stations just south of the coral reefs also indicated that the depth dependence of both the light and temperature differs greatly between the warm summer months and cold winter months. We found that the upper ocean water close to the coral reefs was generally much more opaque to UV and photosynthetically active radiation (PAR) during the cold winter months than during the summer. During the summer, stratification of the water results in clarification of the waters over the coral reefs and much greater UV and PAR exposure of the reefs. This effect is attributable in part to UV-induced decomposition of the CDOM in the water over the reefs. These results suggest that the extensive stratification which occurs under El Nino conditions may be greatly increasing exposure of the reefs to UV and PAR, thus exacerbating corals bleaching. Other research during the case study indicated that decomposing phytoplankton detritus and decaying litter from seagrasses and mangroves are the major sources of UV-absorbing substances over the coral reefs in the Florida Keys. Management strategies designed to protect seagrasses and mangroves should also play an important role in reducing coral reef exposure to harmful effects of solar radiation.