Spatial and temporal variability of solar ultraviolet exposure of coral assemblages in the Florida Keys: Importance of colored dissolved organic matter

Solar ultraviolet (UV) radiation can have deleterious effects on coral assemblages in tropical and subtropical marine environments, but little information is available on UV penetration into ocean waters surrounding corals. Here we provide an extensive data set of optical properties in the UV domain (280[en]400 nm) that were obtained during 1998–2005 at sites located in the Lower and Middle Keys and the Dry Tortugas. Absorption coefficients of the colored component of the dissolved organic carbon (DOC; colored dissolved organic matter [CDOM]) were 63 to 253 larger than particulate absorption coefficients in the UV region, indicating that CDOM controls UV penetration in the inshore coastal waters and reef tract. CDOM absorption coefficients (aCDOM) and DOC were highly correlated to diffuse attenuation coefficients (Kd) in the UV spectral region. Measurements using moored sensors showed that UV penetration at the reef tract in the Lower Keys varies significantly from day to day and diurnally. The diurnal variations were linked to tidal currents that transport CDOM over the reef tract. Summertime stratification of Case 1 bluewaters near the reef tract during periods of low wind resulted in higher temperatures and UV penetration than that observed during well-mixed conditions. This result suggests that higher UV exposure accompanying ocean warming during low-wind doldrums conditions significantly contributes to coral bleaching. Modeling results indicate that changes in underwater sunlight attenuation over the coral reefs can affect UV-induced deoxyribonucleic acid (DNA) damage and inhibition of coral photosynthesis much more strongly than changes in the stratospheric ozone layer. Solar radiation is intimately involved in the physiology of corals. Photosynthetic symbionts (zooxanthellae) in host coral tissues are dependent upon sunlight, especially the visible component known as photosynthetically active radiation (PAR), to drive carbon fixation. However, absorption of the ultraviolet (UV) component of sunlight by corals can inhibit photosynthesis (Fitt and Warner 1995; Shick et al. 1996; Lesser 2000) and influence other processes, such as larval settlement and growth rates of juveniles (Gleason and Wellington 1995; Gleason et al. 1 Corresponding author: ([email protected]). 2 Present address: University of Texas at Austin Marine Science Institute, Port Aransas, Texas. 3 Present address: Everglades National Park, South Florida Natural Resources Center, Daniel Beard Center, 40001 State Road 9336, Homestead, Florida 33033. 4 Present address: Naval Oceanographic Office—Code N35, 1002 Balch Blvd., Stennis Space Center, Mississippi 39522-5001. Acknowledgments We thank K. Potts and the crew of the RV Anderson for their support during cruises along the Florida Keys. We also thank L. Hansen, L. Oliver, D. Santavy, and others from the U.S. Environmental Protection Agency (EPA) National Health and Environmental Effects Laboratory—Gulf Ecology Division, Gulf Breeze, Florida for their help with this research. Thanks go in particular to J. Rogers for providing biological weighting functions for Symbiodinium sp. We thank E. Mueller for his inputs into the experimental work and useful discussions related to this paper. We also thank R. Jones, B. White, H. Fastenau, J. Machula, E. White, and R. Smith for their assistance with the field work. We thank R. Jones for his helpful data analysis and figure preparation. We thank the reviewers for their useful comments on the manuscript, many of which were incorporated. This research was supported in part by EPA Grant 98-NCERQA-R1 and a grant from the Office of Naval Research to R. Zepp (N00014-98-F-0202). This paper has been reviewed in accordance with the U.S. Environmental Protection Agency’s peer and administrative review policies and approved for publication. Mention of trade names or commercial products does not constitute an endorsement or recommendation for use by the U.S. EPA. Limnol. Oceanogr., 53(5), 2008, 1909–1922 E 2008, by the American Society of Limnology and Oceanography, Inc.