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 (a(CDOM)) and DOC were highly correlated to diffuse attenuation coefficients (K-d) 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.