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

Abstract

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 6× to 25× 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 (αCDOM) 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.

Recent decade of growth and calcification of Orbicella faveolata in the Florida Keys: an inshore-offshore comparison

“…Running 30-day mean sea temperature (°C) ing or reduce photo-oxidative damage during warmwater stress via shading (Lirman & Fong 2007, Zepp et al 2008, Ayoub et al 2009, Soto et al 2011. Recently, it was discovered that aragonite saturation state (Ω arag ) is elevated inshore, another possible contributing factor (Manzello et al 2012).…”

Diverse coral communities in mangrove habitats suggest a novel refuge from climate change

“…and reduce the photo-oxidative stress on corals (Fitt and Warner, 1995, Shick et al, 1996, Zepp et al, 2008, Ayoub et al, 2012. It is unlikely that CDOM is providing much additional shading in Hurricane Hole, but it could be a contributing factor.…”

MODIS-derived spatiotemporal water clarity patterns in optically shallow Florida Keys waters: A new approach to remove bottom contamination

“…Under extreme stress, corals will expel the zooxanthellae from their tissues, resulting in a 'bleached' condition, often leading to mortality of the coral polyp (see reviews by Brown, 1997;Hoegh-Guldberg, 1999;Douglas, 2003, and others). The relationship between incident radiation and stress response in corals is also wavelength dependent (Lesser & Lewis, 1996;Zepp et al, 2008), highlighting the necessity for multispectral characterization of the light field reaching coral reefs. In a more general sense, decreases in water clarity are often associated with increased nutrient loading and subsequent increases in water column chlorophyll concentrations (Riley, 1956;Rodhe, 1948).…”

“…In the absence of reliable satellite-derived water clarity data, in situ investigations of the relationship between light and optically shallow ecosystems must rely on localized water clarity monitoring and experimentation (e.g., Fisk & Done, 1985;Gleason et al, 2006;Hoegh-Guldberg & Jones, 1999;Lesser et al, 1990;Moore & Wetzel, 2000; Zepp et al, 2008). Even where long term monitoring programs exist, the extreme costs typically result in repeat sampling frequency and spatial distance between measurements large enough to potentially miss water clarity events and variations.…”