Factors Affecting Ultraviolet Exposure in Coastal Waters of the Florida Keys: Effects on Nearshore and Offshore Coral Reef Tracts

We have investigated how the loss of chromophoric dissolved organic matter (CDOM) in the water column due to photobleaching allows for increased penetration of UV radiation near coral reefs in the Florida Keys. Extended exposure to UV may contribute to coral bleaching episodes. CDOM serves as the primary control on UV exposure of corals in this region because it strongly absorbs UV radiation, especially damaging UVB wavelengths. An important fraction of the CDOM pool in Florida Keys coastal waters is transported from Florida Bay, but local CDOM sources including seagrasses, mangroves, and Sargassum colonies may also be substantial. CDOM samples collected along transects near the reefs and from mangrove leaf and Sargassum incubation experiments were exposed to simulated solar radiation for up to 120 hours. Calculated photobleaching rates (k305) of CDOM produced by mangrove leaf litter and Sargassum colonies (approx. 0.02 hr^-1) were an order of magnitude greater than rates measured for the water column samples (0.002 hr^-1). However, our experiments indicate that photobleaching of CDOM in natural waters near the reefs can still be substantial during summer months and may allow UVB levels at 4 m depth (typical depth of fringing reefs) to increase by as much as 20%. Corals located in shallower waters (2 m) along the reef line may experience up to a 40% increase in UVB exposure due to loss of CDOM.