Penetration of solar UVB radiation in shallow tropical waters and its potential biological effects on coral reefs; results from the central Indian Ocean and Andaman Sea

Abstract

ABSTRACT. This paper presents the first complete data of global downwelling irradiance (Ed) and the diffuse attenuation coefficient (K,,) for solar ultraviolet-B (UVB; 280 to 320 nm) in tropical waters. The penetration of solar UVB into shallow (0 to 5 m) seawater a t 3 sites in the central Indian Ocean and Andaman Sea, adjacent to areas of coral reefs. 1 t . d~ studied using a semi-submersible scanning spectroradiometer. Downwelling global spectral irradiance (E,) w~i s measured at 2 nm intervals over the wavebands 280-320 nm (UVB), 320-400 nm (U\-:\) and 400-700 n m (PAR) above the sea surface (O* m) and at each of 5 depths (1, 2, 3. 4, and 5 m). The 3 sltes consisted of a n ocean atoll in the Maldives (central Indian Ocean), a small (8 km') high island 11 km off the continental coastline at Phuket, Thailand (Andaman Sea), and a n inshore reef at Phuket. E, at each of the depths was integrated over the wavebands as a percentage of the above-watcr irradiance. E,(UVB) at 5 m depth was found to decrease to 12"& of incident ~rradiancye at the mid-occan atoll, to 2 % for the high island site, and to 0.4 %) in the turbid waters of the inshore reef. A 1 'X, Ed(UVB) depth was computed for each site and found to be 11, 6, and 3 m respectively. The diffuse attenuation for downwelling irradiance (Kd) for the depth range 0-m (just below the surface) to 5 m showed a very rapid attenuation with decreasing ~vavelength in the UVB at all sites. Biological damage potential, as weighted by the DNA-damage action spectrum, showed a more rapid attenuation with depth than Ed(UVB), w~t h a 1 % EDN, depth of 9 m for the ocean atoll, 4.7 m for the coastal island, and 2.6 m for the inshore reef.

Photosynthetic responses of corals Mussismilia harttii (Verrill, 1867) from turbid waters to changes in temperature and presence/absence of light

“…There was a gradual reduction of F v /F m as temperature increased, with a further drop in the presence of light, indicating a synergistic effect of these factors -as seen in other studies WARNER, 1995;BROWN, 1996GLYNN, 1996;WARNER et al, 1996;JONES et al, 1998;HOEGH-GULDBERG, 1999;FITT et al, 2001;LESSER;FARRELL, 2004). Similar physiological responses were also observed at low temperatures for Montipora digitata (SAXBY et al, 2003) WARNER et al, 1996), even at subsaturating irradiances (40 µmol photons.m -2 .s -1 ) (IGLESIAS-PRIETO, 1995).…”

Diurnal variations in depth profiles of UV-induced DNA damage and inhibition of bacterioplankton production in tropical coastal waters

“…The observed K d values (0.24 at 305 nm and 0.20 at 320 nm) were comparable to ocean sites in the Gulf of Mexico, Western Greenland, Red Sea and French Polynesia (Kirk 1994, Dunne & Brown 1996. DNA damage in bacterioplankton (< 0.4 µm) induced during the day was found down to only 5 m depth.…”

“…UVBR is more biologically reactive, but is present at low intensity compared to UVAR and photosynthetically active radiation (PAR, 400 to 700 nm). The attenuation of UVR in the water column is a function of wavelength: short wavelength irradiance is more attenuated than longer wavelength irradiance (Kirk 1994, Dunne & Brown 1996.…”

Ultraviolet‐B radiation stimulates shikimate pathway‐dependent accumulation of mycosporine‐like amino acids in the coral Stylophora pistillata despite decreases in its population of symbiotic dinoflagellates

“…Natural bleaching events following prolonged calm sea conditions that allow greater penetration of UVR have occurred at intermediate depths where increases in UVR would be more modest (Goenaga et al 1989;Gleason and Wellington 1993) and where UV fluences increase disproportionally to those of PAR (Gleason and Wellington 1994;Dunne and Brown 1996). Like other sublethal stressors such as temperature (Brown 1997b) and eutrophication (Muscatine et al 1998), such increases in UVR might disrupt the coral symbiosis by affecting the proliferation of algae and their numbers in host cells, a possibility that we have tested.…”