MEPS 144:109-118 (1996)  -  doi:10.3354/meps144109

This paper presents the first complete data of global downwelling irradiance (E_d) and the diffuse attenuation coefficient (K_d) for solar ultraviolet-B (UVB; 280 to 320 nm) in tropical waters. The penetration of solar UVB into shallow (0 to 5 m) seawater at 3 sites in the central Indian Ocean and Andaman Sea, adjacent to areas of coral reefs, was studied using a semi-submersible scanning spectroradiometer. Downwelling global spectral irradiance (E_d) was measured at 2 nm intervals over the wavebands 280-320 nm (UVB), 320-400 nm (UVA) and 400-700 nm (PAR) above the sea surface (0⁺ m) and at each of 5 depths (1, 2, 3, 4, and 5 m). The 3 sites consisted of an ocean atoll in the Maldives (central Indian Ocean), a small (8 km²) high island 11 km off the continental coastline at Phuket, Thailand (Andaman Sea), and an inshore reef at Phuket. E_d at each of the depths was integrated over the wavebands as a percentage of the above-water irradiance. E_d(UVB) at 5 m depth was found to decrease to 12% of incident irradiance at the mid-ocean atoll, to 2% for the high island site, and to 0.4% in the turbid waters of the inshore reef. A 1% E_d(UVB) depth was computed for each site and found to be 11, 6, and 3 m respectively. The diffuse attenuation for downwelling irradiance (K_d) for the depth range 0^- m (just below the surface) to 5 m showed a very rapid attenuation with decreasing wavelength 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 E_d(UVB), with a 1% E_DNA depth of 9 m for the ocean atoll, 4.7 m for the coastal island, and 2.6 m for the inshore reef.

Coral reefs · Diffuse attenuation coefficient · Solar ultraviolet radiation · UVB