PENETRATION OF SOLAR UVB RADIATION IN SHALLOW TROPICAL WATERS AND ITSPOTENTIAL BIOLOGICAL EFFECTS ON CORAL-REEFS - RESULTS FROM THE CENTRAL INDIAN-OCEAN AND ANDAMAN SEA

This paper presents the first complete data of global downwelling irra diance (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 centr al Indian Ocean and Andaman Sea, adjacent to areas of coral reefs, was studied using a semi-submersible scanning spectroradiometer. Downwell ing global spectral irradiance (E(d)) was measured at 2 nm intervals o ver the wavebands 280-320 nm (UVB), 320-400 nm (UVA) and 400-700 nm (P AR) 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 (c entral Indian Ocean), a small (8 km(2)) high island 11 km off the cont inental coastline at Phuket, Thailand (Andaman Sea), and an inshore re ef at Phuket. E(d) at each of the depths was integrated over the waveb ands as a percentage of the above-water irradiance. E(d)(UVB) at 5 m d epth was found to decrease to 12% of incident irradiance at the mid-oc ean atoll, to 2% for the high island site, and to 0.4% in the turbid w aters 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 attenuat ion for downwelling irradiance (K-d) for the depth range 0(-) m (just below the surface) to 5 m showed a very rapid attenuation with decreas ing wavelength in the UVB at all sites. Biological damage potential, a s weighted by the DNA-damage action spectrum, showed a more rapid atte nuation with depth than E(d)(UVB), with a 1% E(DNA) depth of 9 m for t he ocean atoll, 4.7 m for the coastal island, and 2.6 m for the inshor e reef.