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

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 o- 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.