High penetration of ultraviolet radiation in the south east Pacific waters

[1] We investigated the penetration of solar ultraviolet radiation (UVR) in the surface waters of the south east Pacific (08–35°S, 142–73°W) from October to December 2004 during the BIOSOPE cruise. In the hyper-oligotrophic waters of the South Pacific Gyre (near Easter Island), diffuse attenuation coefficients for downward irradiance, K_d(λ), at 305 nm (UV-B), 325, 340 and 380 nm (UV-A) were 0.083, 0.055, 0.039 and 0.021 m⁻¹, respectively. The corresponding 10% irradiance depths, Z_10%(λ), were 28, 42, 59 and 110 m, respectively. These UVR penetrations are the highest ever reported for oceanic waters and are equal to those measured in the clearest fresh waters. UV-extended inherent optical property (IOP) and radiative transfer (RT) models allowed reliable estimations of K_d(λ) with the Case 1 water assumption when two values of chromophoric dissolved organic matter (CDOM) absorption spectral slope coefficient (S) were used, i.e. 0.017 nm⁻¹ at 325, 340 and 380 nm, and 0.023 nm⁻¹ at 305 nm.