Penetration of ultraviolet-B radiation in oligotrophic regions of the oceans during the Malaspina 2010 Expedition

Few studies have investigated ultraviolet (UV) radiation in the open ocean besides its harmful effects on organisms and influence on biogeochemical processes. Here, we assessed UV attenuation, with particular focus on UV-B, across the (sub)tropical ocean during the Malaspina 2010 Circumnavigation. Vertical UV radiometer profiles together with Chl-a concentration, and UV absorption by CDOM (aCDOM(λ)) and by suspended particulate matter (ap(λ)) were measured at 117 stations. At PAR and across UV-A and UV-B wavelengths, the lowest downwelling attenuation coefficients (Kd) during the expedition were recorded in ultra-oligotrophic regions at 5ºS–15ºS (mean Kd(305nm): 0.129 m-1, mean Kd(313nm): 0.107 m-1) in the Indian and South Pacific Oceans. The waters here were comparatively more transparent than at 5ºN–15ºN (mean Kd(305nm): 0.239 m-1, mean Kd(313nm): 0.181 m-1) where Equatorial upwelling occurs. Kd was highest near the Costa Rica Dome (Kd(313nm): 0.226 m-1) and at the confluence of the Benguela and Agulhas currents (Kd(313nm): 0.251 m-1). The contribution of ap(λ) towards non-water absorption (anw(λ)) was significantly lower at 305 nm than at 313 nm and 320 nm, suggesting the contribution of absorption by detritus and phytoplankton particles decreases compared with that of CDOM absorption as UV-B wavelength decreases. Both aCDOM(λ) and ap(λ) at UV-B wavelengths were lowest in the Indian Ocean whereas Kd was lowest in the South Pacific. This finding emphasizes that other factors besides absorption, such as scattering by reflective phytoplankton or inorganic particles, strongly influence UV-B attenuation in open ocean waters.