Ultraviolet radiation and bottom-ice algae: Laboratory and field studies from McMurdo Sound, Antarctica

Abstract

Springtime ozone depletion in the Antarctic has allowed high levels of ultraviolet radiation to reach the Earth's surface, and this may adversely affect the productivity of phytoplankton growing in the Southern Ocean. At this time of the year, sea ice can be surprisingly transpar­ ent to UV radiation. Bottom-ice algae grow in the bottom few centimeters of such ice where they cannot avoid exposure to the UV radiation falling on them, and thus they provide a good model for a preliminary study of the relationship between UV radiation and plankton. We have measured the ultraviolet-B (UV-B, 280-320 nm) radiation that falls onto sea ice in McMurdo Sound, Antarctica, and found that incident fluxes approximately doubled as a result of the ozone hole in the spring of 1990. Multiple scattering due to the high surface albedo meant that polarization of the radiation was low, reaching a maximum of 40%. A simple power law relationship between air mass and intensity was found and used to describe daily averaged total irradiances, once a surface albedo factor was included. The effect of UV-B radiation on sea-ice algae was studied in the laboratory, and a 5% reduction in photosyntheti c production for UV-B levels equivalent to those expected under 1.8-mthick sea ice was observed. In situ studies modifying the UV-B radiation falling onto algae showed that low-level enhancement of UV-B radiation had no effect on chlorophyll-*? con­ tent per cell, but the elimination of UV-B from the incident radiation produced a significant increase. Together these results suggest that the present conditions of elevated UV-B radia­ tion due to ozone depletion will have little effect on bottom sea-ice algae.