Ultraviolet attenuation by dissolved and particulate constituents of first-year ice during late spring in an Arctic polynya

Vertical profiles of ultraviolet radiation (UVR) and photosynthetically available radiation (PAR) were measured under five large ice flees located in the North Water Polynya, northern Baffin Bay, in June 1998. Together with incident irradiance measurements, these profiles were used to assess the irradiance attenuation by the ice and its constituents. We also measured vertical distribution of absorption by colored dissolved organic matter (CDOM) and particulate organic matter (POM) in three melted ice cores. The ice thickness and snow depth varied from 0.5 to 1.3 m and from 1 to 9 cm, respectively. The ice-snow interface was infiltrated by meltwater. About 2-13% of incident UV-B irradiance was transmitted through the snow, ice, and ice algae biomass; transmittance increased to 5-19% for UV-A and to 5-12% for PAR. CDOM and POM contributed significantly to the attenuation of irradiance within the ice. The relatively high UVR transparency found in this study coincided with the seasonal maximum of incident UV irradiance. Hence, the resulting very high UVR:PAR ratio could affect the communities in the sea ice, at the ice-water interface, and in the surface waters underneath the ice cover. In addition, the strong absorption by CDOM found in this high-UVR environment indicates that significant photochemical reactions could occur.