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Vertical profiles of ultraviolet radiation (UVR) and photosynthetically available radiation (PAR) were measured under five large ice floes 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.

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[1] The absorption of ultraviolet and visible radiation by colored or chromophoric dissolved organic matter (CDOM) drives much of marine photochemistry. It also affects the penetration of ultraviolet radiation (UV) into the water column and can confound remote estimates of chlorophyll concentration. Measurements of ocean color from satellites can be used to predict UV attenuation and CDOM absorption spectra from relationships between visible reflectance, UV attenuation, and absorption by CDOM. Samples were taken from the Bering Sea and from the Mid-Atlantic Bight, and water types ranged from turbid, inshore waters to the Gulf Stream. We determined the following relationships between in situ visible radiance reflectance, L u /E d (l) (sr À1 ), and diffuse attenuation of UV,. Consistent with published observations, these empirical relationships predict that the spectral slope coefficient of CDOM absorption increases as diffuse attenuation of UV decreases. Excluding samples from turbid bays, the ratio of the CDOM absorption coefficient to K d is 0.90 at 323 nm, 0.86 at 338 nm, and 0.97 at 380 nm. We applied these relationships to SeaWiFS images of normalized water-leaving radiance to calculate the CDOM absorption and UV attenuation in the Mid-Atlantic Bight in May, July, and August 1998. The images showed a decrease in UV attenuation from May to August of approximately 50%. We also produced images of the areal distribution of the spectral slope coefficient of CDOM absorption in the Georgia Bight. The spectral slope coefficient increased offshore and changed with season.

International audienc

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