Biogenic particulate material in sea ice can substantially influence the spectral irradiance within the ice sheet and underlying seawater. In order to simulate accurately seasonal changes in light conditions in situ, the biomass changes of the sea ice microbial community must be considered. Here we attempt to provide an improved description of the optical regime within sea ice by combining information provided by models of radiative transfer in sea ice and snow and models of solar spectral irradiance with formulations describing the attenuation of spectral irradiance by particulates observed in sea ice in McMurdo Sound, Antarctica. Emphasis has been placed on the role of biogenic particles in visible light attenuation with the intent of developing a bio-optical model that more rigorously describes their influence on radiative transfer processes as they occur in nature. Model results simulating seasonal changes in both photosynthetically active radiation and its spectral distribution agree well with measured under-ice spectral irradiance. Results reveal how changes in microalgal concentrations, as well as their photophysiological characteristics influence both the quantity and quality of downwelled light in sea ice and in the upper layers of the ice-covered oceans. ¿American Geophysical Union 1991