The interrelationships among snow cover, ice structure, optical properties, and biological activity are of critical importance in understanding the response and behavior of sea ice systems. Of particular concern in this regard are the optical properties of the algae and the effect of snow thickness on biomass accumulation. In this paper we present the first direct in situ measurements of biomass specific diffuse attenuation spectra for arctic ice algae. The data show in situ attenuation values that are about 3 times larger than those obtained from corresponding in vivo absorption measurements, apparently reflecting differences in the geometrical distribution of the algae or the influence of skeletal ice and dissolved organic material. Observations also confirm that maximal algal accumulation occurs when there is a thin layer of snow covering the ice. A new technique to separate the effects of snow and algae in observed transmission spectra is presented. The ratio of transmittance between 600 nm and 450 nm is a sensitive indicator of biomass, while the 700 nm to 600 nm ratio is strongly affected by snow depth. ¿ American Geophysical Union 1993