<p>The dramatic loss of sea ice due to global warming is changing light conditions for marine primary production, but exactly how is not well understood. Previous studies revealed that small peaks in the absorption spectrum of liquid water, due to molecular vibrations of H₂O, delineate a series of spectral niches for aquatic photosynthesis. Ice, however, has a smoother absorption spectrum and scatters light much more strongly than liquid water. Here, we show with a radiative transfer model that the loss of sea ice causes a pronounced blue shift, narrowing light spectra in the euphotic zone to shorter wavelengths. Furthermore, ice cover yields a smooth continuum of light spectra, whereas open water creates distinct spectral niches selecting for phytoplankton species with different photosynthetic pigments. These results indicate that the loss of sea ice will cause major changes in both the pigment and species composition of primary producers in polar ecosystems.</p>