LAUSR

In spring 2015, we observed an extensive phytoplankton bloom of Phaeocystis pouchetii, with chlorophyll a concentrations up to 7.5 mg m−3, under compact snow-covered Arctic sea ice at 80-81˚N during the Norwegian young sea ICE (N-ICE2015) expedition. We investigated the influence of the under-ice bloom on inherent optical properties (IOPs) of the upper ocean. Absorption and scattering in the upper 20 m of the water column at visible wavebands increased threefold and tenfold, respectively, relative to pre-bloom conditions. The scattering-to-absorption ratio during the Phaeocystis under-ice bloom was higher than in previous Arctic studies investigating diatom blooms. During the bloom, absorption by colored dissolved organic matter (at 375 nm), seemingly of autochthonous origin, doubled. Total absorption by particles (at 440 nm), dominated by phytoplankton (> 90%), increased tenfold. Measured absorption and scattering in the water were used as inputs for a 1D coupled atmosphere-ice-ocean radiative transfer model (AccuRT) to investigate effects of altered IOPs on the under-ice light field. Multiple scattering between sea ice and phytoplankton in the ocean led to an increase in scalar irradiance in the photosynthetically active radiation range (Eo(PAR)) at the ice-ocean interface by 6–7% compared to pre-bloom situation. This increase could have a positive feedback on ice-algal and under-ice phytoplankton productivity. The ratio between Eo(PAR) and downwelling planar irradiance (Ed(PAR)) below sea ice reached 1.85. Therefore, the use of Ed(PAR) might significantly underestimate the amount of PAR available for photosynthesis underneath sea ice. Our findings could help to improve light parameterizations in primary production models.