| Summary: | Variability in the inherent optical properties (IOPs) of seawater and characteristics of the particle assemblage were examined along a transect in the Chukchi Sea during July 2011. This transect encompassed samples from open waters of the marginal ice zone exhibiting low concentrations of chlorophyll- a (Chl a 1img m -3 ), as well as an extensive phytoplankton bloom (Chl a <30 mg m -3 ) beneath consolidated pack ice. Measurements included the spectral coefficients for particulate beam attenuation, backscattering, and absorption, bulk indicators of particle concentration and composition, and the particle size distribution. Within the bloom microphytoplankton contributed >95% to the total Chl a , and relatively small amounts of nonalgal particles were present. This assemblage exhibited low Chl a -specific phytoplankton absorption coefficients (0.006m 2 mg -1 at 676 nm) indicating a strong influence of pigment packaging, and a weak spectral dependence of the particulate backscattering coefficient. In contrast, the phytoplankton community in nutrient-depleted surface waters outside the sea ice had a strong contribution of picoplankton to Chl a (54%) and an increased abundance of nonalgal particles. The Chl a -specific phytoplankton absorption coefficient approached maximum values at 676 nm (0.023 m 2 mg -1 ) and particle backscattering had much stronger spectral dependence. Additional samples from near the sea floor exhibited characteristics of either mineral-dominated assemblages or a mixture of mineral and organic particles, and also displayed optical differentiation from the surface samples. The marked contrast in absorption, attenuation, and backscattering properties of these ecological regimes suggest that they can be discriminated from optical measurements available on a variety of in situ and remote-sensing platforms.
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