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Concurrent water samples and optical 8rofiles here taken at twelve stations in the Western Sargasso Sea at approximately 34° N and 70° W (Biowatt Mooring) during August 1987. Optical measurements included up- and downwelling scalar and vector irradiance. In situ relative fluorescence and beam transmission at 660 nm revealed a particle layer the center of which varied between 70 and 110 m and had a breadth of 20 to 40 m. Water samples were collected within the particle layer and near surface and analyzed for particle characterization and volume absorption. Absorption coefficients for the near surface and particle layer were calculated using three techniques. Two techniques employed measurements of apparent optical properties. One method utilized scalar and vector irradiances and calculated absorption as the product of the average cosine and the diffuse attenuation coefficient for net irradiance at wavelengths of 441 and 520 nm. The second method used inversion of the irradiance quartet (up- and downwelling vector and scalar irradiances) and also calculation of absorption from only up-and downwelling vector irradiances. In the third method, absorption coefficients were calculated as the sum of particulate, dissolved substances, and molecular water absorption. Some large discrepancies were found between absorption from apparent properties and those calculated from the component sum. Possible sources of error include overestimation of dissolved substance or particulate absorption, inaccurate "pure water absorption, and influence of internal radiance sources, such as Raman scattering, on apparent optical properties. Particles were characterized by three different techniques: relative particle concentration by in situ transmissometry at 660 nm; particle frequency and volume distributions by resistive pulse technique ("Coulter counts"); and particle frequency and size/shape distribution vs. elemental composition by SAX (Scanning electron microscopy with Automated image analysis and X-ray spectroscopy). Total particle number decreased with depth and was largely due to decreasing concentration of organic particles. There was a small size increase in siliceous particles near depths of the fluorescence maxima. In contrast, backscatter values derived from apparent optical properties increased by nearly two-fold from the surface to depth of maximum fluorescence.
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