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The Q-factor—the ratio between upwelling irradiance and upwelling radiance—describes the bidirectional structure of seawater apparent optical properties as a function of geometry and of marine and atmospheric optical characteristics. A 3-yr time series of Qn() measurements—the Q-factor determined by nadir radiance—from the North Adriatic Sea coastal waters, has been analyzed. Scatter plots of Qn() versus sun zenith, 0, for different intervals of the diffuse attenuation coefficient Kd() have shown a consistent exponential trend. In addition, leastsquares regressions of Qn() versus Qn(490) have exhibited determination coefficients R 2 that vary from 0.77 to 0.94 in the spectral range between 412 and 555 nm and R 2 0.50 at 665 nm. To account for these findings, an empirical model for Qn() as a function of 0 and Kd(0) at the reference wavelength 0 490 nm is proposed for North Adriatic coastal waters. The capability of modeling the radiance distribution in marine water is a basic step toward the development of advanced remote-sensing techniques for the accurate determination of optically significant components dissolved or suspended in seawater. In this framework, efforts have been devoted to the study of the nonisotropic character of the light distribution in seawater, conveniently expressed through the Q-factor. The latter, defined as the ratio between the upwelling irradiance Eu(z,) and radiance Lu(,,z,) at wavelength (Tyler 1960; Austin 1979), is a function of (1) the measurement geometry (the sun zenith angle 0, the viewing angle , the azimuth difference between the sun and the observation planes, and depth z), (2) the seawater inherent optical properties (the absorption coefficient a, the scattering coefficient b, and the scattering phase function Pw), (3) the atmospheric optical properties (the aerosol optical thickness a and scattering phase function Pa), and (4) sea state and cloud cover. Simplifying the formalism by explicitly showing the dependence on the observation geometry and wavelength only, the Q-factor, in units of sr, is given by Q(,,z,) Eu(z,)/Lu(,,z,).

Optical measurements performed during the summer of 1997 from an off-shore oceanographic tower in the northern Adriatic Sea in the presence of mucilage matter are presented. These measurements include apparent (diffuse attenuation coefficient, reflectance) and inherent (absorption and beam attenuation coefficients) optical properties of the water column at several wavelengths. The presence of mucilage layers (which was confirmed by divers) was associated with unique, very sharp subsurface maxima in the vertical distribution of both apparent and inherent optical properties. These layers were characterized by very high values of the diffuse attenuation coefficient of upwelling radiance and the beam attenuation coefficient, as well as the ratio of the scattering-to-absorption coefficients at all wavelengths. The mucilage data were compared to nonmucilage observations and two situations came out regarding surface reflectance ratios: (a) When the mucilage was in (vertically narrow) subsurface layers, mucilage events could not be distinguished from nonmucilage situations; and (b) When the mucilage was more homogeneously distributed throughout the water column and, in particular, approaching the surface, there was a detectable difference with respect to nonmucilage water. However, the data set is small, and the detection capability is based on a small difference, so the variance in nonmucilage water properties may still mask the properties of the mucilage-contaminated water.