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Spectral variability in the visible, near-infrared and shortwave directional reflectance factor of beach sands and freshwater sheet flow is examined using principal component and correlation matrix analysis of in situ measurements. In previous work we concluded that the hyperspectral bidirectional reflectance distribution function (BRDF) of beach sands in the absence of sheet flow exhibit weak spectral variability, the majority of which can be described with three broad spectral bands with wavelength ranges of 350-450 nm, 700-1350 nm, and 1450-2400 nm. 1 Observing sheet flow on sand we find that a thin layer of water enhances reflectance in the specular direction at all wavelengths and that spectral variability may be described using four spectral band regions of 350-450 nm, 500-950 nm, 950-1350 nm, and 1450-2400 nm. Spectral variations are more evident in sand surfaces of greater visual roughness than in smooth surfaces, regardless of sheet flow.

It is well-known that in the ocean, where the depth of the ocean floor is large compared with the attenuation length of irradiance, the diffuse attenuation coefficients for vector and scalar irradiance (K-functions) are not affected by the optical properties or proximity of the ocean floor. This is the case of an optically deep ocean where the attenuation coefficients are determined solely by the inherent optical properties of the water and the distribution of radiance. Since, within optical-deep water, variability in the K- functions due to radiance distribution are small relative to the effects of the inherent optical properties of water, K- functions have been treated as quasi-inherent optical properties. Furthermore, when the depth of the ocean floor is shallow enough so that it becomes illuminated by down- welling irradiance, i.e. when the ocean is optically shallow, the in-water light field is modified by the optical properties of the ocean floor. The effect decreases with increasing depth and distance from the ocean floor. It is not generally appreciated, however, that the associated K- functions will also be affected by both the optical properties and the proximity of the ocean floor and, therefore, cannot be treated as quasi-inherent optical properties. If these effects are neglected, large errors, exceeding 25 percent in some cases, can result from modeling the optically shallow scalar irradiance profile as a function of a constant diffuse attenuation coefficient.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In modeling the optical properties of and radiative transfer within ocean water, a common assumption is that the optical properties of the various dissolved and particulate constituents per unit concentration are constant. While this assumption is probably valid for non-living materials, it is frequently not the case with phytoplankton. Living cells are dynamic in their size, shape, and composition and respond readily (time scale of minutes to hours) to changes in the environment. Since each of these factors plays a role in determining the light scatter and absorption properties of the cell, the assignment of optical constants to phytoplankton cells, especially within Case I waters, may introduce significant errors in any theoretical treatment of radiative transfer and may result in erroneous interpretations of bulk optical measurements. Single-cell light scatter and beam attenuation of several species of marine phytoplankton, Thalassiosira pseudonana (clone 3H), an unidentified prasinophyte (clone OMEGA 48-23), Paviova sp. (clone NEP), Emiliania huxleyi (clone BT6), and an unidentified cryptomonad (clone 1D2), respond rapidly to increased light intensities. Flow cytometric determinations of cell refractive index n, measured at 488 nm, was found to decrease under high light conditions while cell size D, expressed as equivalent spherical diameter, increased. Beam attenuation c, measured at 660 nm, was generally found to increase when cultures were exposed to high light intensities. With a constant cell concentration, the observed change in n tends to decrease beam attenuation, while an increase in D tends to increase c. The magnitude and sign of dc/dt will depend upon the relative change in mean cell refractive index ii and the mean cell size i In response to bright light, i5f 3H and NEP was found to increase more rapidly than particulate organic carbon (POC). POC normalized to total cell volume increased within NEP and decreased within 3H', while at the same time, decreased within both cultures, suggesting that the cells swelled. As a result of light induced changes in i and ! optical properties of cells can change independently of biomass.

A Coulter EPICS 741 Flow Cytometer was used at sea to measure distributions in phytoplankton refractive index and size within the North Sea. The optical data gathered by the flow cytometer, multi-band fluorescence and multi-angle light scatter, agreed well with coincident measurements of in vivo fluorescence. By gating on fluorescence, unique subpopulations of phytoplankton may be isolated optically from all other particles within a natural population. At several coastal stations, a unique subpopulation of phytoplankton was found which appeared to contain both chlorophyll and phycoerythrin. Using Mie scatter theory and flow cytometric measurements of light scatter in the near-forward and side directions, this population was found to be composed of cells of refractive index between 1.07 and 1.095 and spherical diameter between 7 and 9 pm (verified using Coulter volume measurements).