Benthic effects on the polarization of light in shallow waters

Abstract

Measurements of the upwelling polarized radiance in relatively shallow waters of varying depths and benthic conditions are compared to simulations, revealing the depolarizing nature of the seafloor. The simulations, executed with the software package RayXP, are solutions to the vector radiative transfer equation, which depends on the incident light field and three types of parameters: inherent optical properties, the scattering matrix, and the benthic reflectance. These were measured directly or calculated from measurements with additional assumptions. Specifically, the Lambertian model used to simulate benthic reflectances is something of a simplification of reality, but the bottoms used in this study are found to be crucial for accurate simulations of polarization. Comparisons of simulations with and without bottom contributions show that only the former corroborate measurements of the Stokes components and the degree of linear polarization (DoLP) collected by the polarimeter developed at the City College of New York. Because this polarimeter is multiangular and hyperspectral, errors can be computed point-wise over a large range of scattering angles and wavelengths. Trends also become apparent. DoLP is highly sensitive to the benthic reflectance and to the incident wavelength, peaking in the red band, but the angle of linear polarization is almost spectrally constant and independent of the bottom. These results can thus facilitate the detection of benthic materials as well as future studies of camouflage by benthic biota; to hide underwater successfully, animals must reflect light just as depolarized as that reflected by benthic materials.

Going Beyond Standard Ocean Color Observations: Lidar and Polarimetry

“…The matrix is calculated for spherical particles. The polarization elements (F 12 /F 11 , F 33 /F 11 , and F 34 /F 11 ) of the scattering matrix of phytoplankton particles are similar to those obtained for Rayleigh scattering, because the relative refractive index is very low, i.e., 1.06, similar to what was presented in Figure 4 of Gilerson et al (2013). In contrast, the shape of the polarized scattering matrix elements of NAPs are significantly different, due to their high refractive index, with exception of the near forward and backward direction where it exhibits a weak polarization effect according to Mie-Lorenz theory.…”

“…Their conclusion is that Mie scattering cannot reproduce the measured scattering matrix due to the limitations of sphericity assumption. Volten Ottaviani et al, 2018 POLRADS 410, 436, 486, 526, 548, and -Boss et al, 2007;Sullivan and Twardowski, 2009;You et al, 2011;Gilerson et al, 2013).…”

Use of Hyperspectral Imagery to Assess Cryptic Color Matching in Sargassum Associated Crabs

“…However, more refined treatment of the spectral absorption and scattering properties of the water column can be important for assessing camouflage in many ecosystems. In optically shallow waters, for example, reflectance from the seafloor impacts the spectral shape, magnitude, and polarization characteristics of the light field [ 2 ]. Organisms that hide in such environments have to match downwelling light from the sun, as well as upwelling light reflected from the seafloor (sediment, seagrass, corals, etc.)…”

“…Organisms that hide in such environments have to match downwelling light from the sun, as well as upwelling light reflected from the seafloor (sediment, seagrass, corals, etc.) and often match the polarization conditions that may be visible to certain predators [ 2 ].…”

“…Biological camouflage, the ability to avoid detection or recognition by an observer, has evolved in multiple phyla as an adaptation to visually-orienting predators and prey. The study of marine camouflage systems has relevance to ecology and animal behavior as well as human naval operations, and to target discrimination in the water column [ 2 , 3 ]. A wide variety of strategies exist, and the classification of camouflage techniques is a complex field [ 4 ].…”