Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations

TitleCharacterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations
Publication TypeJournal Article
Year of Publication2014
AuthorsLi L.H, Stramski D, Reynolds R.A
JournalDeep-Sea Research Part I-Oceanographic Research Papers
Date Published2014/05
Type of ArticleArticle
ISBN Number0967-0637
Accession NumberWOS:000335286400007
KeywordsApparent; asymptotic daylight field; Asymptotic light field; bioluminescence; chlorophyll-a; crustaceans; deep-sea; downwelling irradiance; marine animals; Mesopelagic zone; Ocean optics; optical properties; pure water; Radiative transfer modeling; raman-scattering coefficient; spectral sensitivities; underwater daylight; Underwater light field

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200-1000 m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by similar to 9-10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475 nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565 nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (K-40) indicates that the solar irradiance dominates over the K-40-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400-500 m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly asymptotic regime at depths exceeding similar to 400-500 m. (c) 2014 Elsevier Ltd. All rights reserved.

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