Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean

Y. Huot (1) , A. Morel (1) , M. S. Twardowski (2) , D. Stramski (3) , R. A. Reynolds (3)

1 GEOAZUR 6526 - Géoazur
2 Dept. of Research
3 MPL - Marine Physical Laboratory

The particulate scattering, b_p, and backscattering, b_bp, coefficients are determined by the concentration and physical properties of suspended particles in the ocean. They provide a simple description of the influence of these particles on the scattering of light within the water column. For the remote observation of ocean color, b_bp along with the total absorption coefficient govern the amount and spectral qualities of light leaving the sea surface. However, for the construction and validation of ocean color models measurements of b_bp are still lacking, especially at low chlorophyll a concentrations ([Chl]). Here, we examine the relationships between spectral b_bp and b_p vs. [Chl] along an 8000 km transect crossing the Case 1 waters of the eastern South Pacific Gyre. In these waters, over the entire range of [Chl] encountered (~0.02–2 mg m^-3), both b_bp and b_p can be related to [Chl] by power functions (i.e. b_p or b_bp=a[Chl]^ß) Regression analyses are carried out to provide the parameters a and ß for several wavelengths throughout the visible for both b_bp and b_p. When applied to the data, these functions retrieve the same fraction of variability in b_bp and b_p (determination coefficients between 0.82 and 0.88). The b_bp coefficient fall within the bounds of previous measurements at intermediate and high [Chl] recently published. Its dependence on [Chl] below ~0.1 mg m^-3 is described for the first time with in situ data. At these low and decreasing [Chl] a continuous trend with data at higher [Chl] is observed, i.e. a decrease in b_bp. The backscattering ratio (i.e. b_bp/b_p) with values averaging 0.008 is found to have a weak dependence on [Chl]. These results should foster the development of improved forward models of the mean optical properties for oceanic Case 1 waters as well as inverse models based upon them.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

Y. Huot, A. Morel, M. S. Twardowski, D. Stramski, R. A. Reynolds. Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean. Biogeosciences Discussions, 2007, 4 (6), pp.4571-4604. ⟨hal-00330277⟩