Biogeosciences (Apr 2008)
Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean
Abstract
The particulate scattering, <i>b</i><sub>p</sub>, and backscattering, <i>b</i><sub>bp</sub>, 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, <i>b</i><sub>bp</sub> 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 <i>b</i><sub>bp</sub> are still lacking, especially at low chlorophyll <i>a</i> concentrations ([Chl]). Here, we examine the relationships between spectral <i>b</i><sub>bp</sub> and <i>b</i><sub>p</sub> 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<sup>3</sup>), both <i>b</i><sub>bp</sub> and <i>b</i><sub>p</sub> can be related to [Chl] by power functions (i.e. <i>b</i><sub>p</sub> or <i>b</i><sub>bp</sub>=α[Chl]<sup>β</sup>). Regression analyses are carried out to provide the parameters α and β for several wavelengths throughout the visible for both <i>b</i><sub>bp</sub> and <i>b</i><sub>p</sub>. When applied to the data, these functions retrieve the same fraction of variability in <i>b</i><sub>bp</sub> and <i>b</i><sub>p</sub> (coefficients of determination between 0.82 and 0.88). The <i>b</i><sub>bp</sub> coefficient fall within the bounds of previous measurements at intermediate and high [Chl] recently published. Its dependence on [Chl] below ~0.1 mg m<sup>−3</sup> is described for the first time with in situ data. The backscattering ratio (i.e. <i>b</i><sub>bp</sub>/<i>b</i><sub>p</sub>) with values near 0.01 for all stations appears to be spectrally neutral and not significantly dependent 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.
