Abstract
The backscattering properties of marine phytoplankton, which are assumed to vary widely with differences in size, shape, morphology and internal structure, have been directly measured in the laboratory on a very limited basis. This work presents results from laboratory analysis of the backscattering properties of thirteen phytoplankton species from five major taxa. Optical measurements include portions of the volume scattering function (VSF) and the absorption and attenuation coefficients at nine wavelengths. The VSF was used to obtain the backscattering coefficient for each species, and we focus on intra- and interspecific variability in spectral backscattering in this work. Ancillary measurements included chlorophyll-a concentration, cell concentration, and cell size, shape and morphology via microscopy for each culture. We found that the spectral backscattering properties of phytoplankton deviate from theory at wavelengths where pigment absorption is significant. We were unable to detect an effect of cell size on the spectral shape of backscattering, but we did find a relationship between cell size and both the backscattering ratio and backscattering cross-section. While particulate backscattering at 555 nm was well correlated to chlorophyll-a concentration for any given species, the relationship was highly variable between species. Results from this work indicate that phytoplankton cells may backscatter light at significantly higher efficiencies than what is predicted by Mie theory, which has important implications for closing the underwater and remotely sensed light budget.
Highlights
The structure and function of marine ecosystems are largely dictated by the composition of the phytoplankton community
The volume scattering function (VSF) was used to obtain the backscattering coefficient for each species, and we focused on intra- and interspecific variability in spectral backscattering in this work
While our observations indicated that spectral backscattering differences were not influenced by particle size or shape, the magnitude backscattering ratio, in contrast, was quite sensitive to particle composition
Summary
The structure and function of marine ecosystems are largely dictated by the composition of the phytoplankton community. The bulk inherent optical properties of the ocean (absorption and angular scattering) are determined by the sum of those of it’s constituents (water, particles, and dissolved substances) [1,2]. Our understanding of how diverse phytoplankton taxa contribute to bulk optical measurements remains limited. This is especially true of the backscattering properties of marine phytoplankton, due to a paucity of measurements of this parameter on laboratory cultures [4]. A comprehensive understanding of how marine phytoplankton contribute to the bulk backscattering signal in the ocean is critical for accurate interpretation of remote sensing data because the ocean reflectance is determined by the ratio of backscattering to absorption [1]. The development of inversions of ocean color to retrieve the concentration and type of the underlying particles strongly relies upon information about the factors influencing the backscattering coefficient and spectrum in the ocean
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