Light backscattering efficiency and related properties of some phytoplankters

Abstract

By using a set-up that combines an integrating sphere with a spectroradiometer LI-1800 UW, the backscattering properties of nine different phytoplankters grown in culture have been determined experimentally for the wavelengths domain ν = 400 up to 850 nm. Simultaneously, the absorption and attenuation properties, as well as the size distribution function, have been measured. This set of measurements allowed the spectral values of refractive index, and subsequently the volume scattering functions (VSF) of the cells, to be derived, by operating a scattering model previously developed for spherical and homogeneous cells. The backscattering properties, measured within a restricted angular domain (approximately between 132 and 174°), have been compared to theoretical predictions. Although there appear some discrepancies between experimental and predicted values (probably due to experimental errors as well as deviations of actual cells from computational hypotheses), the overall agreement is good; in particular the observed interspecific variations of backscattering values, as well as the backscattering spectral variation typical of each species, are well accounted for by theory. Using the computed VSF, the measured backscattering properties can be converted (assuming spherical and homogeneous cells) into efficiency factors for backscattering ($\text{Q}{}_{\mathrm{bb}}\text{)}$. Thhe spectral behavior of $\text{Q}{}_{\mathrm{bb}}$ appears to be radically different from that for total scattering $\text{Q}$_b. For small cells, $\text{Q}$ (λ) is practically constant over the spectrum, whereas $\text{Q}$_b(λ) varies approximately according to a power law (λ⁻²). As the cell size increases, $\text{Q}$_bb conversely, becomes increasingly featured, whilst $\text{Q}$_b becomes spectrally flat. The chlorophyll-specific backscattering coefficients ($\text{b}{}_{\mathrm{b}}{}^{\mathrm{\ast }}$ appear highly variable and span nearly two orders of magnitude. The chlorophyll-specific absorption and scattering coefficients, $\text{a}{}^{\mathrm{\ast }}$ and $\text{b}{}^{\mathrm{\ast }}$, are mainly ruled by the interspecific variations in cellssize (D) and intracellular pigment concentration (C_i) (actually by the variations of the product DC_i). Though $\text{b}{}_{\mathrm{b}}{}^{\mathrm{\ast }}$ is involved in the modelling of the diffuse reflectance of waters, the impact of its actual variation is greatly limited because typical $\text{b}{}_{\mathrm{b}}{}^{\mathrm{\ast }}$ values, even at their maximum (10⁻³ m² mg⁻¹), are very low. This result confirms that living algae have a negligible influence on the backscattering process by oceanic waters; other particles (bacteria, detritus, etc.) associated with algae are mainly responsible for this process.