Light backscattering properties of marine phytoplankton: relationships to cell size, chemical composition and taxonomy

Spectral backscattering coefficients were determined for 29 species of cultured marine plankton representing 12 classes using a fixed-angle backscattering meter. Using a multi-angle scattering meter, the volume scattering function was measured and a proportionality constant (chi) between beta(141degrees) and b(b) was determined as 0.82 (+/-0.01 SE), less than the chi value of 1.08 reported for natural waters. Backscattering efficiencies (at 440 nm) of cultures varied between 0.0023 and 0.081 and showed little spectral variation. Plankton backscattering coefficients at 510 nm showed the lowest variability between species when normalized to particulate organic carbon (POC) [4 x 10(-6) (+/- 57% SD) m(2) mg POC-1 at 510 nm], more when normalized to chlorophyll a (Chl a) [8 x 10(-4) (+/- 112% SD) m(2) mg Chl a(-1) at 510 nm], and the greatest when normalized to cell number concentration [9 x 10(-13) (+/- 238% SD) m(2) mg cell(-1) at 510 nm]. There were large variations in the relationships between Chl a, POC and backscattering within and between species. The dinoflagellates were the most efficient backscatterers, owing to their high POCi and D. The diatoms were mid-range in Q(bb) because the presence of the vacuole decreased POCi. The cyanophytes, eustigmatophytes and heterotrophic bacteria were the least efficient scatterers owing to their small cell sizes. Comparison of experimental Q(bb) to those predicted by Mie's scattering model that represents a phytoplankton cell as a homogeneous sphere probably overestimates the value of the real refractive index for these species. Scattering models that consider more complex cellular structure are likely to provide better closure with experimental results.