Size and refractive index of individual marine particulates: a flow cytometric approach

Flow cytometric measurements of light scatter, near-forward light scatter (theta = 1.5-19 degrees ) and side scatter (theta = 73-107 degrees ), from individual marine particles is modeled using Mie theory. Particles are assumed to be homogeneous and nearly spherical in shape. Uniform polystyrene microspheres and oil suspensions are used to estimate model accuracy. Within the particle diameter range of 1-32 microm, the mean error for near-forward scatter and side scatter is 16.9% and 30.1%. respectively. The model is used to estimate size and refractive index of several nannoplankton species and the results are compared with microscopic measurements of cell size and published values of phytoplankton refractive index. Within the refractive-index range of 1.01-1.1, the model may be inverted to yield refractive index with an absolute error of between 0.01 and 0.003. Measurements of particle size distributions in clear ocean water indicate this range accounts for 99% of all nannoplankton and 39% of all particles within the size range from 1 to 10 microm.