Modelling the influence of mineral particles and phytoplankton on chlorophyll estimation from ocean colour remote sensing

The absorption and backscattering properties of phytoplankton were calculated from Mie theory for different slopes of Junge-type size distribution and different complex refractive indices of particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater was analysed as a proxy for reflectance of various concentrations of phytoplankton and mineral particles in seawater. The results indicate that the chlorophyll-specific absorption and backscattering are very sensitive to variations in size distribution and refractive index. Therefore, phytoplankton with a steeper slope of size distribution or stronger absorption ability could cause higher OC4v4-derived chlorophyll concentrations than those with a gentler slope of size distribution or lower absorption properties. Mineral particles also have a strong effect on the determination of chlorophyll concentration. With a gradual increase in mineral concentration, an overall increase in the magnitude of the whole reflectance spectrum is observed, and the spectral maximum tends to shift towards longer wavelengths. The mineral particles may cause an overestimation of OC4v4-derived chlorophyll at low chlorophyll concentrations and an underestimation at high chlorophyll concentrations compared to that in seawater without mineral particles.