Phytoplankton Spectral Absorption and Scattering for Optical Groups discrimination using complete and reduced spectral information

Recent studies on HABs detection by means of absorption signatures highlight the actual possibility to apply these techniques to ocean color remote sensed data. Briefly, ocean color can be described by the back-scattering to absorption ratio. These inherent optical properties are dominated by phytoplankton in case I waters. The aim of this work is to preliminary test for ocean color discrimination capability to reconstruct taxonomical composition of mixed phytoplankton assemblages. Entry data consist in both complete (316 data) and reduced (9 data) spectra independently obtained, the last being used to mimic those achievable from remote sensing applications. Measurements of these coefficients for six phytoplankton monospecific cultures have been carried out both by a spectroradiometer (Li-COR) with integrating-sphere (TR-TFT method) and by a WetLabs ac-9. Both instruments operate from 400 to 715 nm, Li-COR scans 1 nm spectra and ac-9 nine bands spectra. In this test, we have utilized the phytoplankton absorption joined in a single vector with the scattering or back-scattering, to increase the group-specific spectral information. Phytoplankton identification has been obtained evaluating specific spectral contribution, through the use of the Spectral Angle Mapper technique on normalized-ratio and fourth-derivative transformed spectra. We constructed hypothetical mixed spectra of four optical groups with different pigmentary contents and scaled each spectral group contribution at a time from 0 to 1 by 0.2 steps. Similarity indices calculated on single vector and absorption spectra allow a good discrimination of groups. In mixed assemblages determination coefficients of linear regressions between complete absorption similarity indices and different contribution of each group are always higher than 0.9. Reduced spectra too can allow a good differentiation and can quantify different phytoplankton groups also from mixed assemblages. Besides, joining of absorption and scattering spectra allow even to improve the obtained determination coefficients from absorption alone.