Decomposition study of in vivo phytoplankton absorption spectra aimed at identifying the pigments and the phytoplankton group in complex case 2 coastal waters of the Arabian Sea

Phytoplankton modify the optical properties of the seawater by altering the subsurface light field. Information on the accessory pigments present in the phytoplankton helps to differentiate major phytoplankton classes or taxonomic groups. The variability in the absorption spectra of phytoplankton and particulate matter of case 2 coastal waters of the Southeastern Arabian Sea were studied from June 2010 to November 2011. The phytoplankton specific absorption coefficient, at 440 nm and 675 nm, a*_ph (440) and a*_ph (675) varied from 0.018 to 0.32 m² mg^-1 and from 0.0005 to 0.16 m² mg^-1, respectively. The 4^th derivative spectra computed for each in vivo absorption spectrum showed that the amplitude of maxima obtained is proportional to the concentration of the chromoprotein which absorbed that wavelength. Regression of pigment concentration against the 4^th derivative spectral coefficient showed that the measurements of particulate absorption could provide quantitative information on chlorophyll α and other accessory pigment concentrations. Fucoxanthin and diadinoxanthin, the carotenoid pigments found in the diatoms were identified from the derivatives peaks. The study demonstrates the utility of using the 4^th derivative analysis as a tool to identify the dominating phytoplankton group and its pigment composition.