Derivative analysis of spectral absorption by photosynthetic pigments in the western Sargasso Sea

Concurrent measurements of the spectral absorption coefficient and photosynthetic pigmentation of natural particulates were performed to determine the principal pigments responsible for the absorption of spectral irradiance in seawater. The spectral absorption coefficient, Ap(λ), was then analyzed by taking the second and fourth derivatives with respect to wavelength. The wavelength and magnitude of these derivative values provide useful information regarding the identification and quantification of phytoplankton pigments responsible for a given spectral signature. Linear relationships were examined and established between derivative values at selected wavelengths and concentrations of the major tetrapyrrole pigments, specifically chlorophylls a, b, and c. The correlation between derivative values near 526 nm and concentrations of photosynthetic carotenoids was poor and presumably caused by the broad absorption spectra of these pigments. A comparison of the measured particulate absorption coefficient with the absorption coefficient reconstructed for the phytoplankton component revealed that detritus can be a major source of light absorption. The method described here provides a rapid means of obtaining estimates of photosynthetic pigment concentrations in natural samples where absorption can be strongly influenced by detrital matter