An optical classification of coastal and oceanic waters based on the specific spectral absorption curves of phytoplankton pigments, dissolved organic matter, and other particulate materials1

TLDR

Seawater spectral absorption varies mainly with phytoplankton, nonchlorophyllous particles, and yellow substances, quantified by chlorophyll a/pheophytin a (C), total scattering (b), and filtered seawater absorption (Y). The study develops an iterative method to retrieve the specific spectral absorption curves of phytoplankton and nonchlorophyllous particles and proposes an optical classification based on the three coefficients. The method iteratively estimates weighting coefficients from over 80 spectral measurements across marine regions and examines their relationships with C, b, and Y. Reconstruction using the three coefficients achieves a 3 % average error, and absorption efficiency per unit C varies with water type and phytoplankton population.

Abstract

The variations of the spectral absorption coefficient of seawater can be attributed to three principal factors—phytoplankton, nonchlorophyllous particles, and yellow substances— whose concentrations are represented in this study in terms of chlorophyll a and pheophytin a content ( C ), the total scattering coefficient ( b ), and the absorption by filtered seawater at a given wavelength ( Y ). By assuming an exponential absorption by yellow substances, we developed a new iterative method for identifying the in situ apparent specific spectral absorption curves of the other two factors. The weighting coefficients for each of the three components are calculated for over 80 spectral absorption measurements from different representative marine regions. The reconstruction of spectral absorption curves from the three coefficients and the three specific absorption curves fits the original data with an overall average error of 3%. Relationships between these coefficients and C , b, and Y are examined. Results show that, though the spectral form of absorption by pigments can be considered as more or less invariant, the absorption “efficiency” per unit C depends on the nature of the water, and hence on the type of phytoplankton population considered. An optical classification based on the three optical coefficients related to three absorbing agents is proposed.