A model for partitioning particulate absorption into phytoplanktonic and detrital components

Abstract

A model for partitioning total particulate absorption, measured on glass fiber filters, into phytoplanktonic and detrital components is developed. The model reconstructs absorption spectra for living phytoplankton using total particulate absorption at the red absorption maxima for chlorophylls a and b, concentrations of chlorophyll a and pheopigment, and mean normalized absorption spectra for laboratory-grown algal cultures. The model was developed in stages for two types of phytoplankton assemblages. Section A of the model applies to waters dominated by eukaryotic algae and is based on absorption spectra for chromophytic (phytoplankton containing chlorophyll c) and chlorophytic (containing chlorophyll b) species. Section B of the model, allowing more variability in spectral shape, was developed for algal communities with more diverse pigmentation. All spectra are processed through Section A, with an internal evaluation determining whether processing continues through Section B. Detrital spectra, generated as the difference between total particulate and modelled phytoplanktonic spectra, included pheopigment absorption and had high blue absorption. Blind tests on samples of known composition predicted absorption within 8–10% at 436 nm and 1–13% when averaged from 400 to 700 nm, which is within the expected accuracy of the glass fiber filter method. No true measure of phytoplankton absorption in field samples is available for testing the model, but results from methanol-extractions were used for comparison despite inclusion of pheopigment absorption as “phytoplankton”. For samples collected from coastal waters of Washington State, the Sargasso Sea and coastal waters of Norway, modelled absorption (averaged over 400–700 nm) ranged from 25% lower to 0.5% higher than the methanol-extraction results; pheopigment absorption inappropriately included in the phytoplankton component accounts for the higher phytoplanktonic absorption estimated by the methanol technique. Successful application of the model to samples from widespread locations, of different absorption magnitudes, and with varied spectral shapes indicate that the model can be used in diverse environments.