Phytoplankton spectral absorption as influenced by community size structure and pigment composition

Assessments were made of the relative importance of package effects and pigment composition in contributing to variations in spectral absorption in shelf waters off North Carolina during May 1997 and off west Florida during October 1998. Measurements of spectral absorption of size-fractionated particulate material on glass fibre filters were made using two methods, the transmittance–reflectance (T–R) method and the quantitative filter technique (QFT). Spectral absorption of phytoplankton pigments was decomposed into a series of 13 Gaussian absorption bands, and absorption band peak heights were related to concentrations of major pigment classes. Maximum weight-specific pigment absorption coefficients for individual absorption bands (p*m) derived from the fit of a hyperbolic tangent function to the data were found to be similar for North Carolina and west Florida shelf waters. The values were used to reconstruct spectral absorption in the absence of pigment packaging, which was then compared to measured absorption to provide an assessment of pigment packaging. Package effects were found to be responsible for up to a 62% reduction in the amplitude of major absorption bands, particularly for samples from low-salinity waters and for populations dominated by larger (>3 μm) phytoplankton. Variations in pigment composition were also found to have an impact, although it was smaller (10–28%), on variations in total absorption. Potential bio-optical applications of the Gaussian decomposition approach include the estimation of pigment concentrations from in situ or remotely sensed ocean colour observations. Alternatively, where pigment concentrations are known, it may be possible to estimate absorption. Successful application of such techniques may necessitate characterizations of coefficients specific to a given region and time.