Optical properties of some freshwater phytoplanktonic algae

Measurements of absorption and scattering of light by pure cultures of some New Zealand freshwater phytoplankters have been made with a spectrophotometer. An integrating sphere accessory was used to capture most of the light scattered by an algal cell suspension and thus give an indication of the true absorption coefficient, with only a small correction required for residual scattering. The purpose of this study was to investigate the factors affecting the relationships of chlorophyll-a concentration to absorption and scattering by a diverse selection of algae. Qualitative differences in absorption spectra of the different phytoplankton studied here can be related to differences in pigment composition. Quantitative differences in the specific absorption coefficients (absorption coefficient divided by Chl-a concentration) at the Chl-a red peak (676 nm in vivo) are explained in terms of different extents of ‘packaging’ of pigment in cells or cell aggregates in the different cultures. Qualitative differences in scattering spectra are explained in terms of optical size of the particulates comprising the pure cultures. The green and diatom cultures displayed a complex-shaped but non-trending scattering spectrum with minima (troughs) in scattering associated with maxima (peaks) in absorption. The blue-green cultures behaved as optically small particles and displayed a pattern of decreasing scattering with increasing wavelength. Quantitative differences in specific scattering coefficients (scattering coefficient divided by Chl-a concentration) were related mainly to differences in the effective ratio of surface areas to Chl-a content of scattering centres in the different cultures. Overall, however, the specific absorption and scattering coefficients at any given wavelength were less variable between cultures than expected suggesting that the common assumption that absorption and scattering by the algal component of a lake water depends only on the Chl-a concentration may be a justifiable first approximation in field studies.