Measurement of Visible Light Absorption by Particulate Matter in the Ocean1

For the measurement of visible light absorption by particles in sea water, optical arrangements have been devised where scattered light is integrated and the photodetector “sees” mostly diffused light. High spectral resolution has been obtained by removing particulate matter from sea water using a membrane filter and scanning the filter against an unused filter as the reference blank in the Cary spectrophotometer. In this fashion spectral curves for particulate matter in the oceans have been obtained. These curves show pigment banding in the upper 100 m of water characteristic of absorption by chloroplastic pigments. Below this depth the absorption of particulate matter is characterized by a gradual increasing attenuation from long to short wavelengths. Optical density ratios of chloroplastic pigment bands seen in the particulate matter of the upper 100 m suggest the presence of absorbing components not generally seen in algal cultures. Differential filtration through graded filter sizes (5.0 µ ‐0.3 µ ) followed by the spectral scanning of filters, shows that practically all of the chloroplastic pigments are retained by the 5.0 µ filter. Particles passing this filter show little selective attenuation of visible wavelengths. The nature of sea water particulate matter is discussed in terms of its absorbing characteristics as well as the significance to phytoplankton ecology.