<title>Spectral diffuse attenuation of phytoplankton measured by the filter pad technique: are we on the right track?</title>

Abstract

The measurement of light absorption by photosynthetic microalgae is of concern to algal physiologists and plankton ecologists. Interest in this topic by algal physiologists originated with the measurement of quantum efficiency. Recently, plankton ecologists have also felt the need for accurate measurements of phytoplankton quantum efficiency within the submarine light field. In addition, the need for absorption measurements is pivotal to the development of algorithms for global remote sensing. The pioneering research in algal light absorption was done by physiologists such as Shibata and Duysens continued by Latimer and Butler. Their techniques were applied to natural phytoplankton populations by Kiefer and Yentsch. In the course of these developments we may have overlooked the essential problems of absorption measurements. The papers of Shibata and Duysens make the point that spectral definition concerns the ratio of absorption to scattering. The instrumental constraints associated with the measurement technique changes that ratio. It is unclear how much scattering is associated with the phytoplankton as opposed to the optical configuration. Perhaps our present methods are not realistic for optically characterizing natural waters. In this paper we compare spectral absorption measured in a configuration where absorption markedly dominates scattering to situations where scattering is a major component of attenuation. We have placed these extremes in the context of optical closure of light attenuation in natural waters.