IRRADIANCE-INDUCED VARIABILITY IN LIGHT SCATTER FROM MARINE-PHYTOPLANKTON IN CULTURE

A series of laboratory experiments are reported that illustrate the re sponse of beam attenuation (lambda = 660 nm) and single-cell light sca tter (lambda = 488 nm) properties of several species of marine phytopl ankton to light intensity. When unialgal cultures were subjected to an increase in light intensity, the particle-scattering component of bea m attenuation and near-forward single-cell light scatter were found to increase rapidly in response. Cell abundance increased only slightly over the course of the experiments, leading to the conclusion that the response in beam attenuation was due to irradiance-induced changes in the single-cell optical properties. The percent hourly increase in be am attenuation, normalized to cell abundance, and single-cell light sc atter ranged from 5% for a culture of the coccolithophore Emiliania hu xleyi to 25% for a culture of Thalassiosira pseudonana. In a separate set of experiments, carbon-specific beam attenuation (cc; the particu late material component of beam attenuation normalized to the concentr ation of particulate organic carbon) was found to be species specific and, to some extent, sensitive to irradiance. The positive response in phytoplankton light scatter, both at the population and at the single -cell level, to an increase in light intensity is similar to diel patt erns in beam attenuation reported for the near-surface ocean. If a com ponent of the observed diel pattern in beam attenuation is due to irra diance-induced, carbon-independent optical variability in the phytopla nkton assemblage, as the results of the high-light experiments suggest , neglecting such variability can result in either an overestimation o r an underestimation of primary production, depending on the response in cc. Natural variability in c*c is poorly understood and responses to environmental factors, such as irradiance, have yet to be addressed outside of the laboratory.