Irradiance-induced variability in light scatter from marine phytoplankton in culture

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Abstract
A series of laboratory experiments are reported that illustrate the response of beam attenuation (λ = 660 nm) and single-cell light scatter (λ = 488 nm) properties of several species of marine phytoplankton to light intensity. When unialgal cultures were subjected to an increase in light intensity, the particle-scattering component of beam 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 beam attenuation, normalized to cell abundance, and single-cell light scatterranged from 5% for a culture of the coccolithophore Emiliania huxleyi to 25% for a culture of Thalassiosira pseudonana. In a separate set of experiments, carbon-specific beam attenuation (c *c ; the paniculate material component of beam attenuation normalized to the concentration of paniculate 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 diet patterns in beam attenuation reported for the near-surface ocean. If a component of the observed diel pattern in beam attenuation is due to irradiance-induced, carbon-independent optical variability in the phytoplankton assemblage, as the results of the high-light experiments suggest, neglecting such variability can result in either an overestimation or an underestimation of primary production, depending on the response in cc *c . Natural variability in cc *c is poorly understood and responses to environmental factors, such as irTadiance, have yet to be addressed outside of the laboratory.

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