CHANGES IN THE OPTICAL-PROPERTIES OF A PARTICLE SUSPENSION CAUSED BY PROTIST GRAZING
Résumé
The effect of grazing within microbial food webs on the optical properties of seawater was studied in the laboratory by inoculating a culture of cyanobacteria Synechocystis with an oligotrichous ciliate, Strombidium. During the 4 week experiment sequential measurements of spectral absorption and scattering coefficients were supplemented by determinations of the particle size distribution, prey and predator numbers and pigment concentration. All of these parameters changed dramatically during the course of the experiment. Protist grazing caused a large decrease in the concentration of particles that ranged from 0.75 to 2.5-mu-m in size; Synechocystis is within this range. On the other hand, the concentration of particles <0.75-mu-m increased. These fine particles consist of small heterotrophic bacteria and disintegrated detrital material. In addition, the concentration of particles in the range of 2.5-16-mu-m also increased; this size range included a growing population of pigmented flagellates and detritus, Such particle dynamics were associated with large decreases in both the scattering and the absorption coefficient. The spectral shape of the scattering coefficient flattened, and the spectral absorption coefficient shifted to enhanced absorption at shorter wavelengths. These changes suggest that detrital particles and pigmented microflagellates were the dominant contributors to light scattering and absorption at the end of the experiment.