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Scenedesmus and Microcystis usually present in nutrients-enriched freshwater, adapt to the environmental conditions by vertical migration and lead to eutrophication and algal bloom, which results in deteriorated water quality and damaged aquatic ecosystem. The vertical migrations of Scenedesmus and Microcystis with three kinds of light sources were observed in columns. The concentration of chlorophyll a was analyzed to indicate the vertical migration of the algae. The results showed that the extinction coefficients’ rang for Scenedesmus was lower than that for Microcystis, which was due to the algal self shading resulted by the lower concentration of chlorophyll a. Under different illumination, Scenedesmus cells sank down to the bottom due to the gravity and thus were taken away by the samplings, whereas, Microcystis cells floated significantly in the columns all the time. The Stokes’ Law showed that under light sources of I2 and I3 and without light source, the sedimentation coefficients for Scenedesmus were higher than those for Microcystis, while under light source of I1, the sedimentation coefficient was lower than that for Microcystis. The different cell structures lead to different vertical migrating processes: Microcystis keeps balance between buoyancy and gravity using gas-filled structures, whereas, Scenedesmus suspend through clothing a large number of bristles with a complex structure around cells, resulting high viscous forces to help to reducing sinking rates. |
