Motion of diatom chains in steady shear flow

Abstract

Information on the motion of phytoplankton in shear flows is essential for predicting or understanding effects of turbulence on processes such as nutrient uptake, aggregate formation, and phytoplankter-berbivore interactions. Of particular interest is the motion of diatom chains because they are expected to benefit most from enhancement of nutrient flux due to turbulence and are often the most abundant components in aggregates. We studied the motion of two chain-forming diatoms, Skeletonema costatum and Thalassiosira nordenskioldii, in steady shear flow and in the light of available theory for rigid, elongated spheroids. Both species underwent periodic rotation upon exposure to a simple shear flow, as predicted by theory. Whereas the rotational orbits of S. costatum resembled those predicted by theory for rigid spheroids, the rotational motion of T. nordenskioldii was more like the motion of flexible fibers observed in engineering applications. Periods of rotation of S. costatum increased linearly with increasing axis ratio, whereas no clear relationship was observed between periods of rotation and axis ratios for T. nordenskioldii. Measured periods of rotation of both species were similar than predicted by theory for rigid spheroids of similar axis ratios. The diverse behaviors observed imply that fluxes of nutrients and collision frequencies experienced vary greatly with detailed shapes and mechanical properties of chains and their unit cells.