Functional significance of cell size and morphology is a key question in phytoplankton ecology and physiology. Using a mechanistic, organismal approach, I examined the interactions between size and morphology of a phytoplankter and its flow environment. Flow effects on nutrient transfer to cells were studied by applying heat- and mass-transfer theory for stagnant water, steady, uniform flow arising from sinking and swimming, steady shear flow and shear flow arising from dissipation of turbulence. In all cases nutrient transfer increases with increasing cell size and flow velocity. The dependence of nutrient transfer on cell size, however, varies with flow regime.
An emerging issue from this analysis is the motion of a phytoplankter in turbulent flow. I observed the behavior of diatom chains in a shear flow, such as would result from dissipating turbulence, in light of available theory for rigid, elongated spheroids. Both examined species underwent periodic rotation, as predicted by theory. Details of the motion, however, varied between and within species, depending on shape, size and the mechanical properties of the chains (i.e., rigid vs. flexible). Measured periods of rotation for both species were smaller than predicted by theory for rigid spheroids of similar axis ratios.
Shear effects on swimming orientations of dinoflagellates also varied between species. Whereas cells of Glenodinium foliaceum maintained their random swimming orientations upon exposure to shear, swimming orientations of both single cells and chains of Alexandrium catenella were affected by shear. Magnitude of the effect increased with shear rate.
Results endanger assumptions used in models to predict effects of turbulence on nutrient acquisition, migratory behavior of dinoflagellates and predator-prey interactions. Results also carry implications for aggregate formation. Finally, this study points out the diversity of flow regimes experienced by phytoplankton and illuminates some of the connections between morphology, fluid dynamics and individual fitness.