Phytoplankton in flow

Abstract

Phytoplankton are small, unicellular organisms, which form the base of the marine food web and are cumulatively responsible for almost half the global production of oxygen. While phytoplankton live in an environment characterized by ubiquitous fluid motion, the impacts of hydrodynamic conditions on phytoplankton ecology remain poorly understood. In this thesis, we propose two novel biophysical mechanisms that rely on the interaction between phytoplankton motility and fluid shear and demonstrate how these mechanisms can drive thin phytoplankton layers and microscale cell aggregations. First, we consider 'thin phytoplankton layers', important hotspots of ecological activity that are found meters beneath the ocean surface and contain cell concentrations up to two orders of magnitude above ambient. While current interpretations of their formation favor abiotic processes, many phytoplankton species found in these layers are motile. We demonstrate that layers can form when the vertical migration of phytoplankton is disrupted by hydrodynamic shear. Using a combination of experiments, individual-based simulations, and continuum modeling, we show that this mechanism - which we call 'gyrotactic trapping' - is capable of triggering thin phytoplankton layers under hydrodynamic conditions typical of the environments that often harbor thin layers. Second, we explore the potential for turbulent shear to produce patchiness in the spatial distribution of motile phytoplankton. Field measurements have revealed that motile phytoplankton form aggregations at the smallest scales of marine turbulence - the Kolmogorov scale (typically millimeters to centimeters) - whereas non-motile cells do not. We propose a new mechanism for the formation of this small-scale patchiness based on the interplay of gyrotactic motility and turbulent shear. Contrary to intuition, turbulence does not stir a plankton suspension to homogeneity, but instead drives patchiness. Using an analytical model of vortical flow we show that motility can give rise to a striking array of patchiness regimes. We then test this mechanism using both laboratory experiments and isotropic turbulent flows generated via Direct Numerical Simulation. We find that motile phytoplankton cells rapidly form aggregations, whereas non-motile cells remain randomly distributed. In summary, this thesis demonstrates that microhydrodynamic conditions play a fundamental role in phytoplankton ecology and, as a consequence, can contribute to shape macroscale characteristics of the Ocean.