Phytoplankton at fronts are subjected to physical forcing at multiple spatiotemporal scales. To better understand why phytoplankton are distributed where they are in a front, and determine whether phytoplankton undergo net growth or decay, my dissertation focuses on characterizing the physical motions at fronts and the rates of change of phytoplankton. To quantify the rate of change of phytoplankton, I developed a "pseudo-Lagrangian" method that tracks biological tracers, allowing the calculation of net specific rates of growth. The method increases the number and spatial coverage of rate estimates relative to traditional methods. I also derived error estimates for these rates. Applying this method to other tracers will significantly increase the number of rate estimates that can be used, for example, as constraints in biogeochemical budgets. Using high-resolution hydrographic data from a front, I identified fine-scale features in the phytoplankton distribution. I diagnosed cross-frontal vertical velocity shear as the responsible mechanism. A plausible source for this shear was ageostrophic forcing from frontogenesis upstream. Using remote sensing and scaling arguments, I calculated the timescale of the relevant forcing. This shearing mechanism converts existing horizontal gradients into vertical gradients, with consequences for phytoplankton and for zooplankton grazers. Future phytoplankton studies at fronts will have to consider this mechanism and the structures it produces. I also surveyed two submesoscale instabilities at fronts, and diagnosed their impacts on phytoplankton communities. The short timescales of the instabilities have precluded field observation until recently, and modeling studies have only recently resolved their dynamics. Exploration of the biological impacts of these specific instabilities via models or observations remains to be conducted. Therefore, by characterizing the motion, prerequisite conditions, and biological impacts of instabilities, I communicated their dynamics to the community of biological oceanographers in order to stimulate research to address this gap in knowledge. The results from this dissertation can thus be applied in future observational and modeling studies of phytoplankton at fronts: markedly increasing the number of in situ growth rate measurements, aiding in the identification of mechanisms structuring phytoplankton distributions, and providing a guide for investigating the rapid changes of phytoplankton within regions of flow instabilities
