Our view of phytoplankton has historically revolved around their inability to control their location in space. The term phytoplankton itself underscores this particular difference between phytoplankton and their sessile terrestrial counterparts. Yet there are other differences between land plants and the phytoplankton that are perhaps equally important, beyond this sessile-planktonic dichotomy, to their growth, survival, and productivity. For example, phytoplankton are microbes and thus are short-lived, with generational scales on the order of days or less. An intriguing question to ask is how today���s pelagic ecology would differ, had this temporal difference between plants and phytoplankton been initially emphasized, perhaps by naming these microbes phytoephemera instead? This dissertation addresses certain aspects of the ecology of phytoplankton that result from their having short generational scales. Because they are so short lived, phytoplankton need to adjust their photosynthetic physiology to cope with more rapid changes in irradiance than may matter to longer-lived plants. Photoacclimation on the hours-plus time scales has been studied extensively in the phytoplankton, because its temporal scales match those of vertical mixing processes in the ocean. Yet most phytoplankton exhibit faster photosynthetic responses as well, down to the time scales of seconds. These photosynthetic responses have received considerably less attention in phytoplankton ecology. This dissertation specifically examines these rapid, seconds-to-hour scale photosynthetic responses in phytoplankton. First, the physiological bases of rapid photosynthetic regulation were examined using a numerical model that shows how specific physiological changes in phytoplankton photosystems either constrain or enhance light harvesting. This model is stochastic, and thus replicates certain nonlinear aspects of light harvesting better than equation-based analytical models. Also in this dissertation, a laboratory study is described that examined rapid photosynthetic regulation in three model phytoplankton. Results suggest that rapid photosynthetic regulation is not only constrained to higher eukaryotic phytoplankton, but also occurs in the two dominant marine photosynthetic prokaryotes, Synechococcus and Prochlorococcus. Finally, rapid photosynthetic responses were examined in field assemblages at Station ALOHA in the North Pacific. This ocean region experiences considerable cloud cover, which may result in a strong degree of rapid photosynthetic responses, even in near-surface assemblages. / Graduation date: 2007