Aquatic micro-organisms have evolved a bewildering variety of adaptations and exhibit a wealth of strategies for survival in the marine environment. In this colloquium we focus on how the micro-organisms are impacted by the fluid dynamical regime governing their surroundings. Specifically we encourage presentations that investigate how fluid dynamics affects the behaviour (swimming, feeding and hydrodynamic signalling on small scales), population growth (resource competition on intermediate scales) and spatial distribution (aggregations and patchiness over a range of scales) of the Plankton (in its broadest sense from bacteria to fish larvae). Synergistic interactions between modellers and experimentalists are anticipated.