Arctic phytoplankton is the base of the foodweb in the most rapidly changing region on the planet. Within this thesis I show that Arctic coastal diatom communities can sustain stable productivity under future climate scenarios by several mechanisms: though large phenotypic plasticity, sorting between highly diverse genotypes and species shifts. The specific focus of this work lies on the role and functioning of intraspecific selection and population dynamics of the diatom Thalassiosira hyalina. Through application of new population genetic methodologies, I was able to resolve detailed selection dynamics in simplified diatom populations and found that their outcomes are not predictable from responses recorded in single-strain cultures, hinting towards interactions between conspecifics. Furthermore, my experiments and field studies on natural populations suggest that dynamics within populations are likely driven by more complex processes than clonal dominance. Forecasts on the future functioning of phytoplankton populations are therefore bound to include intraspecific diversity and evolutionary adaptation as crucial processes.