Temperature and elemental composition are key factors that affect life history traits in algae. Climate change is predicted to cause major temperature changes in the ocean that may affect algal populations, seasonal dynamic and stratification patterns with profound ecological impact. The determination of how species specific life history traits in algae may vary depending on bioavailable phosphorus (P) and temperature is therefore of vital importance. The growth rate and cell size is of fundamental importance for phytoplankton ecology and evolution. Theory predicts smaller cell size at increasing temperature, either directly related to temperature or indirectly through nutrient scarcity. To address these issues, a factorial experiment with Emiliania huxleyi, Chrysochromulina rotalis and Prymnesium polylepis was conducted. In order to effectively induce P limitation, the cultures with lower P were cultured as chemostats, while the cultures with elevated P were grown as turbidostats. Responses to temperature and P were studied in terms of cellular RNA content, alkaline phosphatase activity, stoichiometry and genome size, combined with quantitative measurements of density and cell size. In general all parameters responded to P and temperature, yet with somewhat different responses for different algae. The results strongly indicate that temperature is the governing factor of plasticity in cell size as predicted from Temperature-Size Rules. The growth rate was primarily affected by P-treatment. The stoichiometric response show that N:P combined with RNA indicate a strong allocation to rRNA rich ribosomes with increased growth rate. The study of change in genome- to cell size was not conclusive, but the results indicate no significant correlation in the study. These findings may indicate a selection for smaller cell size if the predicted climate change results in further increase in oceanic temperature. An increase in stratification and reduced mixing will also affect growth patterns and stoichiometric responses, and potentially seasonal dynamics with profound ecological impact.