The elements carbon (C), nitrogen (N) and phosphorus (P) are the most important constituents of essential biomolecules. P is a crucial element in ribosomal ribonucleic acid (RNA) is indirectly responsible for protein biosynthesis, and thus growth. According to the growth rate hypothesis coined by Elser (1992), we can predict that faster growth must be associated with higher requirement for P, due to allocation of this element into P rich ribosomes. Temperature in turn can further modulate C:P ratio of an organisms, and hence animals requirements. This can potentially be manifested in alterations of life histories whenever temperature changes. Recently, an increase in temperature has been shown to promote P limitation in Daphnia magna, as manifested by changes in somatic growth rate (Persson et al. 2011). However, we still do not know which consequences this might have for life histories and in turn population growth. This thesis is an effort to assess effects of different diet and temperature regimes from a life history perspective. I demonstrate that temperature is significant for the survival probability of D. magna. P limitation affects survival negatively at low temperatures, but not when temperature increase. P limited animals have significantly smaller probability of maturation and lower fecundity in general compared to animals kept on P rich food. Matrix modelling reveals that intrinsic rate of increase (λ) is more sensitive to changes in survival until first reproduction than changes in fecundity. This finding might indicate a trade- off for animals in cold temperatures. The trade- off promotes high survival in longer periods when food quality is diminished, but delaying or stalling reproduction. Such strategies can for instance be used to survive winter in ice covered lakes, when temperatures and food quantity is low. The main finding of this study is that individual body growth rate is not always best predictor for stoichiometric effects on population intrinsic rate of increase. My finding are important to better understand the implications of stoichiometry and temperature change on the overall fitness of D. magna throughout their life.