Increased water temperature due to climate change may affect macrophyte phenology and nutrient content. In experimentally heated mesocosms the emergence and growth of Potamogeton crispus shoots under ambient and increased temperatures (+4.5 °C) were tracked over 55 days. At the end of the experiment we measured the C, N and P content of the P. crispus leaves. The results indicate that warming advanced the emergence of P. crispus shoots by approximately 10 days, whereas the final number of shoots and plant biomass were similar in ambient and heated tanks. Furthermore, warming influenced the ecological stoichiometry of this plant significantly. Leaf C and N content were both less in the heated tanks resulting in an increase in C:N ratios, whereas P content and C:P and N:P ratios were not affected.