Blooms of the marine diatom Skeletonema costatum were initiated in closed-system batch cultures with P-deficient medium under two different initial concentrations of dissolved molecular CO2 ([CO2,aq]: 20.6 and 4.5 µmol L-1). Algal C:N:P ratios strongly increased with decreasing P concentration. In the exponential growth phase, C:N ratios were 1.3 mol mol-1 higher in the low relative to the high [CO2,aq] treatment. There was no [CO2,aq] effect on C:N:P ratios during P-limited growth. Carbon isotope fractionation (ep) was 2-3 per mil higher in the high [CO2,aq] treatment. With growth rate decreasing due to P-limitation, ep increased in both [CO2,aq] treatments by 2-3 per mil despite decreasing [CO2,aq]. Under these conditions the effect of decreasing growth rate on isotope fractionation strongly dominated over that of declining CO2 availability. When extrapolated to the natural environment, these results imply that systematic changes in algal growth, as occurring during the course of phytoplankton blooms, may affect algal isotope fractionation. These results severely complicate the interpretation of carbon isotope measurements in suspended and sedimentary organic matter.