The isotopic fractionation associated with uptake of NO3- by six species of phytoplankton (two diatoms, one cryptophyte, one chlorophyte and two haptophytes) was measured at a variety of steady-state growth rates in nitrogen-limited continuous culture. The magnitude of the isotopic fractionation factor (alpha) declined with growth rate in the diatom Thalassiosira weissflogii, but did not vary consistently with growth rate in the other species. In combination with data from earlier studies, these results suggest that the net isotopic fractionation during growth on NO3- is insensitive to growth rate, but may vary significantly with photon flux density. The six species studied fell into two distinct groups: the two diatoms showed mean isotopic fractionation factors of 1.0090 and 1.0121, which was significantly different from the values of alpha measured for the four flagellate species (mean alpha = 1.0009-1.0032). These results imply that the isotopic perturbation associated with phytoplankton blooms will be greatest when diatoms are the dominant bloom species. Such isotopic perturbations may provide a useful tool for studying the dynamics of nitrogen movement through planktonic food webs.
