ISOTOPIC FRACTIONATION DURING NITRATE UPTAKE BY PHYTOPLANKTON GROWN IN CONTINUOUS-CULTURE

The isotopic fractionation associated with uptake of NO3- by six speci es of phytoplankton (two diatoms, one cryptophyte, one chlorophyte and two haptophytes) was measured at a variety of steady-state growth rat es in nitrogen-limited continuous culture. The magnitude of the isotop ic fractionation factor (alpha) declined with growth rate in the diato m Thalassiosira weissflogii, but did not vary consistently with growth rate in the other species. In combination with data from earlier stud ies, these results suggest that the net isotopic fractionation during growth on NO3- is insensitive to growth rate, but may vary significant ly with photon flux density. The six species studied fell into two dis tinct groups: the two diatoms showed mean isotopic fractionation facto rs of 1.0090 and 1.0121, which was significantly different from the va lues of alpha measured for the four flagellate species (mean alpha = 1 .0009-1.0032). These results imply that the isotopic perturbation asso ciated with phytoplankton blooms will be greatest when diatoms are the dominant bloom species. Such isotopic perturbations may provide a use ful tool for studying the dynamics of nitrogen movement through plankt onic food webs.