We present results from a field study of inorganic carbon (C) acquisition by Ross Sea phytoplankton during Phaeocystis-dominated early season blooms. Isotope disequilibrium experiments revealed that HCO3- was the primary inorganic C source for photosynthesis in all phytoplankton assemblages. From these experiments, we also derived relative enhancement factors for HCO3-/CO2 interconversion as a measure of extracellular carbonic anhydrase activity (eCA). The enhancement factors ranged from 1.0 (no apparent eCA activity) to 6.4, with an overall mean of 2.9. Additional eCA measurements, made using membrane inlet mass spectrometry (MIMS), yielded activities ranging from 2.4 to 6.9 U · [μg chl a]-1 (mean 4.1). Measurements of short-term C-fixation parameters revealed saturation kinetics with respect to external inorganic carbon, with a mean half-saturation constant for inorganic carbon uptake (K1/2) of ∼380 μM. Comparison of our early springtime results with published data from late-season Ross Sea assemblages showed that neither HCO3- utilization nor eCA activity was significantly correlated to ambient CO2 levels or phytoplankton taxonomic composition. We did, however, observe a strong negative relationship between surface water pCO2 and short-term 14C-fixation rates for the early season survey. Direct incubation experiments showed no statistically significant effects of pCO2 (10 to 80 Pa) on relative HCO3- utilization or eCA activity. Our results provide insight into the seasonal regulation of C uptake by Ross Sea phytoplankton across a range of pCO2 and phytoplankton taxonomic composition. © 2010 Phycological Society of America.