Suspended and sea ice particulate organic carbon (POC) collected from the Ross Sea, Antarctica, shows seasonal enrichments in 13C unrelated to atmospheric pCO2 variations. The carbon isotopic composition of phytoplankton bloom-related POC(Δ13CPOC) collected from the southwestern Ross Sea during a 1992 bloom event is strongly correlated with POC concentration. This correlation may be attributed to high growth rates, species-specific fractionation, input from sea ice communities enriched in 13C, increased heterotrophic recycling, or high rates of bloom-related CO2 drawdown in the upper water column. While all these factors likely contribute to POC 13C enrichment, we favor the latter hypothesis because of the link between 13C enrichment and partial isolation of the surface water CO2 pool in areas of sea ice melt-induced stratification of the upper water column. The CO2 drawdown hypothesis is further supported by dissolved inorganic carbon Δ13C enrichments of up to 2--3? in the vicinity of phytoplankton blooms. Patterns of isotopic composition and organic carbon concentration in Ross Sea surface sediments resemble that of the upper water column, particularly in areas of frequent, annually recurrent phytoplankton blooms. Overall, surface sediment Δ13C values are 1--3? enriched relative to surface water POC, with the highest Δ13C values found in areas that receive large amounts of phytoplankton bloom detritus. Future downcore carbon isotopic analyses may prove useful as tracers of bloom dynamics in meltwater-influenced areas, where large enrichments in 13C coincide with high productivity. ¿ 2000 American Geophysical Union