Carbon isotopes may be a useful paleoceanographic tool for reconstructing past pCO2 of surface water, but isotopic composition of particulate organic carbon has been shown in both field and laboratory studies to be affected by pCO2, growth rate, cell size, cell geometry, light availability, carbon metabolism and species composition. To date, field studies have not constrained the dominant factors controlling surface water d¹³C_POC. We present high-resolution time series data from a study in Ryder Bay, Antarctica, which compares seasonal fluctuations in mixed-layer d¹³C_POC with detailed ecological and morphological analysis of phytoplankton communities, community productivity, d¹³C_DIC, nutrient dynamics, and hydrographic parameters. Preliminary results indicate that physical processes such as ocean-atmosphere gas exchange and upwelling do not significantly affect d¹³C_POC signatures. In contrast, speciation shifts of diatom assemblages show strong correlation with changes in d¹³C_POC signals, supporting recent suggestions that taxonomic data are necessary for confident interpretation of sedimentary d¹³C records.