In January-February 1991, in Prydz Bay, phytoplankton bloom was evident in the inner shelf area with the dominant diatoms being represented mainly by pennate species of the Nitzschia-Fragilariopsis group. Dinoflagellates and naked flagellates were most abundant in the centre of the bay; however, larger heterotrophic species prevailed at the southern stations. Cell carbon values (average 317 μg l−1; range 92-1048 μg l−1) found in the bloom in the south were chiefly due to pennate diatoms and larger heterotrophic dinoflagellates. Much lower carbon values (average 51 μg l−1; range 7-147 μg l−1) in the outer shelf region were mainly contributed by large centric diatoms (70-110 mu;m) and small dinoflagellates (5-25 μm). Wide ranges of algal cell sizes were observed in both southern and northern communities; the overlapping of sizes of diatoms and flagellates, the latter containing heterotrophs, suggested complex trophic relationships within the plankton and an enhanced heterotrophic activity in the south. North-to-south variations in surface δ13C of suspended particulate organic matter (SPOM), (range -31.85 to -20.12‰) were directly related to the concentration of particulate matter: this suggested the effect of biomass, and thus of dissolved CO2 limitation on carbon fractionation. Three types of species assemblages were distinguished, corresponding to different narrow ranges of δ13C values (-20.12 to -22.37‰; -24.50 to -26.65‰; -29.73 to -31.85‰); dominant species within each assemblage are the likely major determinants of the carbon isotopic composition and variation of SPOM. Pennate diatoms, such as Nitzschia curta and N.subcurvata appear to have made the major imprint on the highest δ13C values. Phaeocystis, naked flagellates, autotrophic dinofiagellates and centric diatoms are likely to have caused the lower δ13C values of SPOM. It appears that variations in both biomass concentration and in phytoplankton species composition have contributed to the carbon isotopic values of SPOM in Prydz Bay.