| Summary: | 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 mu g l(-1); range 92-1048 mu g 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 mu g l(-1); range 7-147 mu g 1(-1)) in the outer shelf region were mainly contributed by large centric diatoms (70-110 mu m) and small dinoflagellates (5-25 mu m). 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 delta(13)C of suspended particulate organic matter (SPOM), (range -31.85 to -20.12 parts per thousand) 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 delta(13)C values (-20.12 to -22.37 parts per thousand; -24.50 to -26.65 parts per thousand; -29.73 to -31.85 parts per thousand); 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 delta(13)C values. Phaeocystis, naked flagellates, autotrophic dinoflagellates and centric diatoms are Likely to have caused the lower delta(13)C 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. status: published
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