PHYTOPLANKTON COMPOSITION AND CELL CARBON DISTRIBUTION IN PRYDZ BAY, ANTARCTICA - RELATION TO ORGANIC PARTICULATE MATTER AND ITS DELTA-C-13VALUES

In January-February 1991, in Prydz Bay, phytoplankton bloom was eviden t in the inner shelf area with the dominant diatoms being represented mainly by pennate species of the Nitzschia-Fragilariopsis group. Dinof lagellates and naked flagellates were most abundant in the centre of t he bay; however, larger heterotrophic species prevailed at the souther n 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 pennat e 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 sh elf region were mainly contributed by large centric diatoms (70-110 mu m) and small dinoflagellates (5-25 mu m). Wide ranges of algal cell s izes were observed in both southern and northern communities; the over lapping of sizes of diatoms and flagellates, the latter containing het erotrophs, suggested complex trophic relationships within the plankton and an enhanced heterotrophic activity in the south. North-to-south v ariations in surface delta(13)C of suspended particulate organic matte r (SPOM), (range -31.85 to -20.12 parts per thousand) were directly re lated to the concentration of particulate matter: this suggested the e ffect of biomass, and thus of dissolved CO2 limitation on carbon fract ionation. Three types of species assemblages were distinguished, corre sponding to different narrow ranges of delta(13)C values (-20.12 to -2 2.37 parts per thousand; -24.50 to -26.65 parts per thousand; -29.73 t o -31.85 parts per thousand); dominant species within each assemblage are the likely major determinants of the carbon isotopic composition a nd variation of SPOM. Pennate diatoms, such as Nitzschia curta and N.s ubcurvata appear to have made the major imprint on the highest delta(1 3)C values. Phaeocystis, naked flagellates, autotrophic dinoflagellate s and centric diatoms are Likely to have caused the lower delta(13)C v alues of SPOM. It appears that variations in both biomass concentratio n and in phytoplankton species composition have contributed to the car bon isotopic values of SPOM in Prydz Bay.