Primary production, physiological state and composition of phytoplankton in the Atlantic Sector of the Southern Ocean

Phytoplankton species composition and primary production were studied in th e Atlantic Sector of the Southern Ocean in early austral summer 1995/1996. Results from photosynthesis-irradiance experiments (P vs E curves) were use d to examine photosynthetic adaptation in this part of the ocean. The study area comprised 3 different provinces: the Antarctic Polar Front (APF), the Antarctic Circumpolar Current (ACC) beyond the influence of frontal system s, and the marginal ice zone (MIZ). Phytoplankton composition derived from HPLC data, P-m* (maximum biomass-specific photosynthetic production rate) v alues and areal daily primary production (ADP) rates showed different featu res for these zones. The central core of the APF was dominated (60 %) by a bloom of large (>20 mu m) diatoms (Thalassiothrix spp., Pseudonitzschia cf. lineola and Chaetoceros spp.), equal values for P-m* at the surface and 1% light depths and ADP rates exceeding 900 mg C m(-2) d(-1). At the fringes of the APF core, phytoplankton were smaller, diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytes became more important wi thin the community. Chlorophyll a concentrations and ADP rates were low and comparable to values for the ACC outside the front: <0.5 mg m(-3) and <300 mg C m(-2) d(-1) respectively. Beyond the frontal systems, P-m* values fro m the 1% light depth were significantly higher than at the surface. There w as also a bloom of large phytoplankton species within the MIZ, dominated in contrast by Phaeocystis spp.; this province was characterised by ADP rates of 558 mg C m(-2) d(-1). Vertical mixing processes, temperature, silicate concentrations and zooplankton grazing seem to be the factors controlling p roduction and growth of phytoplankton at this time.