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

Phytoplankton species composition and primary production werestudied in the Atlantic Sector of the Southern Ocean in early austral summer1995/1996. Results from photosynthesis-irradiance experiments (P vs E curves)were used to examine photosynthetic adaptation in this part of the ocean. The studyarea comprised 3 different provinces: the Antarctic Polar Front (APF), the AntarcticCircumpolar Current (ACC) beyond the influence of frontal systems, and themarginal ice zone (MIZ). Phytoplankton composition derived from HPLC data,P*m (maximum biomass-specific photosynthetic production rate) values and arealdaily primary production (ADP) rates showed different features for these zones. Thecentral core of the APF was dominated (60%) by a bloom of large (>20 µm)diatoms (Thalassiothrix spp., Pseudonitzschia cf. lineola and Chaetoceros spp.),equal values for P*m at the surface and 1% light depths and ADP rates exceeding900 mg C m-2 d-1. At the fringes of the APF core, phytoplankton were smaller,diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytesbecame more important within the community. Chlorophyll a concentrations andADP rates were low and comparable to values for the ACC outside the front: <0.5mg m-3 and <300 mg C m-2 d-1 respectively. Beyond the frontal systems, P*mvalues from the 1% light depth were significantly higher than at the surface. Therewas also a bloom of large phytoplankton species within the MIZ, dominated incontrast by Phaeocystis spp.; this province was characterised by ADP rates of 558mg C m-2 d-1. Vertical mixing processes, temperature, silicate concentrations andzooplankton grazing seem to be the factors controlling production and growth ofphytoplankton at this time.