PHYTOPLANKTON DISTRIBUTIONS AND PRODUCTION IN THE BELLINGSHAUSEN SEA,AUSTRAL SPRING 1992

Samples were taken from the open water region directly north of the ic e-edge zone between 65.5 and 69.0 degrees S, 83.5 and 88.0 degrees W b y R.R.S. Discovery between 23 November and 8 December 1992 for estimat es of size-fractionated chlorophyll a concentrations and primary produ ction and also for data on phytoplankton taxonomic distributions. The size-fractions (SF) considered were > 18 mu m, 2-18 mu m and 0.2-2 mu m. The data confirmed the presence of a high chlorophyll zone (HCZ) im mediately south of the surface boundary of the Southern Polar Front an d also the absence of a classical stability induced ice-edge bloom, as inferred from a concurrent hydrographic survey. Chlorophyll a concent rations both within and without the HCZ were dominated by the > 18 mu m SF, with this fraction consistently comprising > 75% of the total ch lorophyll biomass within the HCZ. Although maximal productivity also w as associated with the > 18 mu m SF, at the majority of locations its relative contribution to the total was generally less than for the com parable chlorophyll a samples, indicating a reduced photosynthetic eff iciency for this SF compared to the smaller SFs. To the north of the H CZ, primary production was approximately equal between the three SFs. Data from a transect across the HCZ showed the phytoplankton to be dom inated by larger diatoms, although a maximum in Phaeocystis was presen t immediately to the north of the frontal boundary. A succession in th e phytoplankton taxa was inferred northwards from the ice-edge to the frontal boundary. A steady-state modelling approach indicated a loss r ate of the phytoplankton from the HCZ by mesoscale activity of 0.071 d ay(-1). Extrapolation of the data obtained during the survey gave an e stimate of primary production within the HCZ associated with the South ern Polar Front between longitudes 135-75 degrees W in the Bellingshau sen Sea, that is over an approximate distance of 2470 km, of 50 x 10(3 ) tonnes C day(-1).