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

Samples were taken from the open water region directly north of the ice-edge zone between 65.5 and 69.0 degrees S, 83.5 and 88.0 degrees W by R.R.S. Discovery between 23 November and 8 December 1992 for estimates of size-fractionated chlorophyll a concentrations and primary production 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) immediately south of the surface boundary of the Southern Polar Front and also the absence of a classical stability induced ice-edge bloom, as inferred from a concurrent hydrographic survey. Chlorophyll a concentrations both within and without the HCZ were dominated by the > 18 mu m SF, with this fraction consistently comprising > 75% of the total chlorophyll biomass within the HCZ. Although maximal productivity also was associated with the > 18 mu m SF, at the majority of locations its relative contribution to the total was generally less than for the comparable chlorophyll a samples, indicating a reduced photosynthetic efficiency for this SF compared to the smaller SFs. To the north of the HCZ, primary production was approximately equal between the three SFs. Data from a transect across the HCZ showed the phytoplankton to be dominated by larger diatoms, although a maximum in Phaeocystis was present immediately to the north of the frontal boundary. A succession in the phytoplankton taxa was inferred northwards from the ice-edge to the frontal boundary. A steady-state modelling approach indicated a loss rate of the phytoplankton from the HCZ by mesoscale activity of 0.071 day(-1). Extrapolation of the data obtained during the survey gave an estimate of primary production within the HCZ associated with the Southern Polar Front between longitudes 135-75 degrees W in the Bellingshausen Sea, that is over an approximate distance of 2470 km, of 50 x 10(3) tonnes C day(-1).