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°S, 83.5 and 88.0°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 μm, 2–18μm and 0.2–2 μm. 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 μm SF, with this fraction consistently comprising >75% of the total chlorophyll biomass within the HCZ. Although maximal productivity also was associated with the > 18μm 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−. 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°W in the Bellingshausen Sea, that is over an approximate distance of 2470 km, of 50 × 103 tonnes C day−1.