WATER COLUMN AND SEA-ICE PRIMARY PRODUCTION DURING AUSTRAL SPRING IN THE BELLINGSHAUSEN SEA

The findings of a cruise to study the phytoplankton bloom dynamics ass ociated with the marginal ice zone (MIZ) in the Bellingshausen Sea dur ing Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85 degrees W meridian in order to establish the productivity of the microalgae associated with sea-ic e and in the water column. In addition, a series of transects along 85 degrees W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton bi omass and production were noted at ice-covered and ice-edge stations a nd in the open water close to the ice edge. Observations from the tran sects indicated no development of a classical ice edge bloom despite e vidence that sea-ice had retreated more than 100 km during the study p eriod. Survey data along the 85 degrees W line revealed a region of hi gh chlorophyll, centred on 67.5 degrees S, which was initially observe d during brash ice conditions. This feature, which remained geographic ally consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production (C-1 4) in this high chlorophyll region was ca 0.8 g C m(-2) day(-1), more than a times higher than noted in the MIZ. Phytoplankton photosyntheti c characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly f rom oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using C-14 observations suggested that phy toplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be require d to sustain the observed high and constant algal biomass. Ln addition , although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiate d under open water conditions.