Phytoplankton biomass and size fractions in surface waters of the Australian sector of the Southern Ocean

We collected surface water along the 142nd E meridian from Tasmania to Antarctica in December 1999. We measured temperature, salinity and total chlorophyll a; additionally, we collected suspended particle size fractions and used fluorometric analysis to determine the quantity of chlorophyll a in each of four cell size classes: picoplankton (< 3 mu m), two nanoplankton fractions (3-10 mu m and 10-20 mu m) and microplankton (>= 20 mu m). Changes in temperature and salinity show that we crossed 6 water masses separated by 5 fronts. We found low abundance (< 0.2 mg m(-3)) of chlorophyll in all size classes, with the exception of higher values near the continent (0.2 to 0.4 mg m(-3)). Lowest chlorophyll values (< 0.1 mg m(-3)) were found in the Polar Frontal Zone (51 to 54 degrees S). Microplankton made up the largest portion of total chlorophyll throughout most of the region. We conclude that biomass of all phytoplankton fractions, especially pico- and nanoplankton, was constrained by limiting factors, most probably iron, throughout the region and that ecosystem dynamics within a zone are not circumpolar but are regionalized within sectors.