Phytoplankton community structure and the drawdown of nutrients and CO2 inthe Southern Ocean

Data from recent oceanographic cruises show that phytoplankton community st ructure in the floss Sea is related to mixed layer depth. Diatoms dominate in highly stratified waters, whereas Phaeocystis antarctica assemblages dom inate where waters an more deeply mixed. The drawdown of both carbon dioxid e (CO2) and nitrate per mole of phosphate and the rate of new production by diatoms are much Lower than that measured for P. antarctica. Consequently, the capacity of the biological community to draw down atmospheric CO2 and transport it to the deep ocean could diminish dramatically if predicted inc reases in upper ocean stratification due to climate warming should occur.