IMPORTANCE OF IRON FOR PLANKTON BLOOMS AND CARBON-DIOXIDE DRAWDOWN INTHE SOUTHERN-OCEAN

THE iron hypothesis(1-3)-the suggestion that iron is a limiting nutrie nt for plankton productivity and consequent CO2 drawdown-has been test ed by small-scale experiments in incubation bottles in the subarctic P acific(2,4) and Southern(5-7) Oceans, and by a recent large-scale expe riment in the equatorial Pacific Ocean(8,9). Here we test the idea by looking at natural levels of productivity in regions of the Southern O cean with differing iron abundance, In the southerly branch of the Ant arctic circumpolar current (ACC), upwelling of deep waters supplies su fficient iron to the surface to sustain moderate primary production bu t not to permit blooms to develop, In contrast, within the fast-flowin g, iron-rich jet of the polar front (PF), spring blooms produced phyto plankton biomass an order of magnitude greater than that in southern A CC waters, leading to CO2 undersaturation. The plankton-rich PF waters were sharply delineated from adjacent iron-poor waters, indicating th at iron availability was the critical factor in allowing blooms to occ ur.