Iron deficiency limits phytoplankton growth in Antarctic waters

Enrichment experiments were performed in the Ross Sea to test the hypothesis that iron deficiency is responsible for the phytoplankton's failure to use up the luxuriant major nutrient supplies found in these and all other offshore Antarctic ocean waters . Nitrate uptake rates in the controls without added trace elements ranged from 0.58 to 1.22 μmol kg −1 d −1 ; the addition of 1 to 5 nmol of unchelated Fe per liter resulted in rates that were 2 to 10 times higher (2.54 to 6.00 μmol NO 3 kg −1 d −1 ). Rates in bottles with 2 nmol Mn added were identical to those in the controls (0.57 to 1.04 μmol NO 3 kg −1 d −1 ). Total decreases in NO 3 were balanced by increases in particulate organic N. These results suggest that Fe deficiency is the primary reason that the present‐day southern ocean biological pump is shut off. In contrast, iron was 50 times more abundant during the last glacial maximum; greater Fe availability may have stimulated the biological pump and contributed to the ice age drawdown of atmospheric C0 2 . These results also imply that large‐scale southern ocean Fe fertilization is feasible, at least in terms of the total amounts of Fe required; i.e., 100,000 to 500,000 tons yr −1 .