Fertilizing polar oceans with iron to stimulate phytoplankton blooms, which would help draw carbon dioxide from the atmosphere has been suggested as a means of mitigating the effects of rising atmospheric greenhouse gases. Boyd et al. (2004) report results of an experiment in the sub-Arctic ocean near Alaska that raises some previously unreported limitations to this method. These authors find that depletion of silicic acid causes premature termination of the bloom and that the total particulate organic carbon created is only partially transported below the thermocline where it is would be "out of circulation" from possibly returning to the atmosphere as carbon dioxide.
Reference
Boyd, P. W., and Co-Authors, 2004: The decline and fate of an iron-induced sub-arctic phytoplankton bloom. Nature, 428, 549-553.