DESIGN OF A SMALL-SCALE INSITU IRON FERTILIZATION EXPERIMENT

We consider the design of an in situ enrichment experiment to test the hypothesis that Fe deficiency limits primary productivity in some regions of the ocean. A small-scale (10-100 km2) experiment would be preferable for logistical reasons, but the chief practical difficulty is that the patch of enriched water will fragment or streak severely. This problem is made tractable by adding a conservative marker such as sulfur hexafluoride (SF6) along with the Fe release, enabling rapid detection of enriched water despite fragmentation. The concentration of SF6 would indicate the amount of dilution the original injection of Fe had undergone, and regression of parameters such as pCO2 and Chl against SF6 at several times postinjection would enable the degree of fertilization to be assessed. Due to the complexity of Fe chemistry in seawater, we cannot be certain that the added Fe will adequately mimic the input of natural aerosol Fe to the surface. Thus, a null result from the experiment (i.e. no observable fertilization) would be less valuable than a positive result: a positive outcome would be proof that Fe enrichment enhances productivity, but a null result would not be absolute proof of the converse.